||Daniel R. Gold, DO, Departments of Neurology, Ophthalmology, Neurosurgery, Otolaryngology - Head & Neck Surgery, Emergency Medicine, and Medicine, The Johns Hopkins School of Medicine
||This is a 50-year-old woman who underwent resection of a left-sided acoustic neuroma, and post-operatively, she had vertigo, binocular diplopia, left hemi-ataxia and severe gait ataxia. MR diffusion weighted imaging demonstrated an acute stroke involving the left middle cerebellar peduncle (MCP) and lateral pons, which was responsible for her left-sided appendicular and gait ataxia, skew deviation, mild left fascicular 6th nerve palsy, gaze-evoked nystagmus (right-beating nystagmus [RBN] in right gaze and left-beating nystagmus [LBN] in left), and impaired smooth pursuit. There was mild spontaneous RBN that was accentuated with the ‘penlight-cover test'1, where fixation is partially removed by occluding one eye while shining a bright light in the fellow (un-occluded) eye, in this case using the light of the video camera. The RBN increased in right gaze in accordance with Alexander's Law (nystagmus increases in the direction of the fast phase), and there was left unilateral vestibular loss with an abnormal head impulse test to the left due to left cranial nerve 8 injury from the acoustic neuroma and/or its resection. There was gaze-evoked nystagmus (GEN) in left gaze due to the left MCP ischemia (neural integrator injury, especially medial vestibular nucleus and flocculus in this case given their proximity and vascular supply). The RBN in right was likely in part peripheral (related to left unilateral vestibular loss) and in part central (GEN), while the LBN in left was purely GEN; therefore, this is also an example of Bruns nystagmus. MCP strokes have characteristic ocular motor findings, which have been described.2 In a series of 23 patients with MCP strokes, 18 patients had spontaneous horizontal nystagmus, and 15 had horizontal GEN. Eighteen patients had at least one feature of the ocular tilt reaction (OTR, skew deviation [in 14], head tilt, ocular counterroll), and the OTR was ipsiversive in 16 patients and contraversive in 2 patients (both with hemorrhage).2 Our patient had a contraversive OTR (left MCP stroke with left hypertropia and rightward head tilt and ocular counterroll), although the extent of the infarct extended to the pontomedullary junction so that the utricle-ocular motor pathway was likely disrupted just caudal to its decussation. Of note, in the 19 reported patients who had VOG recordings, smooth pursuit was impaired bilaterally in 14.2 This is a particularly characteristic finding, and may relate to the fact that cortical visual areas (e.g., medial temporal) project to pontine nuclei,3 and these fibers then traverse the MCP to synapse in flocculus/paraflocculus and the uvula.4 Additionally, the nucleus reticularis tegmenti pontis (NRTP) receives afferents from frontal eye fields, and fibers from NRTP travel to the dorsal vermis (and then fastigial nucleus) via the MCP. These pontine nuclei project bilaterally through each MCP so that a unilateral lesion commonly causes bilaterally impaired smooth pursuit.2 A likely theory for ipsilateral anterior inferior cerebellar artery (AICA) infarction following acoustic neuroma resection is that distal branches of the AICA are often intimately associated with the capsule of the tumor, thus supplying both the tumor and brainstem/cerebellar parenchyma. If these branches are disturbed, there is the potential for posterior fossa stroke as in this case. Alternatively, direct surgical trauma may play a role in some cases as well.5 1. Newman-Toker DE, Sharma P, Chowdhury M, Clemons TM, Zee DS, Della Santina CC. Penlight-cover test: a new bedside method to unmask nystagmus. J Neurol Neurosurg Psychiatry 2009;80:900-903. 2. Kim SH, Kim JS. Eye movement abnormalities in middle cerebellar peduncle strokes. Acta Neurol Belg 2017. 3. Glickstein M, May JG, 3rd, Mercier BE. Corticopontine projection in the macaque: the distribution of labelled cortical cells after large injections of horseradish peroxidase in the pontine nuclei. J Comp Neurol 1985;235:343-359. 4. Glickstein M, Sultan F, Voogd J. Functional localization in the cerebellum. Cortex 2011;47:59-80. 5. Hegarty JL, Jackler RK, Rigby PL, Pitts LH, Cheung SW. Distal anterior inferior cerebellar artery syndrome after acoustic neuroma surgery. Otol Neurotol 2002;23:560-571.