Myasthenia Gravis one of many causes of weakness

Excerpt from Chapter 12 , Evaluation of Patient with Weakness
Disorders of the Nervous System - Reeves and Swenson

Causes of weakness
There are many potential causes of weakness. Each of these conditions is explained in greater detail in the third section of the book and we will only briefly touch on the differential diagnosis once a decision has been made as to whether the weakness is due to damage to upper motor neurons, lower motor neurons, the neuromuscular junction or the muscle.

Muscle diseases. Muscle diseases are most often productive of proximal and symmetrical weakness. As described in the previous section, there may be EMG abnormalities and creatine kinase levels are often elevated. Some of these conditions are primary, with the pathologic process essentially restricted to the muscle, and some secondary,
where the muscle is affected by the systemic disease. The finding of a secondary myopathy often leads to a specific treatment, so it is important to consider these early in the evaluation. Causes of secondary myopathy include: infection (such as trichanosis), connective tissue disease, endocrine abnormality (particularly thyroid and adrenal dysfunction), paraneoplastic processes or may be related to drug or toxin exposures (for example statins).

Primary myopathies can be broadly broken into subcategories of congenital or hereditary myopathies (dystrophies), inflammatory myopathies, and metabolic myopathies. These are discussed in greater detail in Chapter 21. The inflammatory myopathies are occasionally recognized by systemic signs, along with blood chemistry findings of muscle damage and inflammation (although inclusion body myositis may be insidious and without these other features). The dystrophies and metabolic myopathies (which are due to some deficit in critical muscle enzymes) tend to be more slowly progressive and often require invasive and specific genetic testing for confirmation. Table 21-5 lists conditions that should be considered in the differential diagnosis of myopathy.

Neuromuscular junction. Neuromuscular junction disorders may result from drugs and toxins or from diseases such as myasthenia gravis or Lambert-Eaton myaesthenic syndrome. Toxins such as botulinum toxin, certain snake venom's or tick-borne toxins are rare, but enter the differential diagnosis of neuromuscular disease. Drugs that affect neuromuscular transmission or certain pesticides can work in a similar way to block neuromuscular transmission. However, myasthenia gravis, and LEMS represent the majority of potential causes of neuromuscular conduction blockade. These are described in some detail elsewhere in the book. Briefly, myasthenia gravis, produces a decrement in strength with continued contraction, and most commonly affects the muscles of the eye and pharynx. Lambert-Eaton myasthenic syndrome, which typically affects hip girdle mussels, results in increased strength following repeated contraction.

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Related Reading: Barr's The Human Nervous System: An Anatomical Viewpoint