Keywords
Brainstem malformations - cranial nerve palsies - hindbrain malformations - pontine
tegmental cap dysplasia
Introduction
Brainstem malformations are extremely rare and account for varied clinical symptoms
owing to the presence of multiple cranial nerve palsies and pyramidal tract involvement.[1] Pontine tegmental cap dysplasia (PTCD) is one such under-explored and unfamiliar
hindbrain malformations, characterized by hypoplastic ventral pons, with an ectopic
dorsal pontine white matter band (called as “tegmental cap”), protruding into the
fourth ventricle from the tegmentum of pons.[2] Here, we report an interesting case of this rare pontine malformation and discuss
the associated clinical and radiological features.
Case History
A 4-year-old male child, born out of a non-consanguineous marriage, through a normal
vaginal delivery, presented with facial nerve palsy, deviation of the angle of mouth
towards the left, drooling of saliva from the left angle of mouth and abnormal watering
from the left eye for the last 1 year. There was no history of any speech or swallowing
disturbances. No family history of any neurological deficit was available.
The patient was referred to us for imaging by a pediatric neurologist, to ascertain
the cause of facial palsy. Magnetic resonance imaging (MRI) was performed using 1.5T
scanner (Phillips, Achieva). Axial, coronal and sagittal T1 and T2-weighted images
were obtained. An axial heavily T2-weighted DRIVE sequence was acquired for better
demonstration of cranial nerves. In addition, diffusion tensor imaging (DTI), with
generation of fractional anisotropy (FA) color maps was done, on a 3T MRI scanner
(Phillips, Ingenia) using 32 icosahedral directions. Other parameters used were TE
110 ms, TR 3985 ms, FOV 200–230, slice thickness 3 mm and image matrix 256 × 256.
On axial T1/T2-weighted MRI, we observed hypoplastic left side of ventral pons [Figure 1]A. The ventral pons appeared flattened on the corresponding sagittal images [Figure 1]B. On axial and sagittal T1/T2-weighted MRI, an ectopic aberrant structure, isointense
to the pons, was noted arising from the left side of dorsal pontine tegmentum, at
the pontomedullary junction, protruding into the fourth ventricle posteriorly and
inferiorly [Figure 2]A,[Figure 2]B,[Figure 2]C. On heavily T2-weighted axial DRIVE sequence, this aberrant exophytic dorsal pontine
tissue was seen to impinge upon and compress the left seventh and eighth cranial nerves
at their origin [Figure 3]. However, bilateral inner ear structures and internal auditory meati appeared normal.
Figure 1 (A and B): Axial T2-weighted image (A) shows hypoplastic left side of ventral pons (thick white
arrow), with abnormal lateral orientation of superior cerebellar peduncles (thin white
arrows).Sagittal Tl-weighted image (B) shows flattened ventral pons (dotted white
arrow)
Figure 2 (A-C): An aberrant exophytic structure (thick white arrows) is seen protruding into the
fourth ventricle from the dorsal pontine tegmentum at pontomedullary junction on corresponding
sagittal Tl-weighted (A and B) and axial T2-weighted images (C)
Figure 3: Axial DRIVE sequence shows normal origin of right 7th and 8th cranial nerves (thin white arrow), with impingement at the origin of left 7th and 8th cranial nerve (thick white arrow)
Additional MRI features observed were abnormal morphology and slight lateral orientation
of the superior cerebellar peduncles [Figure 1]A and [Figure 4]A. The middle cerebellar peduncles appeared hypoplastic on the right side [Figure 4]B. However, inferior cerebellar peduncles were observed to be normal. Cerebellar
vermis, cerebellar hemispheres, and medullary olivary nuclei were also normal. Supratentorial
structures appeared normal in morphology.
Figure 4 (A and B): Coronal T2-weighted image (A) shows elongated and laterally oriented superior cerebellar
peduncles (thin white arrows). Axial T2-weighted image (B) shows hypoplastic right
middle cerebellar peduncle (thick white arrow)
DTI, with the cross-sectional FA color maps depicted elongated and laterally oriented
superior cerebellar peduncles [Figure 5]A. The middle cerebellar peduncles appeared hypoplastic, with paucity of white matter
tracts, markedly on the right side [Figure 5]B. An abnormal bundle of white matter axons, directed anteroposteriorly and depicting
green color on color maps, was noted in the dorsal exophytic band of tissue [Figure 5]C. Significantly, the transverse pontine fibers were observed to be virtually absent
[Figure 5]D. The normal transverse pontine fibers in a healthy control are shown in [Figure 5]E.
Figure 5 (A-E): Axial FA color images show elongated and laterally oriented superior cerebellar peduncles
(A) seen as green (thin white arrows) and hypoplastic right middle cerebellar peduncle
(B) (thick white arrow). Sagittal fractional anisotropy color DTI image (C) shows
anteroposteriorly directed white matter axons in ectopic dorsal pontine tissue seen
as green (dotted white arrow). Axial fractional anisotropy color DTI imageshows near
complete absence of transverse pontine fibers (D) and normal transverse pontine fibers
in a healthy control as red (E) (white stars)
Besides MRI, whole spine radiographs were also obtained to exclude any associated
vertebral anomalies, which revealed no obvious abnormality.
Discussion
The term PTCD was coined for the first time in literature by Barth et al.[2] This is a rare, nonprogressive disorder, characterized by developmental delays,
multiple cranial nerve deficits and a unique hindbrain malformation. Associated clinical
features include sensory neural hearing deafness, ataxia, speech disturbances, facial
palsy, spine anomalies, swallowing problems and failure to thrive.[1],[2] The exact etiology of this disorder is yet unexplained, with a possibility of having
a genetic origin.[1],[3]
MRI plays an essential role in adequate assessment of this rare brainstem malformation.[1],[2],[3],[4],[5],[6],[7] The abnormally hypoplastic ventral pons in conjunction with an exophytic ectopic
dorsal tissue projecting from the pontine tegmentum are the key radiological features.
Other associated MRI findings are total or partial absence of middle cerebellar peduncles,
abnormal morphology and orientation of superior cerebellar peduncles, hypoplastic
inferior cerebellar peduncles, distorted appearance of medulla oblongata with absence
of medullary olives and hypoplastic cerebellar vermis and/or cerebellar hemispheres.[1],[2]
The abnormal superior cerebellar peduncles, with MRI appearance of a “molar tooth,”
at times pose difficulty for the radiologist to differentiate it from Joubert syndrome.
However, other typical MRI features seen in PTCD, coupled with cranial nerve palsies,
helps in distinguishing the two entities.[2]
In our case study, MRI and DTI revealed similar features described by previous investigators,
including virtually absent normal transverse pontine fibres and abnormal white matter
axons in the ectopic pontine tegmental cap, that were directed in the anteroposterior
direction and depicted green color on FA images.
In the largest reported series of 16 patients reviewed retrospectively and published
recently in 2016 by Nixon et al., the authors found associated duplication of one or both internal auditory canals
in all but one patient.[8] Associated cochlear nerve aplasia and either atresia or stenosis of internal auditory
canals was also observed in their study. However, our patient did not reveal any temporal
bone and cochlear nerve abnormality.
The possible pathogenesis of this rare condition can be explained by abnormal axonal
guidance and/or neuronal migration as evidenced by absence of transverse pontine fibres,
absent superior cerebellar peduncles decussation and ectopic pontine fibres. All these
findings can be better demonstrated on DTI. Jissendi-Tchofo et al. hypothesized the development of this rare condition at molecular level in one of
the three possible manners. (1) The reduced ventral migration of pontine nuclei may
lead to aberrant dorsal pontine white matter fibres. (2) Increased radial migration
of pontine gray neurons which are then relocated on the surface of pontine tegmentum
in an abnormal manner. (3) Lastly, normal pontine neuronal migration followed by abnormal
axonal guidance away from pontine ventral surface.[2]
Prognostic correlation of neuroimaging findings in PTCD has also been documented.
The degree of brainstem dysplasia on imaging correlates well with clinical disability.
A rounded bump (so called tegmental cap) reflects a mild disease, whereasa beak-shaped
angular brainstem kink, reflects severe form of the disease with poorer outcome.[9]
PTCD has also been included under the umbrella term “pontocerebellar hypoplasia.”
However, an autosomal recessive inheritance in pontocerebellar hypoplasias and an
ectopic dorsal pontine tissue in PTCD aid in adequate differentiation between the
two diagnoses. Besides Jouberts syndrome, PTCD should also be differentiated from
Moebius syndrome. The characteristic MRI findings and substantial developmental delay,
seen invariably in all cases of PTCD, help in distinguishing it from Moebius syndrome.[1],[2],[4],[5] In addition, Moebius syndrome is characterized by the presence of bilateral combined
6th and 7th nerve palsies, which is not seen in PTCD. Moreover, the differentiating MRI features
of Moebius syndrome include predominant hypoplasia of the dorsal pons with absence
of medial colliculus at the pontine level, causing depression in the fourth ventricle
and absent hypoglossal prominence.[5]
Although similar clinical and imaging features of PTCD have been published previously
by a few investigators, a PubMed search did not reveal any report from the Indian
investigators. Our study, thereby adds to the literature in describing the radiological
and clinical features of this rare pontine malformation.
We conclude that the peculiar hallmark MRI appearance of PTCD, together with the abnormal
axonal findings on DTI, can help radiologists, in reaching an accurate diagnosis in
an appropriate clinical setting of neurological dysfunction.
