Ectopic peripontine arcuate fibres, a novel finding in pontine tegmental cap dysplasia

doi:10.1016/j.ejpn.2013.12.007

European Journal of Paediatric Neurology

Volume 18, Issue 3, May 2014, Pages 434-438

https://doi.org/10.1016/j.ejpn.2013.12.007 Get rights and content

Abstract

Background

Pontine Tegmental Cap Dysplasia (PTCD) is a recently described hindbrain malformation presenting hypoplasia of the ventral pons, and a “pontine tegmental cap”. Previous DTI studies identified ectopic transversely oriented nerve fibres in the cap, and absence of transverse fibre bundles in the ventral pons, characterizing PTCD as an embryonic axon guidance defect. A new case with relatively mild symptoms was investigated to identify fibre tracts in the tegmental cap by tracking their connections. In the process a new bilateral ectopic fibre tract was found.

Methods

Routine T1- and T2 weighted images and Diffusion Tensor Imaging (DTI) data were obtained on a 3 T MR scanner. Fractional Anisotropy maps colour coded for orientation were generated. High Angular Resolution Diffusion Imaging (HARDI) data were used for reconstructing maps denoting multiple fibre orientations (i.e. fibre crossings) per voxel through which accurate fibre tracking was performed.

Results

1.
A pontine tegmental cap was discovered on routine MRI and shown to carry transverse oriented fibres on DTI as in previously reported cases of PTCD.
2.
A new finding revealed by DTI with colour coding and fibre tracking, was a robust midline bundle, traced back to the lower brainstem, that ended rostrally in two semi-arcuate loops skirting the outer margins of the pons, to connect to the cerebellar hemispheres close to but outside the middle cerebellar peduncles. Part of this fibre tract was detectable on routine MRI.

Interpretation

Peripontine arcuate fibres were identified, representing a second structural abnormality not previously recorded in PTCD.

Introduction

Pontine Tegmental Cap Dysplasia (PTCD) is a recently discovered hindbrain malformation causing ventral pontine hypoplasia, and “capping” of the pontine tegmentum. The pontine tegmental cap contains ectopic nerve bundles, viewed as an alternative route for a blocked midline in the ventral pons during the embryo-fetal stage.

To date 22 cases have been reported including three publications with DTI and fibre tracking.1, 2, 3Most patients have severe motor and cognitive impairments, variable cranial nerve involvement, with the eighth nerve consistently involved and vertebral anomalies.

A new case with relatively mild symptoms was investigated with parental and patient consent to identify fibre tracts in the cap, tracking their connections, and to examine the ventral pons for midline crossing fibres. In the process a “new” ectopic tract was found.

Section snippets

Methods

Anatomical MRI imaging was performed using a Philips Intera 3.0 T MRI scanner, with a protocol that contained axial, sagittal and coronal T2-weighted MRI, 3D-T1 weighted MRI and DTI. For fibre tract analysis, High Angular Diffusion Tensor Imaging (HARDI) was performed using 92 diffusion directions (four-fold isosahedric tessellation). The scan parameters were: TE 84 m s., TR 10973 m s, b 1600 s/mm², FOV 250 × 250 mm², acquisition matrix 128 × 128, 68 axial slices with slice thickness 2.2 mm.

Results

The male patient was born at term. He had transient neonatal hypertonia and impaired swallowing. Cognitive development was normal. At 11 years he had mild general ataxia, nystagmus, sensory deafness and episodes of hypothermia. No bony abnormalities were detected on routine X-ray survey. EEG, visual evoked potentials and somatosensory evoked response times were normal. The paternal grandfather had adult onset multisystem atrophy. The parents were normal on physical examination. Routine MRI was

Discussion

PTCD is a congenital axon guidance defect involving absence of midline crossing transverse pontine fibres with the distinguishing feature of an ectopic decussation or commissure, non-existent in normal individuals, first reported as distinct entity in 2007.¹ Other neurological findings typically include cranial nerve defects, with the 8th nerve affected in all cases. Non-neurological findings include bony defects, especially the cervical and thoracic spine, less often- visceral malformations.

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Pontine tegmental cap dysplasia: a novel brain malformation with a defect in axonal guidance
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Pontine tegmental cap dysplasia: developmental and cognitive outcome in three adolescent patients
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Pontine tegmental cap dysplasia: MR imaging and diffusion tensor imaging features of impaired axonal navigation
AJNR Am J Neuroradiol
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There are more references available in the full text version of this article.

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It is unclear whether these fibers are continuous with the MCP or not. Recently, a study using HARDI data in PTCD revealed the presence of misoriented fibers connecting the base of the pons with the cerebellar hemispheres through the MCPs (peripontine arcuate fibers), some of which seemed to join the dorsal ectopic band.39 In addition, DEC maps also confirmed the hypoplasia of the MCPs, absence of the ICPs, and mild elongation and lateral displacement of the SCPs resulting in a molar tooth–like appearance.
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While many of these imaging findings are present in PTCD, the most specific characteristic is the tegmental cap [17]. DTI reveals ectopic transverse fibers in the tegmental cap of the dorsal pons which may or may not be connected to the middle cerebellar peduncles [7,18–19]. There are also absent or abnormal transverse fibers in the ventral pons and absent or reduced decussation of the superior cerebellar peduncles, possibly contributing to the hypoplasia of the ventral pons [7].
Pontine tegmental cap dysplasia (PTCD) is a rare neurological syndrome that results in a hypoplastic ventral pons, tegmental cap at the dorsal pons, and cranial nerve dysfunction. The most common symptoms are hearing loss and speech problems. We present a case of a 9-month-old male who presented with developmental delay and hypotonia. Magnetic resonance imaging revealed ectopic dorsal transverse pontine fibers and a cap-like protrusion of the dorsal pons. Diffusion tensor imaging showed that the ventral pontine fibers were absent. The cause of PTCD is undiscovered, but proposed hypotheses include dysfunction in axonal guidance, neuronal migration, and ciliary protein function. PTCD is a rare neurological disorder, but the diagnosis can be suggested with MRI using diffusion tensor imaging as an aid.
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Case studyEctopic peripontine arcuate fibres, a novel finding in pontine tegmental cap dysplasia

Abstract

Background

Methods

Results

Interpretation

Introduction

Section snippets

Methods

Results

Discussion

Neuroimage

Pontine tegmental cap dysplasia: a novel brain malformation with a defect in axonal guidance

Brain

Pontine tegmental cap dysplasia: developmental and cognitive outcome in three adolescent patients

Orphanet J Rare Dis

Pontine tegmental cap dysplasia: MR imaging and diffusion tensor imaging features of impaired axonal navigation

AJNR Am J Neuroradiol

Case study
Ectopic peripontine arcuate fibres, a novel finding in pontine tegmental cap dysplasia