Test & Diagnostic methods

In addition to a complete medical and neurological evaluation, a number of tests may be used to establish a diagnosis of MG. A characteristic of MG is that patients have weakness that comes on with activity and improves following rest. To examine for weakness with activity, a clinician might have a patient do a sustained task, such as looking upwards (which induces the eyelids to remain elevated) to see if the eyelids start to droop (referred to clinically as ptosis) when the eyelids are open for several minutes. To test for recovery of strength after rest, a clinician may examine muscle strength and then have the patient suspected to have MG rest or rest a part of the body and then retest muscle strength after rest to see if strength improves. For example if a person who is suspected to have MG has droopy eyelids, the physician might have the patient lay down with his/her eyes closed for several minutes, perhaps with a cool pack over the eyelids, to see if eyelid function improves after rest.  
 

A diagnosis can be confirmed in several ways, including the following:

• Acetylcholine Receptor Antibody— a blood test for the abnormal antibodies can be performed to see if they are present. Approximately 85% of MG patients have this antibody and, when detected with an elevated concentration the AChR antibody test is strongly indicative of MG.

• Anti-MuSK Antibody testing— a blood test for the remaining 15% of MG patients who have tested negative for the acetylcholine antibody. These patients have seronegative (SN) MG. About 40-70% of patients with SNMG test positive for the anti-MuSK antibody. The remaining patients have unidentified antibodies causing their MG.

• OfficeTests—Sleep, Ice Pack and Edrophonium tests are examinations performed by specialists to evaluate an improvement in strength that may be consistent with MG.

• Electromyography— (EMG) studies can provide support for the diagnosis of MG when characteristic patterns are present. Repetitive Nerve Stimulation is used to check for a pattern of response that is characteristic of MG.

• Single Fiber EMG— studies can provide support for the diagnosis of MG when characteristic patterns are present. The single fiber EMG and AChR antibody test are primary tests used to confirm a clinical diagnosis of MG.

Sometimes all of these tests are negative or equivocal in someone whose story and examination still seem to point to a diagnosis of MG. A clinician skilled in recognizing MG and distinguishing MG from other conditions would need to determine if such a patient has MG or another disorder.

Reviewed by the MGFA’s Medical Advisory Board, November 2010