Superior oblique myokymia—a case report

Sir,

Superior oblique myokymia (SOM) is an unusual, eye movement disorder presenting as episodes of oscillopsia and diplopia.

Case report

A 58-year-old lady presented with a 3-year history of ‘jumping objects’ and ‘difficulty in focusing’. Her visual acuity (unaided) was 6/5 in both eyes. Orthoptic assessment revealed slight updrift of right eye on levoversion and minimal underaction of the right superior oblique muscle with slight overaction of the inferior rectus. Anterior segment and fundus were unremarkable. Fresnels prism (base down) for right eye was prescribed but symptoms continued to persist.

One year later, she presented with a sensation of her right eye moving like a ‘fish in a bowl’! Careful slit-lamp examination with focussed attention to the conjunctival vessels then revealed brief cyclotorsional movements in her right eye (Figure 1). ‘Honda Sign’ was elicited on pressing the diaphragm of the stethoscope over the closed right eyelid.

Figure 1

Cyclotorsional movement-right eye.

Neurological and laboratory investigations including MRI of brain were unremarkable. Diagnosis of SOM was made and treatment with T. Propanolol 20 mg BD was started. Marked improvement in symptoms was noted on 9 months follow-up.

Comment

In 1906 Duane reported the first case with uniocular rotatory nystagmus involving the superior oblique muscle.¹ In 1970 Hoyt and Keane termed SOM as an acquired abnormality in the innervation of the superior oblique muscle causing an episodic oscillation.^{1, 2} They postulated a trochlear nerve disinhibition as the basis of this abnormality.

In 1983 Bringewald suggested that it resulted from vascular compression of the trochlear nerve.³

Aetiology is still unclear, but many factors are found to aggravate these symptoms such as stress, lack of sleep, fatigue, alcohol, etc. It is usually not associated with any underlying systemic disease. However, cases have been identified after fourth nerve palsy⁴, mild head trauma, brainstem stroke in association with posterior fossa mass, dural arterio-venous fistula, astrocytoma in the region of trochlear nerves, and multiple sclerosis.³

Patients complain of vague visual disturbances. On careful slit-lamp examination, low-amplitude cyclo-torsional ocular movements are seen occurring at a very rapid pace (Figure 1).⁵

One can elicit ‘Honda sign’ by placing the stethoscope over the oscillating eye. A distinctive noise that sounds like a motorbike revving is heard. This is due to the rapid movement of the superior oblique tendon back and forth through the trochlea. For an unknown reason, right sided SOM appears to be more frequent than left.¹

Various treatment modalities suggested are summarised in Figure 2.⁶ These treatment options may not be very effective giving rise to unacceptable side effects; but most patients require only reassurance regarding the benign nature of the condition.⁷

Figure 2

Treatment flow-chart.