Superior Oblique Myokymia
Superior Oblique Myokymia (SOM) is a rare condition, characterized by episodes of sudden, rhythmic, monocular contractions of the superior oblique muscle. As a result, patients experience oscillopsia and diplopia, particularly when vision is directed downwards . SOM does not preferentially affect one age group, but right-sided SOM has been found statistically more prevalent in females than left-sided SOM . There is currently no treatment protocol for SOM .
The etiology of SOM is idiopathic. It is possible that it could be caused by the trochlear nerve being compromised at the nerve root exit zone . Compression can be caused by the superior cerebellar artery due to an absence of CSF between the nerve and artery , a cyst, or a brain tumor . Trauma has also been postulated as a cause of SOM, but most cases do not report prior trauma in order to see symptoms .
While compression and trauma have been associated with SOM, there are cases where patients do not display either of these etiologies. Thus, SOM is not defined as a direct cranial nerve irritation disorder .
The superior oblique muscle is innervated by the trochlear nerve and functions to turn the eye inferiorly upon adduction . This muscle typically works in concert with other extraocular muscles, but in isolation, contraction would lead to an inferiorly and laterally displaced eye . Ocular electromyography with electrodes placed in the superior and inferior oblique muscles of an SOM-affected eye results in an atypically long duration of action potentials (7-8 msec) in superior oblique muscles, but not in inferior oblique muscles .
Compromised supranuclear input to the trochlear nerve, such as from vascular compression, results in segmental demyelination of the trochlear nerve and subsequent ephaptic transmission is thought to be more probable, leading to SOM .
While SOM is statistically more common in the right eye than left in women, there are no anatomical asymmetries in the trochlear nerve or surrounding structures when observed by MRI .
SOM must be distinguished from trigeminal neuralgia, hemifacial spasm, the Heimann-Bielschowsky phenomenon, ocular neuromyotonia, and square wave jerks . Oscillopsia caused by eyelid myokymia is also often confused with SOM .
Individuals typically present with vertical diplopia when looking downwards. Patients may also experience image separation and hypertropia when looking away from the affected eye or when the patient’s head is tilted towards the affected eye . Uniocular “shimmering,” “fluttering,” or oscillopsia that last a few seconds at a time are common complaints involving abnormal eye movement. In all cases of reported SOM, the symptoms are unilateral, with the right eye being more commonly affected than the left in women .
SOM can affect patients anywhere from a few hours to weeks at a time before resolution, if at all. Each episode of SOM ranges from a few seconds to hours. The symptoms can also disappear as spontaneously as they appear .
Slit lamp examination may be normal, since episode frequency is random, but eye movements can sometimes be elicited by having the patient move their gaze from inferiorly and laterally to the center . The resulting intermittent vertical and torsional microtremors will be visible in one eye . Otherwise, neuro-ophthalmologic examination is normal .
Oscillations produced by SOM have a low amplitude of less than 4° and high frequencies up to 50 Hz . A “Honda sign” can be heard in patients with SOM when a stethoscope is placed over their oscillating eye. The quick movements of the superior oblique tendon produce a sound similar to a motorcycle engine .
Atypical visual acuity, intraocular pressure, visual fields, pupillary reflexes, appearance of fundus, and ocular movement ranges are not commonly correlated with SOM, and are usually normal in the affected eye .
SOM is a likely differential diagnosis when patients present with localized, involuntary, continuous contractions through the striated superior oblique muscle .
It can be distinguished from the Heimann-Bielschowsky phenomenon, which causes monocular vertical eye movement by the character of the vertical oscillations. In contrast to SOM, the Heimann-Bielschowsky phenomenon has a high amplitude up to 30° and a low frequency of less than 5 Hz.
Furthermore, square wave jerks are high-frequency, low-amplitude eye movements similar to those of SOM, but the former is due to a neurological lesion and patients complain of objects moving side to side rather than “bouncing up and down” in SOM patients .
MRI investigation of the brain is sometimes unremarkable in patients with SOM , but occasionally neurovascular conflict at the nerve exit zone is seen in thinly-sliced MRI images . Visualization by MRI Fourier transform constructive interference along with MRI time-of-flight MRA before and after administration of contrast agents shows greater trochlear nerve visualization by this protocol. By this method, arterial contact can be detected at the trochlear nerve root exit zone .
There is no established treatment for SOM due to two main reasons: a lack of sufficient patient populations to perform a randomized controlled clinical trial, and the unpredictability and variable time course of SOM, making it difficult to evaluate the efficacy of existing medical therapies . Variable success has been seen with topical beta blockers, carbamazepine, phenytoin, baclofen, gabapentin, clonazepam, mirtazapine, memantine, and surgery . Botulinum toxin injections have variable success rates and could potentially only provide temporary relief due to difficulty in isolating the superior oblique muscle from the other extraocular muscles . In many cases, continuous medical therapy at dosages on a case-by-case basis has been correlated with favorable prognosis in SOM patients . However, unfavorable side effects of these drugs make treatment more difficult, and gabapentin is often used as a favorable first-line of treatment due to the relatively few side effects .
Beta blockers such as propranolol, timolol, and betaxolol have recently been reported as successful treatment alternatives due to their ability to reduce blood pressure amplitude. This would mitigate symptoms caused by vascular compression of the trochlear nerve .
When medical management is unsuccessful, surgical intervention may be used . The procedures most commonly used for SOM are superior oblique tenotomy along with inferior oblique myectomy, or microvascular decompression of the trochlear nerve . The latter may only be used when vascular compression is identified as the cause of SOM by MRI.
In some cases, patients with SOM see spontaneous improvement or recovery . In most patients, however, chronic episodes of SOM occur and symptoms can attempt to be controlled using various medical treatments or surgical interventions .
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