Ocular Myasthenia Gravis (OMG) is one of the conditions that an oculoplastic surgeon can correct without surgery or any invasive intervention. It is a very satisfying to treat a patient of OMG provided the diagnosis is accurate. It has the rare distinction of being both under-diagnosed and over-investigated in different settings. Clinical examination is the cornerstone of a diagnosis of OMG. The various fancy but expensive testing including antibody titres of Acetyl Choline Receptor Antibody (AchRAb) can only corroborate a strong clinical suspicion but is not gospel as only about ~65% of patients are seropositive for AchRAb.
This particular case was especially challenging to diagnose. The child came in with erratic lid positions- sometime ptotic, sometimes eyelid retraction. The most sensitive clinical test for OMG- Ice-pack test was equivocal in this child of 5 yrs. AchRAb titres were low and she was euthyroid (strong family history of hypothyroidism-both parents). It was only on the child’s second visit and on re-examination I could elicit the very subtle clinical sign- Cogan’s Lid Twitch Sign.
Cogan’s lid twitch sign is one of the clinical tests for Myasthenia Gravis. This test is known to have a sensitivity of 75% but a specificity of almost 99% in some controlled studies.
This sign is elicited by asking the patient to maintain downward gaze for 10 seconds and then ask them to look upward, finally returning to straight gaze. A positive sign is indicated by the definitive twitch of the upper lid and sometimes overshooting the superior limbus, soon after the patient has returned to straight gaze.
Ice-pack test has been so useful and quite sensitive to test for OMG, that Cogan’s Lid Twitch sign usually take the back seat. Cogan’s sign proved to be the clincher in this case as the other tests were equivocal.
The child was started on oral Pyridostigmine with steroid induction trial after a Neurology consult. Within a couple of weeks the trial was a success. She is now on maintenance therapy with good control and on regular follow up.
Always refer a suspected Myasthenia Gravis patient to the nearest oculoplastic surgeon for best outcomes.
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Ice Pack Test:
It is a simple diagnostic test that can easily be performed in the clinic. It is highly sensitive and specific for MG. The ice test is useful for OMG An ice pack application is done on the more ptotic upper eyelid for around 2 minutes. A positive test is indicated by the improvement of the ptosis by 2mm or more. This timprovement in ptosis is due to the lower temperatures slowing the acetylcholinesterase break-down of acetylcholine at the neuromuscular junction. With more acetylcholine collecting at the junction, the muscle contraction improves for a few fleeting minutes before getting back to previous position. Prolonged cooling, however, can decrease muscle contractility and result in a false negative result.The sensitivity of this test is around 77% and specificity is around 98%. Very rarely is it negative/equivocal in a patient highly suspected to be OMG.
The other clinic based tests that are sometimes required are
1. Edrophonium (Tensilon) Test:
2. Repeatative Nerve Stimulation (RNS) test
3. Single fibre EMG
4. Sleep Tests
The blood tests required
- Serum anti-ACh Receptor Antibody Titer:
This assay measures 3 types of anti-ACh receptor antibodies (AchRAb) found in OMG: binding antibodies, blocking antibodies, and modulating antibodies. Binding antibodies are present in 85- 90% of systemic MG patients but only ~50-65% in most MG patient cohorts. When binding antibodies are negative, blocking and modulating antibodies are then tested.
Although this test is relatively sensitive and specific for MG, 10% to 15% of patients with systemic MG will test negative, as will 30% to 50% of patients with ocular MG. False positives occur in patients with immune liver disorders, thymoma without MG, LEMS, those with primary lung cancer and in a small percentage of older individuals.
- Serum anti-Muscle-Specific Kinase Antibody Titer:
MuSk assays are used when anti-ACh receptor antibody titers are negative but the clinical examination shows a strong clinical suspicion for MG.