Unilateral ocular myasthenia gravis in an acute medical take: is a Tensilon test useful?

A 42-year-old woman presented to an eye clinic with a 2-week history of diplopia. Examination revealed signs of a right sixth nerve palsy. A computed tomography (CT) brain scan was normal. Six weeks later she presented during the acute medical take with increasing diplopia, a drooping right eyelid and unsteadiness. Direct questioning revealed that the drooping became progressively worse throughout the day. She had no relevant past medical history or any regular medication. Examination revealed a partial right ptosis and signs of right third, fourth and sixth nerve palsy. Diplopia on binocular vision resolved on patch testing; neurological examination was otherwise unremarkable, with no evidence of limb weakness. A magnetic resonance imaging scan of the brain and CSF examination were unremarkable. Ocular myasthenia was suspected, anti-acetylcholine receptor antibody test was requested, and a Tensilon test was performed with full resuscitation equipment available. Following the administration of the test dose (2 mg) of intravenous edrophonium (Tensilon) a visible improvement in the right ptosis was observed. Subsequent administration of a further 8 mg edrophonium yielded more marked improvement, with complete correction of the right ptosis (Figure 1) and a subjective decrease in diplopia on right gaze. A diagnosis of unilateral ocular myasthenia gravis was made on the basis of the clinical picture and the positive Tensilon test. A neurological opinion was sought and the patient commenced on pyridostigmine 60 mg four times a day. Further investigations included a CT scan of the thorax which revealed a 2 cm retrosternal mass, compatible with thymic hyperplasia. Five weeks after the positive Tensilon test, a report of strongly positive anti-acetylcholine receptor antibodies (anti-acetylcholine receptor antibodies >8 nmol/litre, normal range 0–0.5 nmol/litre) was received. The diplopia continues to improve and the patient has regular follow up.