By Marcello Cherchi, MD PhD
For patients
In superior oblique myokymia (SOM), one of the muscles that moves the eyeball “twitches,” and this may make you feel as if your vision is “jumping.” Often we do not know why this happens. In some people this problem gets better with medication. If the problem is very severe and does not get better with medication, then some patients may talk with a surgeon about surgical treatment.
For clinicians
Overview
Myokymia of the superior oblique (less commonly inferior oblique) muscle can cause torsional saccades in the affected eye. This condition is typically unilateral, and thus manifests clinically with monocular oscillopsia. Case series report the age of onset to range from 22 – 50 years. Most cases appear to be idiopathic; in some cases microvascular compression is implicated, or traction on the trochlear nerve by a space-occupying lesion. Often the diagnosis is based on the history alone; less commonly the clinician may observe the unilateral torsional eye movements. Proposed treatments have included a variety of oral medications, timolol eye drops, and several surgical procedures.
Introduction
Myokymia of extraocular muscles can cause oscillopsia, which some patients can also experience as a disturbance of equilibrium. The superior oblique muscle is usually implicated, but myokymia of the inferior oblique muscle can also cause this (Smith and Cornblath 2014).
Superior oblique myokymia was first described by Alexander Duane in 1906 (Duane 1906).
Epidemiology
A series of 16 patients with superior oblique myokymia reported the age of onset ranged from 22 to 50 years, with 9 (56%) male and 7 (44%) female (Brazis et al. 1994).
Pathophysiological mechanism of disease
Superior oblique myokymia reflects abnormal activity of the superior oblique muscle, possibly from aberrant firing of the trochlear nerve. In many cases this appears to be idiopathic.
A number of imaging studies support the hypothesis of microvascular compression of the trochlear nerve as the etiology of superior oblique myokymia (Hashimoto et al. 2001; Kang et al. 2013; Mehta and Demer 1994; Yousry et al. 2002a; Yousry et al. 2002b).
Some imaging studies of patients with SOM report that the affected superior oblique muscle is smaller than the contralateral (normal) side, which has led some investigators to conclude that, “These anatomical changes in the superior oblique muscle of patients with myokymia suggest that an antecedent injury to the trochlear nerve has occurred” (Mehta and Demer 1994).
SOM has also been reported in patients with dural arteriovenous malformations (Kommerell and Schaubele 1980).
In some cases of SOM the irritability of the trochlear nerve may be due to traction secondary to a space occupying lesion in the posterior fossa, such as an arachnoid cyst (Donaldson et al. 2020) or neoplasm (Morrow et al. 1990).
Clinical presentation
Superior oblique myokymia (SOM) is generally unilateral, so the most common symptom of SOM is unilateral oscillopsia.
Physical examination
In a patient with superior oblique myokymia who is otherwise healthy, the general neurological examination should be normal.
Ocular motor examination
Given the action of the superior oblique muscle, the ocular motor manifestation of SOM is bursts of torsional saccades. This is sometimes observable on face-to-face examination (Zhao et al. 2024).
Testing: vestibular
Several instrumented ocular motor studies have been conducted on patients with SOM (Hayakawa et al. 2000; Kattah and FitzGibbon 2003; Ma et al. 2021; Suzuki et al. 2003; Thinda et al. 2017; Thurston and Saul 1991).
Testing: other
Rare studies have assessed electromyographic activity of the superior oblique muscle during SOM (Kommerell and Schaubele 1980).
Imaging
Some imaging studies (generally high resolution MRI of the orbits) report microvascular compression of the trochlear nerve (Hashimoto et al. 2001; Kang et al. 2013; Mehta and Demer 1994; Yousry et al. 2002a; Yousry et al. 2002b). Other studies report that the affected superior oblique muscle is smaller than its normal counterpart (Mehta and Demer 1994).
Differential diagnosis
Myokymia of the eyelid (rather than the superior or inferior oblique muscles) can sometimes be experienced as oscillopsia (Krohel and Rosenberg 1986; Reinecke 1973). This is important to recognize because eyelid myokymia may be treated differently, such as with onabotulinum toxin injections.
Treatment
A number of treatments for SOM have been attempted or suggested, including:
- Oral beta blockers:
- Betaxolol (Bibby et al. 1994)
- Levobunol (Zhang et al. 2018)
- Propranolol (Tyler and Ruiz 1990; Williams et al. 2007)
- Topical beta blockers (delivered as eye drops):
- Timolol (Borgman 2014)
- Anti-epileptic medications:
- Carbamazepine (Williams et al. 2007)
- Gabapentin (Deokule et al. 2004; Tomsak et al. 2002)
- Phenytoin (Williams et al. 2007)
- Valproate (Williams et al. 2007)
- Other medications:
- Memantine (Jain et al. 2008)
- Surgery
- Microvascular decompression (Fam et al. 2014; Mikami et al. 2005; Samii et al. 1998; Scharwey et al. 2000)
- Other surgeries (Agarwal and Kushner 2009; de Sa et al. 1992; Kawasaki et al. 2019; Law et al. 2019; Noro et al. 2022; Palmer and Shults 1984; Ruttum and Harris 1988, 2009)
- Prism lenses
Prognosis
Superior oblique myokymia can be a chronic or recurrent condition (Brazis et al. 1994). Although medical therapy is not always successful, we nevertheless advocate starting with lower-risk, non-invasive approaches before entertaining the option of surgical intervention.
References
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