A closely related syndrome, wherein painless contracture is induced by exercise, has been described by Lambert and Goldstein, and by Brody. Muscle contraction is normal but the relaxation phase becomes increasingly slow during exercise. Lambert and Goldstein referred to it as an unusual type of myotonia, and Brody, as a decrease of “relaxing factor”; the slow relaxation has also been attributed to a decreased uptake of calcium by the sarcoplasmic reticulum. In some cases, the disease is transmitted as a recessive trait with a mutation that impairs the function of a sarcoplasmic reticular calcium adenosine triphosphatase (ATPase). In other instances, the disease is transmitted as a dominant trait that is not genetically linked to calcium ATPase. This latter process may be more closely aligned with the muscular dystrophies and is mentioned in Chap. 45 under that heading.
Tetanus (See Tetanus in Chap. 41)
This toxic disorder is characterized by persistent spasms of skeletal muscles, owing to the effect of the tetanus toxin on spinal neurons (Renshaw and other cells), the natural function of which is to inhibit the motor neurons. As the condition develops, activities that normally excite the neurons (i.e., voluntary contraction and startle from visual and auditory stimulation) all evoke involuntary spasms. Sleep tends to quiet them, and they are suppressed by spinal anesthesia and curare. The EMG shows the expected interference pattern of muscle action potentials. Once the muscle is involved in persistent contraction, it is said that the shortened state is not abolished by procaine block or severance of nerve (in animals), but this type of myostatic contracture has not been demonstrated in humans.
The effect of tetanus toxin on the spinal inhibitory neurons is analogous to that of strychnine. There is also an action of the toxin at the neuromuscular junction, which has been more difficult to evaluate in the face of its powerful central action. Having injected this toxin locally in animals, Price and associates demonstrated its localization at motor endplates. It binds with ganglioside in the axon membrane and is transported by retrograde flow to the spinal cord, where it induces local tetanus effects. Neurons that innervate slow-twitch type 1 muscle fibers are more sensitive than those supplying fast-twitch type 2 fibers. Presynaptic vesicles increase in number, acetylcholine (ACh) is blocked, and terminal axon injury may paralyze muscle fibers. Fibrillation potentials and axonal sprouting follow. The similarities to stiff man syndrome are mentioned further on.
The toxin produced by this spider within a few minutes of the bite leads to a striking syndrome of cramps and spasms and then a painful rigidity of abdominal, trunk, and leg muscles. The spasms are followed by weakness. There is also vasoconstriction, hypertension, and autonomic hyperactivity. If death does not occur in the first 24 to 48 h, recovery is complete. The spider venom has a presynaptic localization and rapidly releases quanta of ACh. The vesicles became depleted. There is some evidence that the venom prevents endocytosis of the vesicles by inserting itself into the presynaptic membranes, causing a disturbance of ionic conductance channels (Swift).
Treatment consists of calcium gluconate infusions and diazepine. Intravenous magnesium sulfate also helps to reduce the release of ACh and control the convulsions that sometimes occur. There is antiserum that is available in regions where such envenomation is frequent; it shortens the illness considerably.
Metal or other type of poisoning may simulate an inherently relapsing polyneuropathy.
States of Persistent Muscle Activity