Pontine tegmental cap dysplasia: the role of diffusion tensor imaging

Background

Pontine tegmental cap dysplasia (PTCD) is a rare brainstem malformation characterised by an abnormal orientation of the superior cerebellar peduncle, a flattened pons, a vaulted pontine tegmentum protruding into the fourth ventricle and vermian hypoplasia. In the present case, fibre tractography revealed dorsally ectopic transverse pontine fibres representing the dorsal pontine cap. Patients with PTCD presents with multiple cranial nerve palsies, cerebellar and pyramidal symptoms, and extracranial malformations. Cranial nerve involvement is associated with bilateral deafness, feeding and swallowing difficulties, and corneal ulcers. Severe developmental delays have been reported in most cases with language and speech difficulties. The present case emphasises the role of diffusion tensor imaging (DTI) in PTCD and its differentiation from similar conditions.

Case presentation

A 1-month-old female infant born out of consanguineous marriage with a full-term normal vaginal delivery was brought to the paediatric clinic with complaints of an inability to suck milk since birth with multiple episodes of generalised tonic-clonic convulsion. Neurological examination revealed that the neonate was alert with a vigorous cry. No generalised hypotonia was observed. The response to the suck and gag reflexes was poor, suggesting cranial nerve affection. There was no history of perinatal asphyxia. The patient had no family history of neurological disease, and no significant maternal medical or surgical history was reported. Basic serum electrolytes (sodium, potassium and phosphates), haemoglobin, complete blood counts, liver and renal function test results were within normal ranges. Blood glucose and calcium levels were also within normal range. The patient was administered oral anticonvulsant phenobarbitone (5 mg/kg two times per day). PTCD was diagnosed using MRI and tractography. The patient died after 2 months after multiple episodes of aspiration pneumonia.

Investigations

Radiological investigations included anteroposterior (AP) radiograph of the chest, MRI of the brain with DTI and high-resolution CT of the temporal bone.

MRI was performed using a 1.5T Philips Scanner. Sagittal T1, axial T1 inversion Recovery (IR), T2-axial, diffusion-weighted imaging, fluid attenuation inversion recovery sequence, fast field echo and DTI images were obtained.

The T2 images in the axial plane (figure 1A) at the level of the pontomesencephalic junction revealed laterally oriented superior cerebellar peduncles and shortening of the isthmus at the pontomesencephalic junction, resulting in molar tooth appearance. T1 IR images in the axial plane (figure 1B) at the level of the pons reveal hypoplastic middle cerebellar peduncles.

Midsagittal T1 weighted images (figure 2) revealed a flattening of the ventral pons and inferior vermian hypoplasia in the form of an increased tegmento vermian angle (>18°). The focal bulging of the tegmentum of the pons is protrude into the fourth ventricle, that is, the dorsal pontine tegmental cap.

Further DTI analysis (figure 3A,B) with a colour-coded fractional anisotropy (FA) map revealed the absence of normal transverse pontine fibres and the presence of an ectopic bundle of transverse fibres (coloured red) occupying the place of pontine tegmentum. Colour-coded FA maps are shown with standard DTI conventions (blue for the superior↔inferior direction perpendicular to the plane of section, red for the right↔left direction and green for the anterior↔posterior direction).

Superimposed colour-coded FA images (figure 4A,B) showed tegmental cap (arrows) bundles of transversely oriented fibres, representing ‘ectopic’ pontine transverse fibres. DTI 3D tractography (figure 5A,B) revealed dorsal ectopic transverse pontine fibres (shown in red) that formed the dorsal pontine cap. No longitudinal tracts covering the dorsal pontine fibres on the dorsoventral aspect were noted.

We compared the axial FA map and tractography of a young patient of similar age with those of a normal MRI brain retrospective study to understand the normal anatomy of the brainstem tracts and analyse the ectopic tracts in PTCD.

The 3D projection of brainstem tractography in a normal patient (figure 6A) revealed anteriorly located transverse pontine fibres (red) covered by fibres running longitudinally in the brainstem tegmentum (blue) on dorsoventral aspect. An axial FA colour-coded map (figure 6B) displays normal transverse pontine and longitudinal tracts.

The radiographic chest AP view revealed the butterfly vertebre of the D6, D10 vertebral bodies and bifid ribs (figure 7).

The imaging features of the molar tooth like configuration of the pontomesencephalic junction, dorsal pontine tegmental cap and presence of ectopic transverse pontine fibres, well demonstrated on DTI, enabled the accurate diagnosis of PTCD.

Concluding statement

A molar tooth like appearance of the midbrain, flattened anterior pontine belly and hypoplastic middle cerebellar peduncles with a dorsal pontine tegmental cap showing ectopically located transverse pontine fibres form a constellation of imaging in PTCD. DTI plays a crucial role in PTCD in visualising of ectopic transverse pontine fibres representing the dorsal tegmental cap.