Myasthenia gravis can present with ocular, bulbar, respiratory and limb weakness. Although it is an uncommon condition, it is the most common disorder of neuromuscular transmission (reviewed in Drachman1
and in Vincent et al2
). Myasthenia gravis can present to an acute medical take, particularly where critical symptoms develop, for example, dysphagia and respiratory compromise. Definitive diagnosis is not straightforward and generally depends on a combined clinical, serological and electrophysiological approach – this can be even more difficult in the acute situation.3 4
Confirmatory bedside tests allow a more rapid and definitive diagnosis than is possible from history and clinical examination alone. The Tensilon test is commonly used.5
This involves observing the response to the administration of intravenous edrophonium, an acetylcholinesterase inhibitor. Edrophonium potentiates muscarinic transmission and is contraindicated in patients with cardiac conditions and bronchial asthma; it also requires resuscitation equipment to be readily available.6
Furthermore, there are reports that, although the sensitivity is high, the Tensilon test can result in false negatives.5 7 8
An alternative to edrophonium testing is the ice-test, as described here. It is most useful in patients with ptosis. Both edrophonium and the ice-test are less useful in patients who present with limb weakness only due to the difficulty in quantifying any response.
Cooling improves muscarinic neuromuscular transmission, possibly by decreasing the activity of acetylcholinesterase, and this explains the resolution of ptosis in patients with myasthenia gravis.9–11
For appropriately selected patients, the test is approximately 80% sensitive for myasthenia gravis – this is comparable to the sensitivity of the Tensilon test and actually exceeds the sensitivity of testing for antiacetylcholine receptor antibodies for ocular myasthenia gravis.10–12
The specificity is not known, although it is reported that patients with ptosis not related to myasthenia gravis do not improve with the ice-test.9 12
- Early diagnosis of myasthenia gravis allows prompt initiation of treatment.
- Definitive diagnosis of myasthenia gravis requires serological and electrophysiological investigations which are not generally available in the acute setting.
- In appropriate patients, the ice-test is a non-invasive and sensitive bedside test which can be used to aid diagnosis and direct initial treatment.
- For patients with potentially critical symptoms, starting treatment early is important.