Plasma exchange reversed the syndrome in the case described by Ligouri and colleagues. Some cases have an associated thymoma and its removal may be curative. Weak oligoclonal bands are sometimes found in the spinal fluid. Whether the antibodies are responsible for the hallucinations and other cerebral symptoms is unknown, but the connection is plausible.
An interesting category of disease has emerged as being caused by antibodies to the voltage-gated potassium channel (Thieben et al), sometimes parallel to an idiopathic or paraneoplastic “limbic” encephalitis, which is described in Chap. 30.
In addition to the above disorders, various seizure syndromes (see Chap. 15) and a category of spinocerebellar ataxias are also attributable to mutations of ion channels and are addressed in other sections of the book. Also mentioned here for completeness are several diseases that result from secondary dysfunction of these same ion channels. Most of these conditions are acquired and autoimmune in nature, for example, the Lambert-Eaton myasthenic syndrome that results from an autoimmune attack on calcium channels discussed earlier, the Isaac syndrome that results from an autoimmune attack on potassium channels, producing neuromyotonia, anti- voltage-gated potassium channel and anti-NMDA encephalitis (see Chap. 30), and erythromelalgia that implicates a sodium channel (see Chap. 10). Thus, ion channels, being ubiquitous in all excitable tissues, might be expected to produce a large variety of diseases that affect central and peripheral nervous structures. Despite this, each of the genetic and acquired processes has remarkably specific features that are pertinent only to the channel as expressed in a single tissue.
Muscle Contracture, Pseudomyotonia,
Tetanus, and Related States
Many of these states were discussed in Chap. 45 on diseases of muscle. They are reintroduced here in relation to specific diseases with which they are associated.
Contractures are examples of an entirely different type of painful shortening and hardness of muscle. In both these diseases, an otherwise healthy child, adolescent, or adult begins to complain of weakness and stiffness and sometimes pain on using the limbs. Muscle contraction and relaxation are normal when the patient is in repose, but strenuous activity, especially under conditions of ischemia, causes the muscles to shorten gradually, because of a failure of relaxation. The contracted muscles in these disorders—unlike muscles in cramp, continuous muscular activity syndromes, or myotonia and other involuntary spasms—no longer use energy for which reason they are almost silent electrically in the EMG. This condition is spoken of as physiologic contracture. McArdle and Tarui diseases are discussed more fully in a later section.
This phenomenon is observed in hypothyroidism, where the muscle fibers contract and relax slowly, a response readily demonstrated in the tendon reflexes, particularly the Achilles reflex. The muscles are large and subject to myoedema. When contracted, they may show waves of slow contraction. The basis of this disorder appears to be slowness in the reaccumulation of calcium ions in the endoplasmic reticulum and in the disengagement of actin and myosin filaments. The EMG may show afterpotentials following voluntary contraction, but they do not resemble the typical waning discharges (“myotonic runs”; Fig. 2-19) of true myotonia.