Neuroimaging Findings in Moebius Sequence

Discussion

Moebius sequence refers to malformations arising during development of the mesencephalon-rhombencephalon and is classified within the group of early defects of the dorsoventral pattern.⁵ The term “sequence” is preferred to “syndrome” because it defines a cascade of secondary events after an initial insult during the embryonic period.⁶ In histopathologic studies of patients affected by Moebius sequence, the main abnormality is found in the pontine tegmentum where the nuclei for cranial nerve VI are located and the intra-axial fibers of the seventh cranial nerve are crossing posterior to these nuclei (the facial colliculus).¹⁴

Brain stem ischemia during embryogenesis is the most accepted cause of Moebius sequence. Distribution of branches of the embryonic basilar artery makes the dorsomedial aspect of the brain stem vulnerable to hypoxic-ischemic injury.¹⁵ This feature probably explains why hemorrhage and uterine contractions produced by misoprostol have been associated with Moebius sequence.¹⁴ Concerning the genetic etiology of Moebius sequence, 2 different loci at 3q21-q22 and 10q have been reported. Additionally, de novo mutations have also been reported affecting the PLXND1 and REV3L genes. These result in a defect in facial branchiomotor neuron migration, supporting these genes as causative for at least a portion of Moebius sequence cases.⁶ Clinically, patients have unilateral or bilateral congenital facial paralysis and esotropia with preservation of the vertical gaze.

Known brain MR imaging findings in Moebius sequence are based on small case series. The present study includes the largest number of patients reported to date. In our series, the most frequent finding was bilateral absence of the facial colliculus in the pontine tegmentum with flattening of the floor of the fourth ventricle (100%). In axial images, patients showed depression of the floor of the fourth ventricle leading a V or horseshoe shape, a finding that agrees with reports from other authors (Fig 1).^10,11,16 One of our patients had unilateral absence of the facial colliculus, and this correlated with the clinical manifestation of unilateral facial paralysis.

The second most frequent finding was pons hypoplasia (60.5%), which, for the purpose of this study, was defined as a pontomesencephalic ratio of <2 (craniocaudal pons length/craniocaudal midbrain length) (Fig 2). Pons and brain stem hypoplasia have been also described by other authors.^10,12,13,17 Tectal beaking was present in 39.5% of patients (Fig 2). This finding has only been previously reported in 1 patient and thus is more common than previously believed.¹²

In almost one-third of our patients, we observed a subjective increase in the anteroposterior diameter of the mesencephalon, similar to that described in the previously mentioned case report (Fig 3).¹² We speculate that this finding may be caused by fusion of the inferior mesencephalic tectum colliculus. Its association with a decrease in the amplitude of the interpeduncular cistern remains difficult to explain.

Dysplasia of the corpus callosum and ventriculomegaly were seen with a frequency of 23.7% and 26.3%, respectively. Ventriculomegaly in our study was a less frequent finding than reported by Matsui et al.¹⁷ In that publication, 6 of 10 patients showed ventriculomegaly and 3 exhibited prominence of the frontal horns of the lateral ventricles. We found prominence of the frontal horns of the lateral ventricles in 5 (13.2%), and although we did not measure tissue volume, this finding could be explained by white matter volume loss.

Fusion of the thalami was present in 10 patients (26.3%) (Fig 4). This finding was previously described by Pirmez et al¹⁶ in an isolated case. These authors found that partial fusion of the thalami was associated with agenesis of the septum pellucidum and frontal horns of the lateral ventricles, concluding that it represented a mild form of lobar holoprosencephaly in association with Moebius sequence. In our study, 1 patient who showed thalamic fusion also had agenesis of the septum pellucidum, but the findings did not fall into the spectrum of holoprosencephaly.

Arachnoid cysts are common in the general population and represent up to 1% of intracranial masses.¹⁸ However, in our series, middle cranial fossa and infratentorial arachnoid cysts were observed more frequently in 26.3% and 13.2% of patients with Moebius sequence, respectively. Although the pathogenesis of arachnoid cysts has not been completely elucidated, it has been hypothesized that they could be secondary to an early defect in fetal development coinciding with the appearance of the Moebius sequence.¹⁹

Isolated periventricular nodular heterotopia was observed in 26.3% of patients. These malformations of cortical development are the result of arrested neuronal migration. Clinically, patients with these sporadic malformations do not have major intellectual disabilities, and seizures may not appear until the second or third decade of life.²⁰ Our patients with periventricular nodular heterotopia did not have seizures, but because they are young, seizures may occur later in life.

The differential diagnosis of Moebius sequence includes the congenital cranial dysinnervation disorders. In these disorders, MR imaging can show diverse findings such as the absence of the abducens nerve on the affected side in Duane syndrome (related to mutations in CHN1 or SALL4 genes) or extraocular muscle volume loss in congenital fibrosis of the extraocular muscles (mutations in KIF21A, PHOX2A, and TUBB3 genes).²¹ In our series, we excluded 4 patients with cranial nerve VI and VII palsies because brain MR imaging showed fusion of basal ganglia due to dysgenesis of the anterior arm of the internal capsules, which is a characteristic finding described in the spectrum of tubulinopathies resulting from an abnormal axonal navigation/orientation caused by a mutation in the Tubulin genes.²² Tubulinopathies could not be excluded in those patients because they were not genetically tested; thus, we did not include patients with dysmorphic basal ganglia in this analysis. Last, another condition that should be considered in the differential diagnosis of Moebius sequence is pontine tegmental cap dysplasia (we also excluded another patient with that disorder from the study after analyzing the MR imaging). In this malformation, neuroimaging shows pons hypoplasia associated with tegmental prominence projected into the fourth ventricle (tegmental “cap”) and middle cerebellar peduncle hypoplasia.²³ This malformation can be related to deficiencies of multiple cranial nerves and reorganization of white matter tracts.³

Our study has limitations: mainly its retrospective nature, nonquantitative nature, and the absence of a single researcher-defined protocol for imaging acquisition. Nevertheless, because of the relatively large (largest to date) number of patients, we believe our series is unique and important to report.