Myasthenia Gravis

At a Glance

The key symptom that suggests myasthenia gravis in patients is abnormal, painless, and fluctuating weakness of the proximal musculature. Small muscles, such as the ocular and oropharyngeal musculature, are typically affected first, resulting in diplopia, ptosis, dysphagia, and dysarthria. Another striking characteristic is the diurnal fluctuation of the weakness, typically most pronounced in the afternoon and evening. Periods of rest lead to improvement of symptoms.

Characteristic fatigue can be brought out in simple clinical tests. In the Simpson test, the patient is prompted to gaze upwards for more than 1 minute, leading to weakness of the eyelids.

Infections and certain medications aggravate the symptoms of myasthenia gravis.

It is practical to distinguish purely ocular myasthenia gravis from generalized myasthenia gravis. In generalized myasthenia gravis, the symptoms range from mild extraocular manifestations, such as dysarthria or dysphagia, to massive generalized muscle weakness, including life-threatening respiratory failure. The presence of a thymoma warrants the diagnosis of paraneoplastic myasthenia gravis.

The (often dramatic) improvement of symptoms on application of anticholinergic drugs, namely neostigmin or endrophonium, is diagnostic for the condition.

In the majority of patients with myasthenia gravis, autoimmune antibodies against the acetylcholine receptor (AChR) can be detected and are seen as causal for the disease.

The cause for the production of the pathogenic autoantibodies is thought to be an impaired tolerance induction against AChR in the thymus gland.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

The diagnosis of myasthenia gravis is made from the typical history, clinical examination, and selected laboratory tests.

Clinical examination is typically unremarkable, but exertion tests (e.g., Simpson test or increasing slurring of the voice during longer periods of speaking) can bring out characteristic abnormalities.

A positive endrophonium (or neostigmin) test is diagnostic.

Electromyography with repetitive 3 Hz stimulation shows a characteristic decrement of the amplitude of the MSAPs.

A standard laboratory panel is needed to evaluate possible complicating comorbidities, such as diabetes, nephropathy, or thyroid disease.

Body imaging (CT scan and/or MRI) is used to detect thymoma. (Table 1)

Table 1

Test Results Indicative of the Disorder

Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications - OTC drugs or Herbals - that might affect the lab results?

The most confounding factor in the laboratory evaluation of myasthenia gravis is the presence of autoimmune comorbidities, such as Graves' disease, Hashimoto's thyroiditis, and systemic lupus.

Anti-inflammatory therapy can reduce auto-antibody titers, but, since this is also therapeutic, this rarely causes diagnostic dilemmas.

What Lab Results Are Absolutely Confirmatory?

A positive response to cholestrinesterase inhibitors is virtually diagnostic for the disease. AChR antibodies can be detected in 50% of cases with ocular myasthenia gravis and about 90% of cases of generalized myasthenia. In the presence of a thymoma, the AChR antibodies are detected in nearly 100% of cases. In the absence of AChR antibodies, anti-muscle specific tyrosine kinase (MuSK) antibodies can be detected in 20-40% of the cases.

The diagnosis is further confirmed by the detection of MSAP decrement in the repetitive EMG.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

In addition to the antibody tests and electromyography, sometimes a muscle biopsy is warranted to evaluate whether myositis is also present.

Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications - OTC drugs or Herbals - that might affect the lab results?

Possible confounding factors include auto-immune comorbidities, such as Graves' disease, Hashimoto's thyroiditis, and systemic lupus erythematosis.