Contralateral Upper Motor Neuron Facial Palsy in a Case of Vestibular Schwannoma: A Consequence of an Underrecognized Anatomical Entity

Abstract

Neurological deficits due to vestibular schwannoma (VS) result from either direct compression on the ipsilateral long tracts and cranial nerves, or strain over the contralateral cranial nerves due to angular distortion. However, the occurrence of a contralateral isolated upper motor neuron (UMN) facial palsy in a case of VS is a rare phenomenon, presenting as a clinical localization puzzle. This is the first case report of a contralateral isolated UMN facial palsy due to VS, elucidating the plausible pathophysiological mechanism.

 

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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.

 

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Conflict of Interest

None declared.

• References

• 1 Kumar A, Behari S, Sardhara J. et al. Quantitative assessment of brainstem distortion in vestibular schwannoma and its implication in occurrence of hydrocephalus. Br J Neurosurg 2022; 36 (06) 686-692
• 2 Yamashita M, Yamamoto T. Aberrant pyramidal tract in the medial lemniscus of the human brainstem: normal distribution and pathological changes. Eur Neurol 2001; 45 (02) 75-82
• 3 Shrestha R, Kharel G, Acharya S, Pandit R, Limbu N. A case report: lateral medullary syndrome with facial nerve palsy and hemiparesis. Ann Med Surg (Lond) 2022; 82: 104722

Publication History

Article published online:
19 June 2025