A Case of Vertical Diplopia after Thalamic Deep Brain Stimulation for Essential Tremor

Deep brain stimulation (DBS) of the ventral intermediate nucleus of the thalamus (VIM) is a well-established treatment for essential tremor (ET). Known complications include dysarthria, sensory paresthesias, and gait disturbance.1 Visual disturbances are rarely reported. Here we present a novel case of vertical diplopia after VIM-DBS. A 75-year-old right-handed man presented with longstanding, bilateral, disabling ET refractory to medications. His medical history was significant for thyroid-related orbitopathy, which resolved after bilateral inferior rectus recession surgery and was deemed quiescent for over 2 years. He had no residual misalignment and no diplopia in primary or downgaze as documented by his ophthalmologist, and no radiographic evidence of ongoing orbitopathy on computed tomography (CT) orbits. Other significant comorbidities included glaucoma, prior retinal detachment, and paroxysmal atrial fibrillation; 2 years prior, he had undergone left VIM-DBS with good effect on his right-sided tremor but some mild stimulation-induced dysarthria. He requested treatment of his left-sided tremor, was reevaluated by a multidisciplinary team, and deemed a good candidate for right-sided DBS. He underwent standard awake implantation without microelectrode recording, with therapeutic and side effects assessed using stimulation testing. The implantation target was set using standard atlas coordinates and matched to the contralateral side. The patient appeared mildly dysarthric during stimulation testing, so the electrode was moved 2 mm medially and tested again. However, speech was unchanged, so it was reimplanted at the original target, 14 mm from midline. His tremor significantly improved postoperatively. Speech returned to baseline. Extraocular muscles appeared normal without any enlargement to suggest interval thyroid-related orbitopathy. Postoperative CT showed a 4-mm right-sided subdural fluid collection, which subsequently self-resolved. Postoperative MRI showed no thalamic hemorrhage or infarct (Fig. 1). Electrode reconstruction in Lead-DBS2 revealed expected position within the VIM (Fig. 2). At 1-week follow-up, the patient reported new vertical diplopia with a slight oblique component. He described transient diplopia multiple times daily, especially with distance vision or upright ambulating position, worse with rightward or downward gaze, and better when recumbent. He had no diplopia after his first DBS surgery. Neuro-ophthalmological examination revealed normal afferent visual examination. Efferent visual examination revealed full ocular ductions. Saccades and smooth pursuits were normal. Palpebral fissures measured 10 mm bilaterally, and pupils were isocoric, reactive in bright light and darkness. Alignment testing at near was normal, with normal near-point convergence. Hertel exophthalmometry showed 22-mm measurements in both eyes (base of 117 mm). Alignment testing at distance revealed left hypertropia measuring 4-prism diopters in primary gaze, rightward gaze, leftward gaze, upgaze, and downgaze. The left hypertropia measured 3-prism diopters with rightward head tilt and 5-prism diopters in leftward head tilt. With supine positioning, the left hypertropia measured 2-prism diopters. Intorsion movements were present. There was no fatigability or fluctuation, no pain, proptosis, or chemosis. The patient's diplopia was treated with a 4 base down Fresnel prism, which provided minimal relief; 3-month neuro-ophthalmological follow-up examination revealed no changes, and the patient was fusing well with the prisms. At 6-month follow-up, his symptoms remained unchanged despite several DBS programming adjustments, including a 1-week period where both stimulators were turned off. He is currently undergoing physiotherapy for ocular motility. To the best of our knowledge, this is the first reported case of new, nonstimulation-dependent vertical diplopia after VIM-DBS. Prior reports have described stimulation-induced changes in subjective visual vertical, a measure of one's subjective perception of verticality, in 6 patients with unilateral VIM-DBS,3 and stimulation-induced horizontal diplopia and esophoria after DBS of the centromedian/parafascicular complex for Tourette's syndrome.4 Another report of left gaze palsy after VIM-DBS attributed the complication to a malpositioned lead.5 The exact cause of our patient's diplopia is speculative, although alignment testing suggested skew deviation, namely vertical strabismus resulting from supranuclear lesions. The upright supine test can differentiate skew deviation from trochlear nerve palsy.6 Vertical deviation decreasing by 50% or more from upright to supine suggests skew deviation, especially in conjunction with intorsion, as observed in our patient. His presentation is not consistent with other etiologies, including recurrent thyroid orbitopathy, decompensated strabismus, and myasthenia gravis. Skew deviation is attributed to lesions in the pathway from the utricles of the labyrinth to the interstitial nucleus of Cajal (INC), passing through the medial longitudinal fasciculus.7 Our anatomic reconstruction revealed expected electrode position, with no encroachment of the INC (Fig. 2) nor of functional visual pathways derived from healthy adults (Video 1). However, in a patient with significant ophthalmological antecedents, an unexpected summative effect or abnormal functional anatomy may be considered. Vertical visual disturbances have been described after isolated thalamic strokes, specifically of paramedian arteries arising from the posterior cerebral artery P1 segment.8 Published case presentations similar to our patient's include subtle skew deviation after small left paramedian thalamic infarction,9 and vertical gaze paresis with right skew after unilateral left medial thalamic infarction.10 Detailed examination of such lesions has revealed inferior extension to the INC.11 Although there is no overt evidence of INC involvement in our patient, one possibility may be a microinfarct not captured on postoperative imaging, as diffusion-weighted MRI was not performed. In summary, this case of vertical diplopia after VIM-DBS for ET highlights the potential for unexpected visual disturbances after thalamic DBS, especially in patients with ocular comorbidities. (1) Research project: A. Conception, B. Organization, C. Execution; (2) Statistical analysis: A. Design, B. Execution, C. Review and critique; (3) Manuscript preparation: A. Writing of the first draft, B. Review and critique. R.F.: 1C, 3A S.S.: 1C, 3B C.A.: 1C, 3B F.G.: 1A, 1B, 1C, 3B We wish to thank the patient, as well as his multidisciplinary treatment teams at the University of Calgary Movement Disorders Clinic and Neuro-Ophthalmology Clinic. Ethical Compliance Statement: We confirm that an institutional review board approval was not required for this work. The patient provided informed consent for publication of this case report. We confirm that we have read the journal's position on issues involved in ethical publication and affirm that this work is consistent with those guidelines. Funding Sources and Conflicts of Interest: The authors declare that there are no funding sources or conflicts of interest relevant to this work. Financial Disclosures for the Previous 12 Months: The authors declare that there are no additional disclosures to report. Data sharing not applicable to this article as no datasets were generated or analysed during the current study.

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