Paramyotonia is similar to myotonia in that muscle does not relax normally following contraction (voluntary, percussion), which may prompt a complaint of muscle aching or stiffness, but differs in that repetitive muscle use (e.g., exercise) accentuates the problem, leading to an increased delay in muscle relaxation (worsening stiffness). For example, repeated forced voluntary eyelid closure in a patient with paramyotonia may, after several attempts, lead to a failure of voluntary eyelid opening, the eyes remaining closed for a minute or so. Paramyotonia particularly affects the face and forearms. This type of muscle stiffness may also be sensitive to temperature, being made worse by cooling which may also provoke muscle weakness. Weakness may outlast exposure to cold by several hours.
Neurophysiological studies may assist in the diagnosis of paramyotonia. During the delayed muscle relaxation, electrical activity is not prominent, and after muscle cooling the resting muscle membrane potential may be reduced from around the normal −
80 mV to −40 mV, at which point muscle fibers are inexcitable
Paramyotonia congenita is a channelopathy with mutations affecting the α-subunit of the sodium channel. Mutations in the same gene have been documented in hyperkalaemic periodic paralysis and K+-aggravated myotonia.
Davies NP, Eunson LH, Gregory RP, Mills KR, Morrison PJ, Hanna MG. Clinical, electrophysiological, and molecular genetic studies in a new family with paramyotonia. Journal of Neurology, Neurosurgeryand Psychiatry 2000; 68: 504-507 [erratum: Journal of Neurology,
Neurosurgery and Psychiatry 2000; 69: 139]
Ebers GC, George AL, Barchi RL, et al. Paramyotonia congenita and hyperkaliemic periodic paralysis are linked to the adult muscle sodium channel gene. Annals of Neurology 1991; 30: 810-816