Pontine Tegmental Cap Dysplasia- the Role of MRI and DTI in Diagnosis
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.289019
Source of Support: None, Conflict of Interest: None
Pontine tegmental cap dysplasia is a rare disorder of the brainstem with reduced number of axons, impaired axonal navigation, and abnormal course with ectopic track in dorsal pons ectopic track in dorsal pons. The underlying cause is genetic mutation. The patient presents with difficulty swallowing difficulty and cranial nerve palsies. It is a radiological diagnosis and identification and classification in correct subgroup on MRI is essential.
Magnetic Resonance Imaging [MRI] with Diffusion Tensor Imaging [DTI] is the investigation of choice. It pinpoints ventral pontine flattening, smaller middle cerebellar peduncles and ectopic dorsal pontine fibers causing bump on dorsal aspect of pons.
A 2.5-month-old child presented with respiratory distress, hypoglycemia, difficulty in swallowing and failure to thrive. Birth history was uneventful. Chest X-ray was suggestive of bronchiolitis. We were clinically suspicious of a tracheo-oesophageal fistulata, but the barium study was conducted and proved normal. To rule out metabolic disorder, a blood investigation was performed which turned out to be normal. The child was further investigated with MRI brain to rule out any underlying causes.
MRI brain revealed normal supra-tentorial brain parenchyma with brain stem and cerebellar abnormality. On T2w sagittal image, the pons was small with flattened ventral pons and band-kike structures in the dorso-superior aspect of the pons giving cap-like appearance [Figure 1]a. Junction of midbrain and pons was short [Figure 1]a. Pons immediately caudal to band as extremely thin [Figure 1]a. Dorsal band was projecting into the fourth ventricle [Figure 1]a.
On T2w axial images, the dorsal pons was round in shape. Middle and inferior cerebellar peduncles are hypoplastic. On axial and sagittal images superior cerebellar peduncles are elongated and running laterally giving molar tooth appearance.
Cerebellar vermis was hypoplastic. Cerebellar hemispheres were normal.
Diffusion tensor imaging with color-coded fractional anisotropy maps and tractography were obtained at 3T.
Coronal FA image shows light blue-colored transverse bundle in the dorsal pons with the absence of ventral and middle transverse fibers [Figure 1]b, [Figure 1]c, [Figure 1]d. These ectopic light blue-colored bundles are not connecting with middle cerebellar peduncles. There were elongated blue- and pink-colored superior cerebellar peduncles and hypoplastic middle cerebellar peduncles [Figure 1]b, [Figure 1]c, [Figure 1]d. Dark blue-colored cortico-cerebral fibers in ventral pons also appeared hypoplastic [Figure 1]b, [Figure 1]c, [Figure 1]d.
Malformation of the brainstem is poorly understood both radiologically and pathologically hence it is not well described in medical literature. Barth et al. first named the disease as PTCD., The exact genetic mutation is not understood but autosomal recessive and dominant inheritance both have been described., It is detected sporadically in males and females without any affected siblings and consanguinity as in our case.
Patients with this disorder present with pyramidal and cerebellar signs. There will be multiple cranial nerve palsies especially V, VI, VII and VIII nerves in affected child which leads to neuro-sensorial hearing loss, swallowing impairment and oculomotor palsy. Our patient presented with swallowing difficulty, which leads to reflux, aspiration pneumonitis and failure to thrive. Ataxia, speech disorder, developmental delay and other neurological investigation could not be investigated because due to the early age of presentation.
The molecular basis is responsible for this disease.,,, Etiology underlying this disease is abnormality of the HOX gene, loss of Unc5h3 receptor functions and/or abnormality in ciliary protein function. This causes reduced numbers of axons with impaired navigation and migration. Moreover, developing neurons undergo apoptosis if synaptic connections are not established.
Abnormal axonal navigation and migration causes misorientation of descending pathway of the dorsal long tracks, that is, medial lemniscus, lateral lemniscus, medial longitudinal fasciculus, posterior longitudinal fasciculus, central tegmental, trigeminothalamic, spinothalamic, and tectospinal tracts and ectopic fibers on dorsal aspect of pons., On morphological MRI study, this ectopic track appears to be continuous with the MCPs but color FA map of DTI study reveals this track does not join MCP.
Reduced number of axons, their abnormal migration and subsequent apoptosis leads to a smaller size of cortico-spinal and cortico-pontine tracts in the ventral pons, which are smaller as compared to normal patients.,,,,,,,, The smaller size of these tracts explains the motor problems observed in our patient.,,,,,,,, Three other major findings were identified on the color FA map of our patient: absence of the SCP decussation, markedly diminished size of the MCPs, and poor or non-visualization of the ICPs.
Abnormality in ciliary protein function causes abnormality in decussation of both the SCPs. The cortico-spinal tracts are also affect, giving the appearance of molar tooth like midbrain like in Joubert syndrome.,,,,,,
Prognosis of PTCD appears to be very variable. Language disorders are a constant feature in PTCD and may result in complete absence of verbal language, sign language only, understandable speech with moderate to severe problems in both being able to understand and to express one-self. The neurological outcome of PTCD patients is also variable, ranging from severe intellectual disability up to normal intelligence.
Thus, PTCD is a newly characterized entity of brain-stem and cerebellar malformation. The exact etiology is still under research but likely to be genetic cause. However, MRI findings are fairly diagnostic of disorder.
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