AJR 2005; 184:S4-S6
© American Roentgen Ray Society
Partially Uncrossed Pyramidal Tracts Shown by Tractography in Horizontal Gaze Palsy and Scoliosis
Harushi Mori1,
Takashi Fijishiro2,
Naoto Hayashi1,
Yoshitaka Masutani1,
Shigeki Aoki1,
Kuni Ohtomo1,
Makoto Aihara2 and
Masato Wakakura3
1 Department of Radiology, Graduate School of Medicine and Faculty of Medicine,
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8655, Japan.
2 Department of Ophthalmology, Graduate School of Medicine and Faculty of
Medicine, The University of Tokyo, Tokyo, Japan.
3 Department of Neuro-ophthalmology, Inouye Eye Hospital, Tokyo, Japan.
Received February 21, 2004;
accepted after revision May 6, 2004.
Address correspondence to H. Mori
(hmori-tky{at}umin.ac.jp).
Introduction
Congenital defects of the midline structures of the brainstem in the
absence of disorders of the cerebellum are rare
[1-4].
The primary clinical finding is the restriction of bilateral horizontal gaze.
Brainstem morphology suggests a defect associated with pyramidal decussation;
however, relatively few reports have shown this characteristic via
transcranial magnetic stimulation, functional MRI, and sensory evoked
potentials [1]. We describe a
patient with sporadic isolated horizontal gaze palsy and scoliosis in whom
tractography revealed partially uncrossed pyramidal tracts.
Case Report
A 27-year-old right-handed woman was referred to our institution for
evaluation of abnormal eye movements, which had been present since birth.
Neurologic and ophthalmologic examinations revealed total absence of
horizontal gaze (Fig. 1),
preserved convergence, latent primary position nystagmus that was not manifest
in any gaze position, and an otherwise normal sensory-motor system. Evidence
of Moebius syndrome or other musculoskeletal abnormalities, except previously
treated scoliosis, was lacking.
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Fig. 1. —27-year-old right-handed woman with abnormal eye movements.
Photographs acquired in the primary position and in all attempted positions of
gaze (arrows) exhibit total defect of horizontal gaze and
preservation of vertical gaze. No esotropia is present.
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MRI studies were performed on a 1.5-Tesla Signa Horizon LX MRI system (GE
Yokogawa Medical Systems), consisting of routine multiplanar sequences and
diffusion tensor imaging (DTI)
[5]. A single-shot, spinecho
echo-planar sequence was used (TR/TE 5,000/96, acquisition of 30 interleaved
contiguous 5-mm axial images covering the entire brain, field of view of 24
x 24 cm, and matrix of 128 x 128 interpolated to 256 x 256).
Diffusion gradients were applied in 13 noncollinear directions with b = 1,000
sec/mm2 as the peak diffusion gradient. Deterioration collection
software was used for echo-planar imaging. Tractography was generated with our
original software (Volume One and VizDT-II). The seed points were placed on
the hyperintense foci of the posterior internal capsule on T2-weighted images.
The cut-off value for the fractional anisotropy was 0.18 (default value).
Routine MR images exhibited sagittal clefts or wedge-shaped notches of the
pons and medulla oblongata in the midline (Figs.
2A,
2B and
2C); images of the upper
cervical cord were unremarkable. Tractography showed uncrossed right pyramidal
tracts and incompletely crossed left pyramidal tracts
(Fig. 2D). The cerebellum and
cerebral hemispheres were normal. Volumes of the lateral rectus muscles were
grossly preserved with respect to those of other extraocular muscles.
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Fig. 2A. —27-year-old right-handed woman with abnormal eye movements.
T1-weighted midsagittal image depicts dorsal cleft of pons and medulla
oblongata. The ventral cleavage is subtle on sagittal image.
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Fig. 2B. —27-year-old right-handed woman with abnormal eye movements.
T2-weighted image (T2WI) at the level of the pons shows pontine volume
reduction and a dorsal wedge-shaped notch (arrow). The wedge-shaped
configuration is partially formed by missing prominence of the abducens
nuclei.
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Fig. 2C. —27-year-old right-handed woman with abnormal eye movements.
T2WI at the level of the medulla oblongata displays a rectangular-shaped
medulla oblongata accompanied by a deep anterior median fissure (black
arrow) and dorsal wedge-shaped midline cleavage (black
arrowhead). The anterior protuberances are characterized by reduced
volume. The inferior olivary nuclei (white arrows) are rather
prominent with respect to pyramid equivalent structures (white
arrowheads).
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Fig. 2D. —27-year-old right-handed woman with abnormal eye movements.
Head-on view tractography shows that right side of pyramidal tracts
(arrows) travels downward independently through entire distance
between primary motor cortex and upper cervical cord (open
arrowheads). The left side exclusively of pyramidal tracts forms an
incomplete pyramid shape (arrowheads) at level of medulla oblongata;
this structure fuses to right side, uncrossed pyramidal tracts (Barnes'
ventrolateral pyramidal tract).
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Discussion
This report describes the first documented example, to our knowledge, of in
vivo 3D visualization of partially uncrossed pyramidal tracts using diffusion
tensor tractography. DTI, an MR technique for analysis of diffusion-anisotropy
of the brain, can illustrate subtle white matter anatomy
[5]. Before development of DTI,
examination of neurofibers fell into the domain of neuroanatomy using
postmortem or tracer studies in animal models. In humans, motor movements are
predominantly innervated by the contralateral motor cortex. Several anatomic
and clinical investigations showed ipsilateral projection from the primary
motor cortex [1,
6]; however, to our knowledge,
no descriptions of this nature involving tractography appear in the
literature. Normal tractography indicates that pyramidal tracts originate in
the primary motor cortex, converge in the central semiovale, bundle in the
posterior limb of the internal capsule, pass through the cerebral peduncles,
separate at the level of the pons, converge again at the pontomedullary
junction, and, finally, cross at the pyramidal decussation. In the current
case, the right pyramidal relevant tracts traveled straight downward without
crossing, forming so-called Barnes' ventrolateral pyramidal tracts
[6]
(Fig. 2D).
In the early stage of pyramidal tract genesis, the anterior median fissure
separates the primitive tracts on either side. During ontogenesis, a point at
which the boundary of the medulla oblongata and the spinal cord displays an
angled appearance, the anterior median fissure focally disappears and the
pyramidal tracts cross to the contralateral side until the 16th week of
gestation. After reversal of brainstem flexion to a straight configuration,
neurofibers in the pyramidal tracts that later reach the medulla oblongata
pass through with no intersection
[6]. Some brainstem
abnormalities may interfere with the flexion process of the medulla oblongata,
resulting in the formation of uncrossed pyramidal tracts. We expect that
failure to form a complete brainstem is consequent to defective gene
expression, which produces and/or regulates adhesion or chemotactic molecules.
Vascular involvement has been implicated, given that the vulnerable watershed
zone can exist in the fetal tegmentum and medulla oblongata due to
end-vascular supply with little anastomosis during the fourth to sixth week of
gestation [7]. The vascular
hypoperfusion leads to disturbed abducens nuclei, which form during the sixth
to eighth week of gestation. This hypothesis, however, cannot fully explain
why vascular insult to the midline structures of the brainstem damages
particular entities (abducens nuclei) while sparing other adjacent structures
(internal genua of the facial nerve).
A wedge-shaped dorsal cleft at the level of the pons is responsible for the
absence of protrusions of the abducens nuclei. This situation accounts for the
bilateral abduction deficit. However, loss of esotropia and preservation of
lateral rectus muscle volume in the present case suggest that all functions of
the abducens nuclei were not lost. Thus, involvement of cross-neuronal
connections (including the medial longitudinal fasciculi) between the abducens
nucleus and contralateral oculomotor nucleus, and/or involvement of the
paramedian pontine reticular formations, rather than the absence of bilateral
abducens nuclei, caused the bilateral horizontal gaze palsy, lack of
adduction, and normal convergence. Vertical eye movements were spared
consequent to the preservation of the rostral pons and midbrain. Horizontal
gaze palsy and abnormalities of the paramedian dorsal tegmentum of the pons
and medulla oblongata are features of Moebius syndrome; however, additional
evidence indicative of Moebius syndrome was not observed in this patient.
Patients with horizontal gaze palsy and congenital scoliosis similar to the
current case have been documented
[1-3].
Lower brainstem lesions may cause scoliosis associated with horizontal gaze
palsy, as shown in rats. Damage to brainstem structures (gracilis nuclei,
lateral vestibular nuclei, and superior colliculus) and lesions of the dorsal
longitudinal fasciculi (DLF) resulted in scoliosis in rats
[8]. MRI of this patient
revealed gracilis and vestibular nuclei; the DLF were probably involved in the
brainstem abnormality, which led to scoliosis.
Patients presenting with abnormal decussation of the pyramidal tracts
reportedly exhibit brain malformations, including X-linked Kallman's syndrome,
Dandy-Walker's syndrome, agenesis of the corpus callosum, Fukuyamatype
congenital muscular dystrophy, cortical dysplasia, lissencephaly,
schizencephaly, and encephalocele. None of these abnormalities was observed in
the current case.
In conclusion, we describe a patient with partially uncrossed pyramidal
tracts shown by tractography associated with horizontal gaze palsy and
scoliosis. Tractography can disclose physiologic connection of neurofibers;
moreover, it is a potent tool with respect to analysis of brain anomalies in
vivo.
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Introduction
Case Report
