Superior oblique myokymia (SOM) is a rare condition characterized as a “monocular movement disorder involving rapid torsional, low-amplitude contractions of the superior oblique muscle that causes monocular oscillopsia and diplopia.”^{Reference Kattah and FitzGibbon1} Multiple etiologies, like neurovascular compression of the trochlear nerve and brainstem tumor, have been identified as potential causes; however, most cases remain idiopathic.^{Reference Patel and Malhotra2} Treatment of SOM is not well established due to its rarity and its idiopathic nature.^{Reference Patel and Malhotra2} Topical beta-blockers and carbamazepine are the usual first-line treatment.^{Reference Patel and Malhotra2} In this article, we present a patient with SOM who treated herself with oral cyclobenzaprine, which had been originally prescribed to her for a cervical sprain. We thus argue for a potentially new therapeutic option for SOM.

A 42-year-old woman presented to our neuro-ophthalmology clinic with episodic oscillopsia. She had no past personal medical or surgical history for neurological or ophthalmologic disorders, except for migraine and benign paroxysmal positional vertigo. Family history was unremarkable. She had been taking cyclobenzaprine 10 mg each night for a year, ever since she was diagnosed with a cervical sprain. The patient described her oscillopsia as painless, very rapid and taking on the shape of an “inverse C” in her left eye. These eye movements were frequent, occurring in episodes lasting several seconds at most, and worsening with fatigue. These episodes began around three years prior to her visit and would initially occur almost daily, with up to a hundred episodes per day. When she began taking cyclobenzaprine a year ago, she quickly noticed a drop in the frequency of attacks, down to ten to twenty episodes per day, every day or so. She continued her cyclobenzaprine use even after her cervical sprain had resolved as a form of self-treatment. Twice she attempted to wean off her cyclobenzaprine over two-week periods. During these trials, the frequency of her attacks would increase to her baseline of up to a hundred per day.

During the exam, we witnessed a few of her typical episodes, during which we noted high-frequency low-amplitude left eye incyclotorsion movements lasting several seconds at most. Her neuro-ophthalmologic exam was otherwise unremarkable. She had a normal brain MRI with and without contrast. We diagnosed her on clinical grounds with left SOM. We prescribed her timolol eye drops, which, upon follow-up, did not seem to have a significant effect on her symptoms.

SOM is a rare diagnosis, and its pathophysiology remains to be fully elucidated. The diagnosis is clinical and can be made with confidence based on visualization of characteristic sporadic incyclotorsion movements of the eye. Brainstem tumors, head trauma, superior oblique palsy and neurovascular compression of the trochlear nerve have all been reported as possible etiologies of SOM (see Figure 1).^{Reference Patel and Malhotra2} The lattermost etiology refers to the compression of the trochlear nerve by an adjacent artery, which would lead to demyelination of the nerve; ephaptic transmissions would then erupt and cause SOM.^{Reference Patel and Malhotra2} The current treatment strategy for SOM overlaps with that of other neurovascular compression syndromes, with membrane stabilizer medications such as carbamazepine, gabapentin, baclofen and phenytoin being used.^{Reference Patel and Malhotra2–Reference Shi, Zhang, Xu and Xu4} Topical or systemic beta-blockers, botulinum toxin injections or neurovascular decompression surgery are other lines of treatment shown to have an effect.^{Reference Patel and Malhotra2,Reference Zhang, Gilbert and Hunter3}

Figure 1. Anatomy and pathogenesis of superior oblique myokymia. Superior oblique myokymia (SOM) represents an abnormality of the superior oblique muscle or its innervation, from the trochlear nucleus in the midbrain to the trochlear nerve itself. Its various etiologies are represented here, although cases are often tagged as idiopathic even after neuroimaging is performed.^{Reference Patel and Malhotra2} The mechanism of action of cyclobenzaprine is not well understood; it is mostly described as a muscle relaxant that acts on the central nervous system directly. It is unknown if cyclobenzaprine acts on peripheral nerves or muscles themselves.^{Reference Khan and Kahwaji5}

Cyclobenzaprine is a cyclical antidepressant that acts on the central nervous system as a skeletal muscle relaxant.^{Reference Khan and Kahwaji5} Its exact mechanism of action is not well understood, and its indications include muscle spasms in acute painful musculoskeletal conditions and fibromyalgia.^{Reference Khan and Kahwaji5} Cyclobenzaprine is not typically used in SOM. Nevertheless, as cyclobenzaprine is known to reduce muscle hyperexcitability through CNS effects, and SOM consists essentially of involuntary muscular contractions, we believe that this medication may have genuinely alleviated our patient’s symptoms.^{Reference Khan and Kahwaji5} The tight timing of the change of frequency of our patient’s symptoms in relation to her intake of cyclobenzaprine, in which the two times she interrupted cyclobenzaprine, her episodes increased, followed by an improvement of her symptoms after restarting cyclobenzaprine, supports this theory. With a half-life of 18 hours, its once-daily oral dosing is advantageous.^{Reference Khan and Kahwaji5} Its side effects are mainly somnolence and dizziness; cyclobenzaprine is usually prescribed for short periods of time, but there are studies suggesting that it can be well-tolerated in long-term settings.^{Reference Oldfield, Gleeson and Morford6} This compares to carbamazepine, the general first-line treatment of SOM, which has many drug interactions and more significant side effects such as ataxia, hyponatremia, aplastic anemia, agranulocytosis and SJS/TEN.^{Reference Maan, vi Duong and Saadabadi7} Thus, cyclobenzaprine could become a more secure treatment option for certain patients.

A few limitations and caveats must be noted. As this is only a case report of one patient’s experience with cyclobenzaprine, no causal association between cyclobenzaprine and SOM can be asserted. It also remains unclear whether this treatment would work on all identified etiologies of SOM. Still, given the rarity of SOM, we hope our report may encourage more research on the role of cyclobenzaprine or similar compounds in this disorder.

In this case report, we presented a patient with typical manifestations of SOM. We argued for the potential use of cyclobenzaprine in her condition, highlighting cyclobenzaprine’s mechanism in reducing muscle hyperexcitability as a possible explanation for its effect on her symptoms. Further research is needed to establish a causal link between cyclobenzaprine and SOM.