Overview

Myopathies are a diverse and complex cluster of disorders associated with terms that may be confusing due to their similarity. Some brief definitions may be in order.

Myo- as a prefix, refers to muscle, as in myocardial (referring to heart muscle).

Myopathy can refer to any disease or abnormal condition of striated muscle.

Myopathic is a term that simply refers to a disease of muscle tissue.

Myotonia refers to a general class of myopathies characterized by myotonus, or tonic (characterized by tension or contraction) spasms, or temporary rigidity of muscle.

Hypotonia refers to reduced tension, relaxation, and loss of tone.

Hypertonia refers to increased tension, a stiffening or rigidity. The term tetany is also used to describe intermittent tonic spasms or rigidity.

Myoclonus refers to a twitching or clonic (characterized by alternating contraction and relaxation) spasm of a muscle or group of muscles, as in myoclonic jerks.

Most of the disorders addressed at this site are myopathies in some sense. The exception being those disorders which have a predominantly neurological origin, as in Sleep Paralysis.

In general, these disorders have been divided into major categories for more convenient reference. The primary division is between the Periodic Paralyses, a category dedicated to those forms of Periodic Paralysis that have traditionally been clinically associated with blood potassium levels, and the Myopathies (where you are now). Depending on one's willingness to break with established nomenclature, even sleep paralysis could be considered a myopathy in the sense that REM inhibition ultimately results in a temporary denervation of skeletal muscle. As discussed elsewhere in this site, there are a significant number of clinicians who still have difficulty differentiating between these two classes of disease. Because of the apparent overlap of much of the information associated with these disorders, it is not always easy to maintain clean distinctions in the presentation of information.

The myotonias, for example, have a close historical association with the classic periodic paralyses due to the commonly held assumption that paramyotonia and hyperkalemic periodic paralysis were the same disease. See The Hereditary Myotonias and Periodic Paralyses: An Evolving Perspective.

The Myotonias

The more commonly recognized myotonias include:

Acetazolamide-responsive myotonia
Becker, or generalized myotonia
Myotonia congenita (Thomsen Disease)
Myotonia fluctuans
Myotonia levior
Myotonia permanens
Paramyotonia congenita

Perhaps the earliest defined of the myotonias is Myotonia Congenita. This disease was first described in 1876, by Julius Thomsen, a Danish physician who suffered from the disease. There is evidence that an additional 20 members, through at least four generations of Thomsen's family, also suffered from the disease he named ataxia muscularis. The name myotonia congenita was provided by Strumpell in 1881. Strumpell's contribution, along with Westphal's 1883 reference to Thomsen's disease seem to have stuck, as the names have been equally associated with this disease for the last one hundred and fourteen years.

The Myopathies

Those diseases officially designated as "myopathies" include the Idiopathic Inflammatory Myopathies (IIM).The Idiopathic Inflammatory Myopathies are described as a group of rare diseases of undefined, or unknown cause characterized by symmetrical proximal muscle weakness and inflammation of skeletal muscle.

Some of the recognized diagnoses that have been characterized by the term Idiopathic Inflammatory Myopathy include polymyositis, dermatomyositis, cancer-associated myositis, miositis associtated with other connective tissue disease, and inclusion-body myositis. The standard criteria used by most physicians to define Idiopathic Inflammatory Myopathy were provided by Bohan and Peter in 1975: