Keywords
Upper motor neuron facial palsy - vestibular schwannoma - aberrant pyramidal tract
Case Description
A 36-year-old gentleman presented with left cerebellar symptoms, ipsilateral hearing
loss, and left grade two lower motor neuron facial palsy as evident when the patient
could not hold air inside the oral cavity on providing pressure with examiner's finger
on tapping of the cheek. However, while smiling, a right-side upper motor neuron (UMN)
facial palsy was apparent which was further confirmed with usual clinical tests for
facial nerve weakness ([Fig. 1]). Imaging confirmed a left-sided giant vestibular schwannoma (VS; [Fig. 2]). The contralateral UMN facial palsy improved following tumor resection.[1]
Fig. 1 The 36-year-old gentleman presented with right CP angle syndrome with left UMN-type
facial palsy. (A) Clinical photograph showing facial asymmetry, diminished right nasolabial fold,
and deviation of angle of mouth to left. (B) Clinical photograph showing preserved forehead creases on right side as compared
with left side. (C) Clinical photograph showing normal eye closure. CP, cerebellopontine; UMN, upper
motor neuron.
Fig. 2 Radiological images of the CE-MRI head of the patient. (A) T2 axial image showing a 4.6 × 4.2 cm left-sided cerebellopontine angle mass with
typical ice cream cone appearance with enlargement of the internal acoustic meatus.
(B) Postgadolinium contrast axial images showing contrast uptake in the tumor. (C, D) T2 flair and diffusion-weighted imaging to rule out supratentorial vascular insult.
CE-MRI, contrast-enhanced magnetic resonance imaging.
This presentation raised two enigmas regarding lesion localization. First, the occurrence
of a contralateral UMN facial palsy in the absence of hemiparesis (contralateral isolated
central facial palsy). Second, a supranuclear facial palsy due to a lesion having
a mass effect at the pontomedullary junction (infranuclear). The first query is explained
by the presence of an aberrant corticofacial tract that separates from main pyramidal
tract, runs in pontine tegmentum in the paralemniscal zone, and crosses contralaterally
at midbrain or upper pons ([Fig. 3]). Thus, the more ventromedial-located pyramidal fibers are spared while the laterally
located corticofacial fibers get compressed. Rarely, does the crossover happen more
caudally. The loop of the corticofacial fibers is formed in lower pons or upper medulla.[2] This aberration has been hypothesized based on the frequent findings of central
facial palsy in cases of lateral medullary syndrome due to isolated medullary infarcts.[3] It also explains the occurrence of contralateral central facial palsy due to compression
by a VS at the lower pons or medulla ([Fig. 4]).
Fig. 3 Pathway of the normal and aberrant corticofacial fibers.
Fig. 4 Image shows the compression of the aberrant corticofacial fibers explaining the phenomena
of contralateral UMN facial palsy in case of VS. UMN, upper motor neuron; VS, vestibular
schwannoma.
While the presence of an aberrant pyramidal tract (APT) has been validated by recent
studies, there exists no definite evidence of this tract. The present clinical scenario
could not be explained unless both hypotheses regarding the APT were true.
