Superior oblique myokymia is a rare disorder characterised by unilateral rotatory oscillopsia, which may be due to neurovascular compression of the trochlear nerve (cranial nerve IV) transition zone from branches of the superior cerebellar artery.

Epidemiology

The exact incidence of superior oblique myokymia is not known, but it is considered rare ¹. It may present across the adult demographic and does not have a sex predilection ¹. For reasons unclear, it is described slightly more commonly on the right side, especially in females ¹.

Clinical presentation

Clinical features are usually paroxysmal, lasting seconds to hours in duration ¹. Paroxysms can often, but not always, be triggered by stress, illness, alcohol intake, fatigue, or looking downwards ^1,2,6.

Clinical examination during a paroxysm classically reveals a subtle, ipsilateral, low-amplitude and high-frequency incyclotorsional nystagmoid movement or microtremor ^1-6. Auscultation over the affected eye can reveal a sound resembling the rev of a motorcycle, known as the ‘Honda sign’ ^1,2. On electromyography, there are myokymic-like discharges ³.

The resultant clinical symptoms during paroxysms include ^1-6:

• ipsilateral rotatory oscillopsia

• oblique binocular diplopia

If there is concurrent weakness of the superior oblique muscle, which can develop in longstanding cases, there may be additional ^6,9:

• oblique diplopia present independently of an episode, especially when looking downwards

• compensatory contralateral head tilt

Pathology

Superior oblique myokymia describes paroxysmal contractions of the superior oblique muscle ^1-6. While the exact aetiopathogenesis of superior oblique myokymia is not known, many cases have been associated with neurovascular compression to the transition zone of the trochlear nerve (cranial nerve IV) from branches of the superior cerebellar artery ^1-7. Other causes described include brainstem disease (e.g. stroke, demyelination), posterior fossa tumour, and head trauma ^1-6. Most cases, however, are idiopathic ¹.

Radiographic features

MRI

MRI with heavily T2 weighted (e.g. CISS) thin section images of the brainstem and post-gadolinium contrast enhanced sequences, should be performed, especially in pharmacologically-refractory cases ¹. In cases of neurovascular compression, an implicated branch of the superior cerebellar artery may be identified to be in contact with the transition zone of the trochlear nerve (cranial nerve IV) ^1,3,4,7.

Treatment and prognosis

Pharmacological management may include beta-blockers (topical or systemic) or antiseizure medications (e.g. carbamazepine, gabapentin, clonazepam) ¹. Other treatment options include botulinum toxin A injections to the superior oblique muscle, or surgery (e.g. extraocular muscle surgery, microvascular decompression) ¹.

History and etymology

Superior oblique myokymia was first described by Alexander Duane (1858-1926), American ophthalmologist, in 1906 ⁸. However, the condition was given the name ‘super oblique myokymia’ in 1970 by William F Hoyt (1926-2019), American neuro-ophthalmologist, and James R Keane, American neurologist ⁵.

Differential diagnosis

• superior oblique neuromyotonia ^1,6

• eyelid myokymia ¹

• Heimann-Bielschowsky phenomenon ¹

• other phenomena of extraocular motility (e.g. nystagmus, square wave jerks) ¹