Superior oblique myokymia (SOM) is a rare neurological disorder characterized by involuntary eye movements. Despite the unsettling sensations it can cause, SOM is generally considered a benign condition.
Defining Superior Oblique Myokymia
Superior oblique myokymia involves an intermittent, involuntary, high-frequency tremor of the superior oblique muscle. This muscle plays a unique role in eye movement, facilitating depression (looking down), abduction (looking outward), and intorsion (rotating the top of the eye inward). The involuntary contractions of this muscle lead to distinct visual disturbances. Typically, SOM affects only one eye at a time.
Recognizing the Symptoms
Individuals with superior oblique myokymia often report a range of subjective visual disturbances. Oscillopsia is a common symptom, where the environment appears to briefly move, often described as a jerky, up-and-down or torsional movement. Double vision, or diplopia, can also occur, making objects appear as two instead of one. Patients may also experience blurred vision or a characteristic “shimmering” or “quivering” sensation in the affected eye.
Symptoms are intermittent, lasting from a few seconds to several hours, and occur when the patient is awake. They can be triggered or worsened by specific eye positions, fatigue, stress, or the use of nicotine or caffeine. It is important to distinguish these symptoms from common eyelid twitches, as SOM involves the deeper eye-moving muscle rather than the eyelid itself.
Exploring the Underlying Causes
Superior oblique myokymia is often idiopathic, meaning there is no identifiable underlying cause. However, some theories suggest it may arise from irritation or demyelination of the trochlear nerve (cranial nerve IV). This nerve innervates the superior oblique muscle. One postulation involves neurovascular compression of the trochlear nerve, where a blood vessel presses against the nerve at its exit point from the midbrain.
In very rare instances, SOM has been associated with more serious underlying conditions such as brainstem lesions, including those related to multiple sclerosis or tumors. Such associations are uncommon. Most cases of SOM are not linked to other disorders and do not show abnormalities on imaging.
The Diagnostic Process
Diagnosing superior oblique myokymia begins with a detailed patient history, where the individual describes their symptoms, including their onset, frequency, and any potential triggers. A comprehensive eye examination is then performed, often by a neuro-ophthalmologist, a specialist in vision problems related to the nervous system. During this examination, subtle, high-frequency oscillations of the eye, known as “objective myokymia,” may be visible.
Due to the episodic nature of SOM, multiple examinations may be necessary to observe the symptoms. Imaging techniques, such as magnetic resonance imaging (MRI) of the brain, are used to rule out more serious neurological conditions with similar symptoms, such as brainstem tumors or multiple sclerosis. However, in the majority of SOM cases, these imaging studies do not reveal any abnormalities.
Treatment Options and Management
Treatment for superior oblique myokymia aims to manage symptoms and improve the patient’s quality of life. Oral medications are a common approach, including carbamazepine, which can significantly improve or resolve symptoms in some patients. Other medications such as gabapentin, memantine, beta-blockers, baclofen, clonazepam, mirtazapine, and phenytoin may also be considered. Topical beta-blockers and systemic propranolol are often preferred as initial treatments due to their minimal side effects.
For individuals with persistent and severe symptoms, botulinum toxin injections can be used to temporarily weaken or paralyze the superior oblique muscle, thereby reducing spasms. Surgical options are rarely pursued and typically reserved for severe, intractable cases. These procedures might involve a tenectomy or recession of the superior oblique muscle. Beyond medical interventions, non-pharmacological strategies like managing fatigue and stress can also help mitigate symptom frequency. Many individuals with mild symptoms may not require treatment, and for others, symptoms can be effectively managed, with some experiencing a decrease or even disappearance of symptoms over time.