Superior Oblique Myokymia
Superior oblique myokymia (SOM) is characterized by monocular, paroxysmal, high-frequency bursts of superior oblique muscle contraction. These bursts typically last for seconds, occur numerous times per day, and produce vertical and torsional eye movements. The movements are of very small amplitude and usually require magnification (obtained, for instance, with a slit lamp or 20.00 D lens) to observe them. Using the slit lamp to focus on a conjunctival vessel can help identify these movements. The abnormal movements may occur either spontaneously or immediately after a downward eye movement or blinking. Patients may experience a combination of oscillopsia, vertical/torsional diplopia, blurry vision, and tremulous ocular sensations.
The etiology of SOM is unknown. Some clinical and neuroimaging findings suggest that this disorder may occur when the superior cerebellar or posterior cerebral artery compresses the cranial nerve (CN) IV root exit zone, similar to the neurovascular compression that occurs with hemifacial spasm and trigeminal neuralgia. In rare cases, SOM may occur prior to or after a CN IV palsy. Thus, CN IV compression, aberrant regeneration, and ephaptic transmission are implicated in the pathophysiology of this disorder.
Although most patients with SOM are otherwise healthy, in rare cases, SOM may be associated with MS, posterior fossa tumor, stroke, or trauma. Neuroimaging should be considered to identify any associated conditions.
The clinical course of SOM is variable. Some patients recover spontaneously or experience only brief spells of symptoms. Other patients experience chronic oscillopsia or intermittent diplopia. Treatment with β-blockers (topical or systemic), carbamazepine, phenytoin, baclofen, oxcarbazepine, or gabapentin may be helpful for some patients. A superior oblique tenectomy (to alleviate the oscillopsia) combined with an ipsilateral inferior oblique myectomy (to treat the iatrogenic superior oblique weakness) may help patients with more severe symptoms. In patients with neurovascular compression, intracranial surgery to decompress CN IV may alleviate symptoms. However, most patients choose a less invasive treatment option.
Agarwal S, Kushner BJ. Results of extraocular muscle surgery for superior oblique myokymia. J AAPOS. 2009;13(5):472–476.
Brazis PW, Miller NR, Henderer JD, Lee AG. The natural history and results of treatment of superior oblique myokymia. Arch Ophthalmol. 1994;112(8):1063–1067.
Hashimoto M, Ohtsuka K, Suzuki Y, Minamida Y, Houkin K. Superior oblique myokymia caused by vascular compression. J Neuroophthalmol. 2004;24(3):237–239.
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.