Diagnosis of MG
MG affects people initially in a variety of ways so that no two cases have exactly the same symptoms. The onset can be sudden, or more commonly, it starts gradually and develops slowly or intermittently. This fact makes it easily missed or only diagnosed after some time. Likewise, because the symptoms are similar to many other situations in which the limbs feel heavy and weak, it can be confused with other ailments.
Positive diagnosis however is now available, even though this is normally only achieved over a period of time and in some cases, requires confirmation by more than one test.
A preliminary test which suggests further investigation is the test for muscle strength and fatigue-ability. The limbs are tested for muscle strength after which the patient is asked to make multiple repetitive movements and then strength again tested to see if unusual muscle fatigue and weakness occurs.
We all know well how stupid one feels flapping our elbows up and down when this test is frequently carried out after diagnosis. However a treating neurologist suggests that this is a kinder method of assessing the degree of weakness present and hence the need for treatment than repetitive stimulation testing on every visit!
If MG is suspected a blood test is carried out to try and establish the presence of AChR antibodies if they are present this is indicative of the common form of autoimmune MG.
On other occasions, where the problem is not clearly confirmed, your practitioner may determine that further tests are necessary. One such test is the Edrophonium (Tensilon) Test. This test is carried out under controlled conditions requiring the injection of a myasthenia treatment similar to Mestinon in the veins which has the effect of greatly stimulating the muscle for a brief period. Muscle strength, eyelid position, or eye movements are assessed before and after injection, and where a marked improvement is obvious, the result is considered positive. As Tensilon is only useful for testing for myasthenia it is no longer supported in the market by the manufacturer, so additional government paper work is required for this test.
Another test is called repetitive stimulation, where the nerve to the muscle is repetitively stimulated (shocked); this can help determine whether there is neuromuscular junction fatiguability present as the size of the muscle response gets smaller with successive stimulations.
In some cases an extra test called SFEMG (single fibre electro myography) is needed, where a tiny needle is placed within a number of individual muscle units (of which there are hundreds or thousands in each muscle) and the firing of each muscle unit is observed.
In about 20% of patients, who present with all the symptoms of generalised MG, tests show that they do not have AChR Antibodies. It has been recently discovered that about half of these patients have antibodies to another protein present at the neuromuscular junction, called Muscle Specific Tyrosine Kinase (MuSK). The role of the MuSK protein is still being clarified.