Suboccipital craniectomy (hindbrain decompression) and cervical laminectomy are perhaps the most widely performed procedures for treatment of symptomatic Chiari I malformation. Furthermore, the bony resection can be performed without opening the underlying dura—either in conjunction with durotomy while leaving the arachnoid intact or with arachnoidolysis along with duraplasty. On the one hand, preserving the arachnoid prevents blood from entering the subarachnoid space and reduces the chance of postoperative chemical meningitis [17]. On the other hand, opening the arachnoid enables division of subarachnoid adhesions and possible tonsillar shrinkage or resection, as well as investigation of the 4th ventricle for retained rhomboid roof or other reasons (e.g., for impaired CSF flow through the foramen magnum or Magendie foramen). Ultimately, the rationale for these procedures is to increase the volume of the posterior fossa and widen the foramen magnum, resulting in a “neoforamen” magnum. Such procedures lead to improved CSF flow across the craniocervical junction, which can manifest as expansion of the CSF spaces surrounding the inferior cerebellum, medulla, and upper spinal cord; restoration of CSF flow across the craniocervical junction, visible on phase contrast images; and decrease in size of associated syringohydromyelia (Fig. 1). Indeed, the cerebellar tonsils and brainstem can assume a normal appearance at MRI within 6 months after craniocervical decompression and the procedure can lead to a significant reduction in tonsillar pulsatility and CSF flow velocities across the neoforamen magnum [15, 18, 19]. At least in pediatric patients, upward shifting of the tip of cerebellar tonsil is significantly correlated with improvement in associated syringohydromyelia [20]. However, a decrease in size of Chiari-associated syringohydromyelia at MRI can lag behind clinical improvement by up to 10 months [21]. In addition, although an improved appearance at MRI may not necessarily correlate with postoperative clinical amelioration, drop attacks and headaches are most likely to respond to hindbrain decompression [22].

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Fig. 1A —Foramen magnum decompression in 12-year-old girl with history of Chiari I malformation and syringohydromyelia who experienced improvement of symptoms after surgery. A, Sagittal T2-weighted (A) and phase contrast (B) MR images before surgery show low-lying cerebellar tonsils with pointed configuration and associated crowding of foramen magnum, with paucity of CSF flow posteriorly and constricted CSF flow anteriorly (arrow, B). There is also partially imaged large syrinx in cervical spine.

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Fig. 1B —Foramen magnum decompression in 12-year-old girl with history of Chiari I malformation and syringohydromyelia who experienced improvement of symptoms after surgery. B, Sagittal T2-weighted (A) and phase contrast (B) MR images before surgery show low-lying cerebellar tonsils with pointed configuration and associated crowding of foramen magnum, with paucity of CSF flow posteriorly and constricted CSF flow anteriorly (arrow, B). There is also partially imaged large syrinx in cervical spine.

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Fig. 1C —Foramen magnum decompression in 12-year-old girl with history of Chiari I malformation and syringohydromyelia who experienced improvement of symptoms after surgery. C, Sagittal T2-weighted (C) and phase contrast (D) MR images after surgery (partial suboccipital craniectomy and C1 laminectomy) show resultant increased CSF spaces and improved CSF flow at craniocervical junction due to capacious neoforamen magnum. In addition, syrinx has decreased in size.

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Fig. 1D —Foramen magnum decompression in 12-year-old girl with history of Chiari I malformation and syringohydromyelia who experienced improvement of symptoms after surgery. D, Sagittal T2-weighted (C) and phase contrast (D) MR images after surgery (partial suboccipital craniectomy and C1 laminectomy) show resultant increased CSF spaces and improved CSF flow at craniocervical junction due to capacious neoforamen magnum. In addition, syrinx has decreased in size.

Stenting of the 4th ventricle requires opening the dura and is currently reserved for treatment of patients with refractory syringohydromyelia [23]. The goal of the stent is to enable CSF to flow freely out of the 4th ventricle into the cervical subarachnoid space. Occasionally, the 4th ventricular CSF can be shunted to other spaces, such as the peritoneal or pleural cavities. The silastic tubes that are generally used for this purpose are visible on conventional MRI sequences as low-signal-intensity structures on T1- and T2-weighted sequences, and CSF flow can often be delineated through patent tubes on phase contrast sequences (Fig. 2).

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Fig. 2A —4th ventricular stenting in 31-year-old woman with history of Chiari I malformation. A, Sagittal T1-weighted (A) and phase contrast (B) MR images show tube (arrows, A and B) within 4th ventricle with discernable CSF flow.

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Fig. 2B —4th ventricular stenting in 31-year-old woman with history of Chiari I malformation. B, Sagittal T1-weighted (A) and phase contrast (B) MR images show tube (arrows, A and B) within 4th ventricle with discernable CSF flow.

Endoscopic 3rd ventriculostomy (ETV) can be an effective treatment option for noncommunicating hydrocephalus associated with Chiari I malformation. ETV is often performed for initial management before craniocervical decompression and is considered to be an effective alternative to ventriculoperitoneal shunt insertion [24–28]. The rationale of this procedure is to decrease the effects of increased intracranial pressure on the structures crowding the foramen magnum, thereby improving symptoms attributable to Chiari I malformation and facilitating normalization of CSF circulation [24]. The procedure consists of creating a defect in the anterior floor of the 3rd ventricle between the retrochiasmatic recess and the mammillary bodies and subsequently opening the Liliequist membrane to allow free flow of CSF into the prepontine cistern. On MRI, successful ETV results in a CSF flow jet across the floor of the 3rd ventricle, which can be evident on multiple sequences, particularly phase contrast sequences, and a defect in the 3rd ventricular floor can be best discerned on the high-resolution T2-weighted sequences (Fig. 3) [27].

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Fig. 3A —Endoscopic 3rd ventriculostomy in 27-year-old woman with Chiari I malformation and hydrocephalus. A, Sagittal phase contrast (A) and fast imaging employing steady-state acquisition (FIESTA) (B) MR images show jet of CSF across defect (arrow, A and B) in floor of 3rd ventricle, indicating patency of 3rd ventriculostomy.

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Fig. 3B —Endoscopic 3rd ventriculostomy in 27-year-old woman with Chiari I malformation and hydrocephalus. B, Sagittal phase contrast (A) and fast imaging employing steady-state acquisition (FIESTA) (B) MR images show jet of CSF across defect (arrow, A and B) in floor of 3rd ventricle, indicating patency of 3rd ventriculostomy.

Selective cerebellar reposition, reduction, or resection (“triple R”) has been proposed as a means of opening the obex and facilitating the CSF outflow from the 4th ventricle in patients with Chiari I malformation [24, 28, 29]. Cerebellar tonsil reduction is performed by surface electrocautery, and tonsillectomy is performed by standard subpial resection. The integrity of the pia is maintained as much as possible to avoid adhesion formation. Cerebellar tonsillar resection can be performed with or without craniectomy or laminectomy [29]. The rationale for triple R is that persistence of syringohydromyelia in patients with apparently normal cine flow MRI findings after craniocervical decompression without tonsillar resection suggests that tonsillar ectopia plays a role in the formation of syringohydromyelia in addition to obstructing the flow at the craniocervical junction [30]. On MR images obtained during the early postoperative period, cerebellar tonsillar cauterization produces small areas of restricted diffusion at the inferior pole of the cerebellum owing to ischemia and can ultimately result in decreased crowding of the neoforamen magnum (Fig. 4).

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Fig. 4A —Cerebellar tonsillectomy in 15-year-old boy with history of Chiari I malformation. A, Axial FLAIR image (A), DW image (B), and apparent diffusion coefficient map (C) show bilateral inferomedial cerebellar hemisphere signal abnormality (arrows, A) and restricted diffusion (arrows, B and C) due to cauterization during early postoperative period.

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Fig. 4B —Cerebellar tonsillectomy in 15-year-old boy with history of Chiari I malformation. B, Axial FLAIR image (A), DW image (B), and apparent diffusion coefficient map (C) show bilateral inferomedial cerebellar hemisphere signal abnormality (arrows, A) and restricted diffusion (arrows, B and C) due to cauterization during early postoperative period.

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Fig. 4C —Cerebellar tonsillectomy in 15-year-old boy with history of Chiari I malformation. C, Axial FLAIR image (A), DW image (B), and apparent diffusion coefficient map (C) show bilateral inferomedial cerebellar hemisphere signal abnormality (arrows, A) and restricted diffusion (arrows, B and C) due to cauterization during early postoperative period.

Direct intervention for syringohydromyelia may be considered a rescue procedure, particularly when MRI shows a large syrinx with significant thinning of the spinal cord tissue and obliteration of the spinal subarachnoid space and for patients with syrinx-related symptoms [31–33]. Syringosubarachnoid shunting consists of performing a myelotomy and inserting a silastic tube that extends from the syrinx cavity to the subarachnoid space. To minimize complications from arachnoiditis, the surgeon preferentially positions the distal end of the shunt tube in the ventral subarachnoid space [34]. Alternatively, CSF can be diverted from the syrinx cavity to the pleural or peritoneal spaces. The tubes are most readily discernable on high-resolution T2-weighted MRI sequences (Fig. 5). Flow artifacts at the tube opening suggest patency of the shunt tube [35].

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Fig. 5 —Syringosubarachnoid shunting in 42-year-old woman with Chiari I malformation and syringohydromyelia who had persistent symptoms. Sagittal T2-weighted MR image shows tube (arrow) situated within cervical spinal cord syrinx cavity.

Pseudomeningoceles consist of fluid collections that communicate with the intracranial or intrathecal CSF spaces through dural defects and essentially represent CSF leaks. These are perhaps the most common complication of posterior fossa decompression, reported in 18.5% of decompressions with duraplasty versus 1.8% of decompressions without duraplasty [12, 36]. On MR images, pseudomeningoceles typically appear as CSF-signal-intensity fluid collections superficial to the dura or duraplasty at the site of hindbrain decompression. Pseudomeningoceles can attain large sizes, dissect into the posterior neck subcutaneous tissues, and exert mass effect on the craniocervical junction contents (Fig. 6). Pseudomeningoceles have also been reported to extend into the diploic space [37]. Treatment of the pseudomeningocele consists of primary closure, temporary lumbar drainage if needed, and, rarely, extracranial CSF shunting if refractory to the other treatments.

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Fig. 6A —Pseudomeningocele in 33-year-old woman with posterior neck swelling after craniocervical decompression for Chiari I malformation. A, Sagittal T2-weighted (A) and T1-weighted (B) MR images show large extradural fluid collection (asterisk, A and B) that extends into posterior neck subcutaneous tissues.

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Fig. 6B —Pseudomeningocele in 33-year-old woman with posterior neck swelling after craniocervical decompression for Chiari I malformation. B, Sagittal T2-weighted (A) and T1-weighted (B) MR images show large extradural fluid collection (asterisk, A and B) that extends into posterior neck subcutaneous tissues.

Cerebral, brainstem, or cerebellar infarction is an uncommon complication of Chiari I malformation decompression and has been reported to occur in 0.5% of patients [12]. The posterior inferior cerebellar artery territory is most often involved, presumably owing to injury to distal arterial branches in the region of the operative zone, which may be more likely during complex revision surgeries. DWI and FLAIR MRI are useful for evaluating acute postoperative infarcts (Fig. 7), which should not be mistaken for the expected postoperative restricted diffusion in patients intentionally treated via cerebellar cauterization.

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Fig. 7A —Postoperative stroke in 45-year-old woman with new symptom of postoperative ataxia and history of Chiari I malformation previously treated with craniocervical decompression. A, Axial FLAIR (A), DW image (B), and apparent diffusion coefficient map (C) show bilateral posterior inferior cerebellar artery territory acute infarcts after reexploration of Chiari decompression, midline myelotomy, and 4th ventricular stenting.

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Fig. 7B —Postoperative stroke in 45-year-old woman with new symptom of postoperative ataxia and history of Chiari I malformation previously treated with craniocervical decompression. B, Axial FLAIR (A), DW image (B), and apparent diffusion coefficient map (C) show bilateral posterior inferior cerebellar artery territory acute infarcts after reexploration of Chiari decompression, midline myelotomy, and 4th ventricular stenting.

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Fig. 7C —Postoperative stroke in 45-year-old woman with new symptom of postoperative ataxia and history of Chiari I malformation previously treated with craniocervical decompression. C, Axial FLAIR (A), DW image (B), and apparent diffusion coefficient map (C) show bilateral posterior inferior cerebellar artery territory acute infarcts after reexploration of Chiari decompression, midline myelotomy, and 4th ventricular stenting.

Infections after craniocervical junction decompression can be characterized as superficial or deep wound infections and have an incidence in the range of 0.5%–3.7% but can be as high as 11% [38–43]. Superficial wound infections, such as cellulitis, are clinically apparent and generally managed without imaging unless they become complicated and an associated deep component is suspected. Deep wound infections can manifest as abscess or meningitis. On MR images, abscesses can appear as rim-enhancing fluid collections, whereas meningitis can manifest as leptomeningeal enhancement predominantly within the posterior fossa; however, both processes can coexist (Fig. 8). The main differential diagnosis for superficial wound abscess is pseudomeningocele, which can predispose to infection. The main differential diagnosis for bacterial meningitis is aseptic or chemical meningitis, which can be differentiated via CSF fluid analysis and has been reported to occur in 3.8–44% of patients with Chiari I malformation [12, 38, 39, 44].

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Fig. 8A —Postoperative infection in two different patients with postoperative fever. A, Sagittal fat-suppressed contrast-enhanced T1-weighted MR image in 34-year-old woman shows peripherally enhancing fluid collection in posterior neck, consistent with abscess.

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Fig. 8B —Postoperative infection in two different patients with postoperative fever. B, Coronal contrast-enhanced T1-weighted MR image in different patient (23-year-old man) shows diffuse leptomeningeal enhancement in posterior fossa, consistent with meningitis.

In approximately 0.5% of cases, arachnoid adhesions form after Chiari I malformation surgery that involves substitution of the dura [45]. These adhesions can tether the CNS tissue to overlying dura, impede CSF flow, and cause symptom recurrence. Adhesions appear as low-signal-intensity bands that distort the parenchyma and are often located posterior to the cerebellum or at the craniocervical junction (Fig. 9). Adhesions are best depicted on thin-section T2-weighted images. Arachnoid adhesions can be treated by surgical lysis.

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Fig. 9A —Postoperative adhesion in 27-year-old man with worsening neurologic symptoms after surgery. A, Axial (A) and sagittal (B) T2-weighted MR images show linear hypointense band (arrow, A and B) that wraps around spinal cord, which is kinked.

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Fig. 9B —Postoperative adhesion in 27-year-old man with worsening neurologic symptoms after surgery. B, Axial (A) and sagittal (B) T2-weighted MR images show linear hypointense band (arrow, A and B) that wraps around spinal cord, which is kinked.

The incidence of elevated postoperative CSF pressure or hydrocephalus (or both) has been reported to be generally 1.0–3.6% but may be as high as 8.7% [7, 12, 38, 42, 46]. Patients presenting with new or recurrent symptoms or CSF leakage from the surgical wound after foramen magnum decompression require investigation to exclude the presence of pseudotumor cerebri or hydrocephalus. Potential manifestations of hydrocephalus include ventriculomegaly or subdural hygromas (external hydrocephalus), which can be infratentorial or supratentorial (or both) (Fig. 10). The pathogenesis for the development of this condition is controversial, but it may result from defects in the arachnoid created during surgery. Subdural CSF collections can cause rapid neurologic deterioration, leading to coma due to compression of the brainstem and posterior fossa structures. Treatment of symptomatic elevated CSF pressure that is medically intractable consists of CSF shunting.

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Fig. 10 —External hydrocephalus in 21-year-old man with headache after craniocervical decompression for Chiari I malformation. Coronal T2-weighted MR image shows bilateral posterior fossa and left cerebral convexity subdural fluid collections (arrows).

Further downward displacement of the hindbrain (cerebellar slump) after decompression surgery for Chiari I malformation is not rare, particularly if the neoforamen magnum is too large (greater than ~ 4 × 4 cm) [47]. Although the degree of cerebellar slump is often not severe enough to produce symptoms, it can lead to craniospinal CSF pressure dissociation. Both conventional and phase contrast MRI sequences can be used to assess the degree of hindbrain migration. In particular, there can be associated mass effect on the medulla and spinal cord by the migrated cerebellum with disruption of CSF flow across the neoforamen magnum in symptomatic cases (Fig. 11).

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Fig. 11A —Cerebellar slump syndrome in 30-year-old woman with worsening neurologic deficits after surgery for Chiari I malformation. A, Sagittal T1-weighted MR image shows low-lying tonsils that obliterate neoforamen magnum.

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Fig. 11B —Cerebellar slump syndrome in 30-year-old woman with worsening neurologic deficits after surgery for Chiari I malformation. B, Corresponding phase contrast image shows absent CSF flow across neoforamen magnum (arrow).