December 2013, VOLUME 201
NUMBER 6

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December 2013, Volume 201, Number 6

Residents' Section

Review

Diagnostic Imaging of Fetal and Pediatric Orbital Abnormalities

+ Affiliations:
1 Department of Radiology, University of Washington School of Medicine, Seattle, WA.

2 Department of Radiology, Seattle Children's Hospital, 4800 Sand Point Way NE, MA.7.220, Seattle, WA 98105.

3 Department of Radiology, BC Children's Hospital, Vancouver, BC, Canada.

Citation: American Journal of Roentgenology. 2013;201: W797-W808. 10.2214/AJR.13.10949

ABSTRACT
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OBJECTIVE. The orbit contains structures from which a wide spectrum of disease can arise. This article focuses on orbital anatomy and a simple compartmental approach to evaluating the orbit on diagnostic imaging. The characteristic findings of key fetal structural diseases and a wide spectrum of pediatric orbital disorders, including inflammatory disorders and developmental lesions, and the differential diagnosis of benign versus malignant masses will be discussed.

CONCLUSION. Orbital abnormalities in fetuses may be recognized using ultrasound and MRI. Anophthalmia, hypertelorism, and hypotelorism either may be part of a genetic syndrome or may be related to a developmental abnormality of the fetal skull. In the pediatric population, cross-sectional imaging with CT and MRI offers a means to assess which compartments of the orbit are affected. Aggressive masses have characteristic features and must be evaluated for intracranial extension.

Keywords: developmental abnormalities, fetal imaging, infection, neoplasm, ocular imaging, orbit, pediatric imaging

The orbit is a bony space containing the globe, extraocular muscles, and neurovascular structures from which a wide spectrum of disease can arise. Abnormalities affecting the pediatric orbit may be symptomatic or may be incidentally detected on clinical, ophthalmologic, or imaging examinations. Evaluation of the orbits is a routine component of prenatal ultrasound, fetal MRI, and any pediatric head and neck imaging study. The role of the radiologist includes determining the correct imaging modality to use, diagnosing abnormalities, establishing the extent of disease, and assessing response to therapy. Although some disease processes overlap with those seen in the adult population, many orbital abnormalities in children are unique to this population. The purposes of this article are, first, to provide the reader with a systematic approach for evaluating the fetal orbit and the pediatric orbit and globe based on the intraorbital compartments; second, to review orbital anatomy and highlight important structural relationships; third, to describe the optimal imaging modality for a suspected abnormality; fourth, to describe key imaging findings for common orbital abnormalities and discuss how to develop a reasonable differential diagnosis; and, fifth, to explain how to differentiate aggressive from nonaggressive tumors of the orbit.

Anatomy
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The orbit is a complex compartment within the skull containing the globe, extraocular muscles, fat, neurovascular structures, and lacrimal gland. Its anatomy and close relationship to both the intracranial contents and the paranasal sinuses have important implications for pathology.

The shape of the orbit is similar to a pyramid lying on its side so that the base of the pyramid is the anterior portion and the apex is the posterior aspect [1] (Fig. 1). The orbit is rotated so that the lateral wall is the most exposed to external forces [1]. This wall is the thickest and therefore is the most protective wall of the orbit; it is composed of the zygoma, the greater wing of the sphenoid, and the frontal bone [1, 2]. The inferior orbital fissure separates the lateral wall from the floor of the orbit, and the superior orbital fissure separates the lateral wall from the roof [1, 2]. The medial wall is composed mostly of the ethmoid bone or lamina papyracea and is extremely thin. The thinness of the medial wall renders the orbit vulnerable to processes such as the spread of infection from the adjacent ethmoid sinuses, medial wall fractures, or injury during surgery. The floor is composed of the orbital plate of the maxilla, the zygoma, and the palatine bone. Finally, the orbital roof is formed from the frontal bone and a small portion of the lesser wing of the sphenoid [1, 2].

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Fig. 1A —Anatomy of the bony orbit and conal structures.

A, Three-dimensional surface-rendered CT image of face of 3-month-old girl shows pyramidal shape of bony orbit. Optic foramen and superior and inferior fissures are at posterior apex of orbit leading to cranial fossa. Fr = frontal bone, Sp = sphenoid bone, Et = ethmoid bone, Zy = zygoma, Max = maxilla.

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Fig. 1B —Anatomy of the bony orbit and conal structures.

B, Schematic of orbit in axial plane shows extraocular muscles arising from tight fibrous ring, called annulus of Zinn, through which optic nerve and orbital vessels pass. (Adapted by Patricia Chapman with permission from Patrick J. Lynch)

The orbital apex is the site of communication between the orbit and the pterygopalatine fossa and middle cranial fossa via the superior and inferior orbital fissures and the optic canal. The optic nerve, ophthalmic artery, and central retinal vein travel in the optic canal, whereas the superior orbital fissure contains cranial nerves III, IV, V1, and VI and the superior ophthalmic vein. The inferior orbital fissure contains the maxillary division of the trigeminal nerve (cranial nerve V2) and the infraorbital artery and vein [1]. There is a tight fibrous ring at the orbital apex, which is called the “annulus of Zinn” or “common tendinous ring,” from which the four rectus muscles arise. These muscles are the landmark boundaries defining the extra and intraconal spaces [1] (Fig. 1).

Assessment of the Fetal Orbit by Ultrasound and MRI

Because ocular abnormalities of the fetus may be bilateral and symmetric and thus easy to overlook, a simple checklist is imperative when reviewing any prenatal imaging study. This checklist includes determining if the eyes are present, observing the morphology of the lens and vitreous, and measuring ocular biometry.

Both the normal fetal lens and vitreous appear hypoechoic by ultrasound. On endovaginal ultrasound, the fetal lens may be visualized as early as 14 weeks’ gestational age [3]. The outline of the lens may be visible as a thin, echogenic line in the anterior aspect of the globe (Fig. 2A); however, the lens may be difficult to see depending on the angle of the insonation beam relative to the lens position. On T2-weighted imaging, the lens is low in signal intensity relative to the high-signal intensity vitreous (Fig. 2B). Ocular biometry using ultrasound to measure the bony landmarks of the medial and lateral orbital walls in the axial plane has been well established [47]. Measurements include the binocular distance between the two malar margins, the interocular distance between the two bony ethmoidal margins, and the ocular distance itself. Biometry using fetal MRI to measure the interface of the high-signal intensity vitreous and the low-signal intensity orbital margins in either the transverse or coronal plane has also been established [810] (Fig. 2C). These measurements can be compared with growth charts to determine if the globes are abnormally small or large and to determine if the interocular distance is abnormally narrow or wide (Table 1).

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Fig. 2A —Normal-appearing orbits in fetus at 19 weeks’ gestational age.

A, Ultrasound image in coronal plane shows lens. Fetal lens (arrows) is identified in anterior aspect of globe by thin echogenic line.

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Fig. 2B —Normal-appearing orbits in fetus at 19 weeks’ gestational age.

B, Single-shot fast spin-echo T2-weighted image shows lens is dark against high-signal intensity vitreous. Arrowheads point to globes.

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Fig. 2C —Normal-appearing orbits in fetus at 19 weeks’ gestational age.

C, Single-shot fast spin-echo T2-weighted image shows binocular distance (BOD) and interocular distance (IOD). BOD and IOD are measured using well-defined interface of vitreous and low-signal intensity orbital margins. These measurements can be made in either axial or coronal plane.

TABLE 1: Compiled Biometry of Fetal Orbits From Three Published Sources

Hypotelorism refers to an interocular distance below the 5th percentile [11]. Early embryologic development of the orbits involves migration of paired nasal swellings medially and inferiorly, followed by fusion with the midline frontal swelling to form the nose; overmigration leads to primary hypotelorism. The underlying genetic programming that governs this maldevelopment of the two halves of the face also influences the development of the brain, and therefore hypotelorism most commonly is associated with the spectrum of holoprosencephaly [11, 12]. Secondary hypotelorism relates to abnormal skull formation like that seen in microcephaly and metopic synostosis [12].

Hypertelorism refers to an interocular distance above the 95th percentile [12]. During early development of the primordial face, if the nasal swellings do not migrate far enough medially and inferiorly, the result is primary hypertelorism. The underlying cause is usually chromosomal anomalies and syndromes such as frontonasal dysplasia [3], which is characterized by hypertelorism, facial clefting, and callosal agenesis.

Anophthalmia refers to the complete absence of the globe, although the eyelids, conjunctiva, and lacrimal apparatus will be present (Fig. 3). Primary anophthalmia is usually associated with chromosomal abnormalities and genetic syndromes, such as trisomy 13, SOX2-related eye disorders, Walker-Warburg syndrome, and CHARGE (coloboma, heart anomaly, choanal atresia, retardation, genital and ear anomalies) syndrome [3, 13]. Secondary anophthalmia is caused by an in utero event, such as a toxic or metabolic insult (i.e., abnormally low or high vitamin A levels), a vascular event, or infection (i.e., rubella). With secondary anophthalmia, a small amount of ocular tissue may be present, and the ocular diameter will be below the 5th percentile [11].

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Fig. 3A —Anophthalmia in fetus at 20 weeks’ gestational age.

A, Coronal ultrasound image of orbits (A) and axial T2-weighted image obtained through orbits (B) show normal globe and lens on one side (arrowhead) and lack of any clearly defined globe in contralateral orbit (arrow). Other images (not shown) also depicted extensive cerebral and cerebellar malformations including agenesis of corpus callosum, cerebellar hypoplasia, severe ventriculomegaly, schizencephaly, and extensive polymicrogyria. Pregnancy was terminated and findings of anophthalmia and brain malformations were confirmed by autopsy.

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Fig. 3B —Anophthalmia in fetus at 20 weeks’ gestational age.

B, Coronal ultrasound image of orbits (A) and axial T2-weighted image obtained through orbits (B) show normal globe and lens on one side (arrowhead) and lack of any clearly defined globe in contralateral orbit (arrow). Other images (not shown) also depicted extensive cerebral and cerebellar malformations including agenesis of corpus callosum, cerebellar hypoplasia, severe ventriculomegaly, schizencephaly, and extensive polymicrogyria. Pregnancy was terminated and findings of anophthalmia and brain malformations were confirmed by autopsy.

Assessment of the Pediatric Orbit on Cross-Sectional Imaging

When evaluating the orbit on postnatal imaging, the globe size, position, and symmetry should be evaluated. To assess for proptosis using CT or MRI, an axial image including the lens bilaterally is chosen, and a transverse line between the two zygomatic processes is drawn. A perpendicular line from the interzygomatic line to the anterior margin of the globe should measure less than 21 mm [14] (Fig. 4A). Alternatively, a line can be drawn from the medial canthus to the lateral canthus of each orbit and approximately one third of the globe should be posterior to this intercanthal line (Fig. 4B). The optic nerves are isointense to white matter on all sequences with no enhancement after contrast administration [15] (Fig. 4B). The size and contour of the extraocular muscles should be symmetric and the myotendinous junctions should be smaller than their muscle belly [16]. The superior ophthalmic veins are symmetric and thin (Figs. 4C and 4D). Finally, the orbital apex should be assessed for normal, noninfiltrated fat, and the intraorbital and periorbital soft tissues should be assessed for infiltration or masses.

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Fig. 4A —Assessment of orbital structures in axial plane on cross-sectional imaging.

A, Appearance of normal orbits in 8-year-old boy on axial T2-weighted MR image (A) and in 5-year-old boy on axial unenhanced CT image (B). Cross-sectional imaging shows bilateral globe position is symmetric within bony orbit. Relative position of anterior aspect of globe to interzygomatic line (horizontal line) is normally less than 21 mm (distance depicted by vertical lines), as shown in A, and approximately one third to one half of globe should be posterior to intercanthal line (oblique lines), as shown in B. T2-weighted image (A) shows lens is readily outlined by bright vitreous in anterior globe, and MRI shows optic nerves (arrowheads in A) are symmetric in diameter. In B, arrow depicts myotendinous junction of medial rectus muscle.

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Fig. 4B —Assessment of orbital structures in axial plane on cross-sectional imaging.

B, Appearance of normal orbits in 8-year-old boy on axial T2-weighted MR image (A) and in 5-year-old boy on axial unenhanced CT image (B). Cross-sectional imaging shows bilateral globe position is symmetric within bony orbit. Relative position of anterior aspect of globe to interzygomatic line (horizontal line) is normally less than 21 mm (distance depicted by vertical lines), as shown in A, and approximately one third to one half of globe should be posterior to intercanthal line (oblique lines), as shown in B. T2-weighted image (A) shows lens is readily outlined by bright vitreous in anterior globe, and MRI shows optic nerves (arrowheads in A) are symmetric in diameter. In B, arrow depicts myotendinous junction of medial rectus muscle.

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Fig. 4C —Assessment of orbital structures in axial plane on cross-sectional imaging.

C, Contrast-enhanced CT images of healthy 4-year-old girl (C) and 4-year-old girl with carotid-cavernous fistula caused by head trauma (D). Ophthalmic veins (arrows), located in superior aspect of intraconal space, should be uniformly thin, as shown in C, in contrast to abnormally dilated superior ophthalmic veins shown in D.

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Fig. 4D —Assessment of orbital structures in axial plane on cross-sectional imaging.

D, Contrast-enhanced CT images of healthy 4-year-old girl (C) and 4-year-old girl with carotid-cavernous fistula caused by head trauma (D). Ophthalmic veins (arrows), located in superior aspect of intraconal space, should be uniformly thin, as shown in C, in contrast to abnormally dilated superior ophthalmic veins shown in D.

Development of a Differential Diagnosis by Anatomic Compartment

After a systematic evaluation of the orbits using CT or MRI, an appropriate differential diagnosis can be established by identifying the anatomic compartment within which an abnormality is centered. It is useful to consider the native structures within the separate spaces of the orbit and then contemplate the pathologic processes that can arise from these tissues. The orbit can be divided into compartments using the extraocular muscles to define the cone of the orbit. Table 2 summarizes the abnormalities arising from the extraconal, conal, and intraconal spaces of the pediatric orbit. Next, we will detail the entities that are most relevant for the radiology resident and general radiologist.

TABLE 2: Abnormalities Arising From the Extraconal, Conal, and Intraconal Spaces of the Pediatric Orbit
Inflammatory Disorders

Orbital cellulitis—The orbital septum is a fibrous membrane that arises anteriorly from the orbital rim and is continuous with the frontal and periorbital periosteum [1719]. Superiorly, it joins with the levator aponeurosis just proximal to the superior tarsal plate and inferiorly with the lower eyelid retractors just proximal to the inferior tarsal plate [18] (Fig. 5A). It acts as a fibrous barrier between the orbit and the soft tissues external to the orbit [1719]. Preseptal infection is limited to the eyelid and facial soft tissues anterior to the orbital septum, and postseptal cellulitis also involves orbital structures posterior to the orbital septum [20]. Preseptal infection may present as erythema and edema, whereas patients with postseptal infection can also show proptosis and restricted ocular motion [21]. Risk factors for the development of postseptal cellulitis include spread of infection from the ethmoid sinuses, penetrating trauma, and progression of preseptal infection [22]. In preseptal cellulitis, there is soft-tissue thickening and stranding exclusively anterior to the orbital septum (Fig. 5B), whereas in postseptal cellulitis, the inflammation extends to the intraorbital soft tissues and may manifest with muscle thickening, an abscess, or bony erosion. MRI may show diffuse T1 hypointensity and T2 hyperintensity of the orbital soft tissues (Fig. 5C). Complications of postseptal cellulitis can include a subperiosteal abscess and thrombosis of ophthalmic veins and the cavernous sinus [21]. In the case of a subperiosteal abscess, a rim-enhancing collection confined to the periosteum is present [20].

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Fig. 5A —Cellulitis of orbit.

A, Drawing shows fibrous connection between bony orbit and tarsal plates of eyelids, dividing preseptal and postseptal spaces. (Adapted with permission from A.D.A.M. Inc. by Patricia Chapman)

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Fig. 5B —Cellulitis of orbit.

B, Preseptal cellulitis in 4-year-old boy is diagnosed using contrast-enhanced CT. Axial image shows diffuse left preseptal edema (arrows). Note absence of inflammatory changes posterior to left globe in preseptal cellulitis.

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Fig. 5C —Cellulitis of orbit.

C, Orbital cellulitis in 3-year-old boy with pansinusitis is diagnosed using unenhanced CT. Coronal image shows fluid collection (arrow) within right orbital medial extraconal compartment; this finding is consistent with subperiosteal abscess. There is displacement of right medial rectus and superior oblique muscles. Note fluid attenuation throughout paranasal sinuses.

Nonspecific orbital inflammatory disorder—When faced with enlargement of one or more of the extraocular muscles, there are two inflammatory disorders one should consider: thyroid ophthalmopathy and nonspecific orbital inflammation (NSOI). Thyroid ophthalmopathy is an autoimmune reaction in patients with Graves disease that leads to inflammation, the accumulation of glycosaminoglycans in the extraocular muscles, and fibrosis of the orbital fat [23]. In one study by Chan and colleagues [24], 12% of children with systemic Graves disease were found to have proptosis. The classic imaging findings are proptosis, enlargement of the extraocular muscles, and sparing of the myotendinous insertions.

NSOI, also known as orbital pseudotumor, is caused by infiltration of the orbit by a mixture of inflammatory cells, including plasma cells, lymphocytes, and histiocytes [25, 26]. Pediatric cases account for approximately 11.5% of the total number of NSOI cases [27]. In contrast to thyroid ophthalmopathy, typical imaging findings include extraocular muscle enlargement with infiltration of the orbital fat leading to shaggy, ill-defined muscular borders and enlarged myotendinous insertions [25, 26] (Fig. 6). The acute form responds very well to steroids, whereas the chronic form may have a poor response secondary to fibrosis and may even require methotrexate and radiation therapy [25, 26]. Important differential considerations include infectious myositis, Graves disease, and lymphoma.

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Fig. 6A —11-year-old boy with nonspecific orbital inflammatory disorder (orbital pseudotumor).

A, Axial (A) and coronal (B) contrast-enhanced CT images show profound enlargement of all left extraocular muscles. Right orbital muscular cone is normal. Note involvement of myotendinous junction of left medial rectus muscle (arrow, A) compared with normal contralateral side (arrowhead, A). As is typical of this disorder, there is infiltration of orbital fat by soft-tissue–fluid density (arrowheads, B).

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Fig. 6B —11-year-old boy with nonspecific orbital inflammatory disorder (orbital pseudotumor).

B, Axial (A) and coronal (B) contrast-enhanced CT images show profound enlargement of all left extraocular muscles. Right orbital muscular cone is normal. Note involvement of myotendinous junction of left medial rectus muscle (arrow, A) compared with normal contralateral side (arrowhead, A). As is typical of this disorder, there is infiltration of orbital fat by soft-tissue–fluid density (arrowheads, B).

Developmental Disorders

Coloboma—A coloboma is a gap or defect in the globe resulting from the failure of the choroidal fissure to close; the choroidal fissure is the embryologic structure through which the hyaline vasculature passes [20, 22, 28] (Fig. 7A). It can affect any part of the globe and may lead to the retrobulbar herniation of vitreous fluid [20, 22, 28]. Colobomas can be unilateral or bilateral and range from small defects in the posterior aspect of the globe to large retrobulbar cystic masses [20]. They occur in 1 in 1000 infants [28]. CT can show an intraconal cystic mass closely associated with the posterior aspect of the globe [20, 22]. On MRI, cystic contents usually exhibit simple fluid signal intensity (Fig. 7D). Microphthalmos is usually present, although the defect might create an asymmetrically large globe [20]. Clinical differential considerations include duplication cyst or advanced myopia [22]. Coloboma can occur as part of the CHARGE syndrome, consisting of coloboma, heart anomalies, choanal atresia, growth retardation, genital and ear anomalies, which is a set of associations usually secondary to CHD7 mutations [29]. Colobomas may also be diagnosed prenatally using fetal MRI [30].

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Fig. 7A —Developmental lesions of eye.

A, Schematic diagram of developing eye at approximately 5–6 weeks’ gestational age. Primitive vessels delivering blood supply to mesoderm derivatives enter optic stalk through choroidal fissure, which normally completely closes during development. (Adapted by Patricia Chapman from Converse JM, McCarthy JG, Wood-Smith G. Symposium on diagnosis and treatment of craniofacial anomalies. St. Louis, MO: CV Mosby, 1979; p.15, with permission from Elsevier Ltd.)

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Fig. 7B —Developmental lesions of eye.

B, 4-year-old girl with persistent fetal vasculature and microphthalmos. Axial T2-weighted image shows conical low-signal intensity retrolental structure in right globe (arrow). Note small size of right globe compared with normal left globe.

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Fig. 7C —Developmental lesions of eye.

C, 15-year-old boy with end-stage retrolental fibrodysplasia (retinopathy of prematurity). Axial unenhanced CT image shows substantial bilateral microphthalmos with ocular calcifications and diffuse atrophy of optic nerves and extraocular muscles.

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Fig. 7D —Developmental lesions of eye.

D, 2-year-old girl with left coloboma. Axial T1-weighted MR image through orbits at level of optic nerves shows distortion of shape of left globe. Vitreous (arrow) protrudes posteriorly and is consistent with coloboma. Coloboma elongates anteroposterior dimension of globe and may create appearance of buphthalmos (enlargement of eye). Coloboma may also present as rounded retrobulbar cystic mass and microphthalmos (not shown).

Persistent fetal vasculature—Persistent fetal vasculature (PFV, formerly termed “persistent hyperplastic primary vitreous” or “PHPV”) is a developmental anomaly of the eye caused by hyperplasia of the primary vitreous and its failure to regress [28, 3133]. The fetal eye begins development during the first month of gestation and is supplied by the hyaloid artery, a terminal branch of the ophthalmic artery. During the ninth week of gestation, the hyaloid artery begins to regress, forming the secondary, avascular vitreous that replaces the primary vitreous [33, 34]. The primary vitreous and hyaloid artery condense and atrophy along a course extending from the optic disc to the posterior lens called the “Cloquet canal” [33, 34]. A white fibrovascular membrane can remain behind the lens, a remnant of the tunica vasculosa lentis, and can lead to lens distortion and leukoria [28, 33, 35, 36]. This condition is the second most common cause of leukoria. PFV leads to visual impairment by causing swelling of the lens and secondary angle closure glaucoma; therefore, early lens replacement is critical [28, 31, 33]. PFV is classified as either anterior (the most common subtype), in which the remnants are present just behind the lens, or posterior, at the optic disc [28]. CT findings may include a dense vitreous; enhancement of the abnormal vitreous tissue; a small lens; microphthalmia; and possibly vitreous fluid-fluid levels, resulting from intraocular hemorrhage. Unenhanced CT is helpful to show lack of calcification, a distinguishing characteristic from retinoblastoma. MRI may show the Cloquet canal, appearing as a cone-shaped tubular structure posterior to the lens (Fig. 7B); retinal detachment; and fluid-fluid levels [28, 32, 35, 37].

Retrolental fibrodysplasia (retinopathy of prematurity)—Retrolental fibrodysplasia is a vascular disorder of the retina affecting premature infants that is seen with increasing prevalence in infants with younger gestational age and low birth weight (< 1.5 kg) and who have been treated using supplemental oxygen [28, 38]. Up to 82% of infants with birth weights less than 1 kg and 47% of infants with birth weights between 1 and 1.25 kg develop a form of this disease [28]. Although oxygen therapy in these premature infants is thought to play a role in the development of retrolental fibrodysplasia, the exact pathophysiology remains unknown [28].

Retinal vascular development begins around 2 months’ gestational age, with hyaloid vessels growing outward from the optic disc. At approximately 5–6 months of gestation, apoptosis of the hyaloid vasculature occurs, followed by growth of retinal arcades toward the periphery. Apoptosis should be completed by the time that the retinal arcades reach the periphery; however, in premature infants the regressing hyaloid vasculature interferes with the developing retinal vasculature, creating hypoxia and abnormal dilatation and growth of the retinal capillaries [38, 39]. At this junction, an avascular zone forms anteriorly and tortuous dilated vessels posteriorly. With progression, vascular scarring and retinal detachment develop [28, 38, 39]. Screening by ophthalmoscopy should occur during 4–6 weeks after birth, which balances the need to detect significant disease early, but allows a clear view of the retina, which can be obscured by vitreous haze in the first few weeks of life [38]. Treatment with photocoagulation of the abnormal hyaloid vasculature can be performed [39]. This entity is a preventable form of blindness, and as the survival rate of premature infants rises, it continues to be a significant problem [22, 28]. Important imaging findings include an abnormal density posterior to the lens, retinal detachment, calcifications, and microphthalmia secondary to scarring (Fig. 7C). MRI may also show fluid within the subretinal space [28].

Benign Masses

Dermoid cyst—Dermoid cysts and capillary hemangiomas are the most common extraconal masses in children younger than 10 years old [25, 40]. Dermoids are formed by the isolation of ectodermal contents in bone during bone development [25]. Epidermoid cysts are similar but do not contain accessory ectodermal components, such as hair and sebaceous glands [20]. Dermoid cysts usually present as subcutaneous nodules near sutures (e.g., zygomaticofrontal sutures) (Fig. 8A). Clues for identification are its cystic nature, T1 hyperintensity (attributable to fat and protein contents), fluid-fluid levels, and internal calcification; however, macroscopic fat or fluid levels may not always be present [20, 41]. In addition, they show water restriction on DWI sequences [42]. Rim enhancement is occasionally seen. Dermoid cysts can mimic rhabdomyosarcoma especially if they rupture, leading to inflammation that can obscure the primary nodule. Dermoid cysts typically occur near the zygomaticofrontal suture and cause bony remodeling there because they grow slowly, whereas rhabdomyosarcomas tend to cause bony erosion [20, 41] (Fig. 8B).

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Fig. 8A —10-month-old boy with dermoid cyst.

A, Coronal unenhanced CT image reveals ovoid, well-circumscribed, fat- and fluid-density mass (arrow) along superolateral left orbit.

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Fig. 8B —10-month-old boy with dermoid cyst.

B, Axial image in bone windows from same study as A illustrates nonaggressive osseous remodeling subjacent to lesion (arrow). Compare this appearance with appearance of normal right lateral orbital wall.

Hemangioma—A hemangioma is a benign vascular tumor composed of endothelial cell–lined channels [41]. Capillary hemangiomas, also called infantile hemangiomas, are the most common orbital tumor in infancy. They are usually located anteriorly and patients present with chemosis and may have strawberrylike cutaneous lesions or a bluish skin color when the tumor is located deeper in the orbit [20, 41]. These tumors characteristically grow for the first year of life and then involute over the next several months to years. On imaging, they are usually extraconal, are in the anterior orbit, and may be either poorly marginated or well defined. Ultrasound can show hemangiomas as hyperechoic and hypervascular on Doppler ultrasound because of the large number of acoustic interfaces, the increased arterial flow velocity, and low arterial resistance [20, 41]. On contrast-enhanced CT, hemangiomas are well defined and enhance homogeneously. MRI reveals heterogeneous T1 signal intensity and hyperintense T2 signal intensity with flow voids (Fig. 9). Occasionally the presence of cutaneous ipsilateral hemangiomas or visceral hemangiomas helps to differentiate them from vascular malignancies such as rhabdomyosarcomas [20, 41].

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Fig. 9A —3-year-old girl with orbital hemangioma.

A, Coronal T2-weighted MR image shows homogeneously hyperintense well-demarcated mass (arrows) in superficial soft tissues lateral to left orbit. Mass shows curvilinear low-signal intensity structures coursing through mass, which are consistent with small vessels.

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Fig. 9B —3-year-old girl with orbital hemangioma.

B, Axial gadolinium-enhanced T1-weighted fat-suppressed MR image shows typical homogeneous and avid enhancement of these hemangiomas (arrows).

Lymphatic-venous malformation—A lymphatic-venous malformation is a developmental anomaly consisting of dysplastic vessels lined by a single flat layer of endothelium within a stroma of lymphatic tissue, smaller vessels, and connective tissue [25, 41]. This vascular malformation is classified as a lowflow vascular malformation because of its lack of an arterial component [43]. Like hemangiomas, these malformations are present at birth, but they do not undergo rapid expansion and subsequent involution; instead, they enlarge in proportion to the patient's age [41, 43]. They may also present later in childhood than hemangiomas. Acute proptosis may occur if hemorrhage or infection leads to rapid expansion [20, 25, 41]. Lymphatic-venous malformations are multilobulated, predominantly composed of macroscopic cysts, and poorly circumscribed, characteristically crossing anatomic boundaries and involving both the pre- and postseptal tissues of the orbit [25, 41]. CT shows bony remodeling, hyperostosis, and sometimes venous phleboliths [41]. MRI, however, is the preferred method to evaluate these malformations because MRI best shows the characteristic fluid-fluid levels and anatomic extent [20] (Fig. 10). MRI usually shows the lesions as T2 hyperintense and iso- to mildly T1 hyperintense because of the intralesional blood products and proteinaceous fluid [20, 41]. Enhancement is heterogeneous because the venous channels do enhance, but the cystic lymphatic channels do not enhance [20, 25, 41].

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Fig. 10 —15-year-old boy with venolymphatic malformation. Axial T2-weighted image shows multicystic infiltrative mass with multiple fluid-fluid levels (arrows). Findings are typical of venolymphatic malformation. Fluid-fluid levels often reflect blood products layering with simple fluid. Note how mass infiltrates multiple compartments of orbit.

Optic nerve glioma—Optic nerve gliomas account for approximately 3–5% of pediatric intracranial tumors [44, 45]. Most of these cases are sporadic; however, they are commonly seen in patients with neurofibromatosis type 1 (NF1). Approximately 11–30% of children with NF1 are found to have optic nerve gliomas [46] and therefore published guidelines recommend annual screening eye examinations in all NF1 patients up to the age of 8 years [47]. Most (90%) patients with optic nerve gliomas present before the age of 20 years [48] with loss of visual acuity, loss of color vision, proptosis, an afferent papillary defect, or strabismus [44, 45]. Interestingly, optic nerve gliomas arising sporadically are more likely to extend posteriorly to involve the optic chiasm and intracranial structures, whereas those seen in NF1 patients tend to be confined to the optic nerve [49]. The presence of bilateral optic nerve gliomas without involvement of the optic chiasm strongly suggests a diagnosis of NF1 [50]. These tumors are considered low grade and may even spontaneously regress, but treatment with chemotherapy and later radiotherapy may be required for tumors that progress [51].

On CT and MRI, an optic nerve glioma presents as a fusiform mass centered in the optic nerve, causing enlargement and deformity of the nerve (Fig. 11). If the tumor extends posteriorly toward the chiasm, there may be enlargement of the optic canal. An optic nerve glioma is usually hypointense or isointense on T1-weighted sequences and hyperintense on T2-weighted sequences; most enhance homogeneously. Another observed pattern of enhancement is peripheral enhancement with necrosis or cyst formation centrally [52]. The main differential consideration on imaging is an optic sheath meningioma, which arises from the meningothelial cells lining the nerve sheath. An optic sheath meningioma can cause fusiform enlargement of the optic nerve, as does the glioma. An imaging feature that could help distinguish between the two entities is calcification, which is seen in a meningioma [53]. Most important to know, however, is that a meningioma is rare in children and is seen only in the context of neurofibromatosis type 2 (not type 1) or may develop secondary to remote radiation therapy.

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Fig. 11A —8-year-old girl with neurofibromatosis type 1 and left optic nerve glioma.

A, Axial T2-weighted image (A) and axial gadolinium-enhanced T1-weighted image (B) through mid aspects of orbits show fusiform enlargement and homogeneously intense enhancement of left optic nerve (arrows). Contrast-enhanced image also shows abnormal enhancement tracking along margins of left optic nerve through optic canal, which indicates further extension toward middle cranial fossa.

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Fig. 11B —8-year-old girl with neurofibromatosis type 1 and left optic nerve glioma.

B, Axial T2-weighted image (A) and axial gadolinium-enhanced T1-weighted image (B) through mid aspects of orbits show fusiform enlargement and homogeneously intense enhancement of left optic nerve (arrows). Contrast-enhanced image also shows abnormal enhancement tracking along margins of left optic nerve through optic canal, which indicates further extension toward middle cranial fossa.

Malignant Tumors

Retinoblastoma—Retinoblastoma is a tumor of the multipotent retinal progenitor cells and is the most common pediatric intraocular tumor. It develops from mutations in the tumor suppressor RB gene on chromosome 13q14, which may be inherited or sporadic [41, 54, 55]. Most patients present before the age of 4 years and may present even earlier within the first 12 months of life if they are born with the RB mutation. Presentation after the age of 6 years is rare. Leukocoria and strabismus are the most common clinical manifestations at the time of diagnosis.

Patients who have inherited the RB mutation are at increased risk for bilateral retinoblastomas and pineoblastoma, which is termed the “trilateral retinoblastoma.” They are also at increased risk of other tumors elsewhere in the body, including rhabdomyosarcoma and osteogenic and soft-tissue sarcomas.

CT or ultrasound will show a mass in the posterior globe with calcification and possibly retinal detachment. On CT, the mass is hyperdense and enhancing (Fig. 12A). Although vitreous seeding is best identified by ophthalmoscopy, MRI is superior for evaluation of the optic nerve and choroidal extension [56]. Enhancement of the optic nerve could indicate tumor involvement, but it can also be caused by increased intraocular pressure, hypervascularity, or inflammatory cell infiltrates. Observing both optic nerve enhancement and thickening of the nerve is more specific than nerve enhancement alone, and a lack of optic nerve enhancement is a good negative predictor of the absence of tumor extension [56] (Fig. 12B). DWI may show restricted diffusion attributable to the tumor's hypercellularity [42] (Fig. 12C).

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Fig. 12A —7-month-old boy with multifocal (“trilateral”) retinoblastoma.

A, Sagittal contrast-enhanced CT image displays partially enhancing, partially calcified mass in posterior globe (arrows).

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Fig. 12B —7-month-old boy with multifocal (“trilateral”) retinoblastoma.

B, Axial gadolinium-enhanced T1-weighted fat-suppressed image through orbits shows bilateral mildly enhancing masses (arrows) in posterior aspects of globes.

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Fig. 12C —7-month-old boy with multifocal (“trilateral”) retinoblastoma.

C, Apparent diffusion coefficient map from same examination as A reveals restricted diffusion within masses (arrows); restricted diffusion reflects high cellular density of these tumors.

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Fig. 12D —7-month-old boy with multifocal (“trilateral”) retinoblastoma.

D, Axial gadolinium-enhanced T1-weighted image through brain shows heterogeneously enhancing pineoblastoma (arrow). There is mild dilatation of lateral and third ventricles, which indicates obstructive hydrocephalus at level of cerebral aqueduct.

Rhabdomyosarcoma—Rhabdomyosarcoma is the most common extraocular tumor occurring in the pediatric orbit [57]. This aggressive tumor arises from pluripotent mesenchymal cells, which differentiate into skeletal muscle. There are three main subtypes of rhabdomyosarcoma, which differ histologically, genetically, and in location [58]. The most common subtype is the embryonal subtype, accounting for 60–70% of all cases and occurring in children younger than 10 years old. The embryonal subtype is typically located in the head, neck, retroperitoneum, and genitourinary tract. In contrast, the alveolar subtype comprises 20% of all cases, usually presents in adolescence, and is poorly differentiated. It often is found on the chest or extremities, but this subtype may present in the orbit. If it does present in the orbit, it has a predilection for the inferior orbit. Finally, the pleomorphic type usually occurs in adults. Orbital rhabdomyosarcoma (embryonal subtype) presents with rapid unilateral proptosis, chemosis, or preseptal swelling [41, 57]. On imaging, these masses are extraconal and may arise from the paranasal sinuses, lacrimal gland, or extraocular muscles. CT usually shows a homogeneous mass with either irregular or well-defined margins that is similar in density to muscle. There may be associated calcification, bony erosion, and eyelid thickening. Again, MRI is the best modality for detecting intracranial extension. Rhabdomyosarcomas tend to be T1 isointense and T2 hyperintense relative to muscle. The tumor enhances on both CT and MRI and can have a heterogeneous appearance (Fig. 13A). Although rhabdomyosarcoma is a soft-tissue tumor that can occur throughout the body, primary orbital rhabdomyosarcoma is the type of rhabdomyosarcoma that has the best prognosis [20, 41, 58].

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Fig. 13 —6-year-old girl with rhabdomyosarcoma. Coronal T2-weighted fat-suppressed image illustrates well-demarcated mass (arrows) situated in right superolateral orbit, arising from extraocular muscles.

Lymphoma and leukemia—Lymphoma of the orbit can occur primarily or may be a secondary manifestation of systemic lymphoma. Non-Hodgkin lymphoma (NHL) is the most common systemic lymphoma with orbital manifestation, and interestingly, many patients who present with primary orbital lymphoma will eventually develop systemic NHL [25]. It is most common in adults in the sixth and seventh decades of life; however, it is also seen in older children. The typical presentation is painless proptosis [25].

Lymphoma is usually extraconal and can be nodular, typically involving the superolateral quadrant of the eye and lacrimal gland, or it may be infiltrative with intraconal extension and encasement of the posterior globe. Bilateral involvement raises the suspicion for lymphoma. There may be variable enhancement [41, 59]. On MRI, lymphoma is T1 isointense and T2 isointense to hyperintense; however, the signal intensity is variable, and low ADC values on DWI have been shown [42]. Posttransplantation lymphoproliferative disorder (PTLD) occurs as a result of a combination of immunosuppression and Epstein-Barr virus infection. PTLD is uncommon and may rarely involve the orbit as an aggressive mass, including bony changes, which is not typical of NHL [60]. Inclusion of this entity in a differential diagnosis for an extensive intraorbital process is reasonable in a patient who has undergone organ or hematopoietic cell transplantation.

Granulocytic sarcomas, also called chloromas, are masses of primitive granulocyte precursor cells and are typically seen in cases of myelogenous leukemia. Soft-tissue involvement of the orbit by leukemic masses may include the extraocular muscles, the optic nerve, and the globe [59] (Fig. 14).

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Fig. 14A —4-year-old boy with acute myelogenous leukemia who presented with mild right-sided proptosis.

A, Contrast-enhanced CT images in axial (A) and coronal (B) planes show bilateral homogeneous masses (arrows) in superolateral aspect of right orbit and in apex and inferior aspects of left orbit with extension into middle cranial fossa and infratemporal fossa (arrowheads). Further imaging of body for staging by PET/CT (not shown) showed no other sites of disease.

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Fig. 14B —4-year-old boy with acute myelogenous leukemia who presented with mild right-sided proptosis.

B, Contrast-enhanced CT images in axial (A) and coronal (B) planes show bilateral homogeneous masses (arrows) in superolateral aspect of right orbit and in apex and inferior aspects of left orbit with extension into middle cranial fossa and infratemporal fossa (arrowheads). Further imaging of body for staging by PET/CT (not shown) showed no other sites of disease.

Conclusion
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Orbital abnormalities in fetuses may be recognized using ultrasound and MRI. Anophthalmia, hypertelorism, and hypotelorism either may be part of a genetic syndrome or may be related to a developmental abnormality of the fetal skull. In the pediatric population, cross-sectional imaging with CT and MRI offers a means to assess which compartments of the orbit are affected. Aggressive masses have characteristic features and must be evaluated for intracranial extension.

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