Paramyotonia congenita

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Paramyotonia congenita (PC) is inherited as an autosomal dominant (MIM 168300). Signs are present at birth and often remain unchanged throughout life. The cardinal symptom is cold-induced muscle stiffness that increases with continued activity (paradoxical myoto-nia). On repeated strong contractions of the orbicularis oculi, the opening of the eyelids is increasingly impeded; finally the eyes cannot be opened to more than a slit. As a rule, muscles are bilaterally and symmetrically affected. Many patients exhibit the lid-lag phenomenon and some have percussion myotonia. The motility of the eyeballs may be hampered, which may lead to short bouts of diplopia. Also, swallowing may be impeded for short periods of time. These symptoms, however, tend to be transient. In rare cases, the paramyotonic muscles seem to be somewhat swollen. Muscle atrophy or hypertrophy are not typical for the disease.

In the cold (even in just a cool wind), the face may appear mask-like, and the eyes cannot be opened for several seconds or minutes (Fig. 3.6). Working in the cold makes the fingers so stiff that the patient cannot move them for several minutes. Under warm conditions, most patients have no complaints because impaired muscle relaxation improves at higher temperatures. Other patients have stiff limb muscles in a warm environment; the stiffness improves on continued exercise and displays the paradoxical reaction only on cooling. On the whole, the duration and degree of the paramyotonic reaction of muscles depends on the duration and intensity of cooling, but there are also individual differences in susceptibility.

A few patients claim that emotional factors or hunger aggravate their condition. In many cases alcohol has an obvious beneficial effect. Some patients believe that they are more susceptible to paramyotonia when they have a cold. Paramyotonia may become more severe during pregnancy, so that the leg muscles stiffen even under warm conditions. Hypothyroidism also causes generalization of paramyotonia and aggravates both muscle stiffness and weakness. All movements are then severely hampered, even independently of cooling.

An estimate of the prevalence of paramyotonia congenita seems almost impossible to obtain, because most of the affected individuals never consult a doctor for their symptoms. Moreover, when a paramyotonic patient requires medical help for another reason, they hardly mention their paramyotonic symptoms. Although paramyotonia can be troublesome, it is often a harmless abnormality or a familiar peculiarity that the sufferer simply tolerates. Patients feel that they must make the best of their condition, as did their ancestors, an opinion reinforced when they encounter medical ignorance. On the whole, patients tend to hide their

Paramyotonia Congenita Eye
Fig. 3.6. Effects of local cooling on a paramyotonia congenita patient. After the patient's right eye was cooled for 10 min, she was asked to close her eyes forcefully (A) and the open them fast (B). The cooled right eyelid remained involuntarily closed for almost a minute.

family anomaly as much as possible, even from close relatives, because they have often experienced embarrassing situations and been ridiculed. On the other hand, paramyotonia patients readily share their experiences with each other. Older patients report that their para-myotonia improved with age. In many of these cases, however, it was not clear whether the paramyotonia had really improved or whether the patients had learned to adapt to it by avoiding exposure to cold and by taking advantage of improving standards of living. Life expectancy is not decreased by paramyotonia.

In most families, the stiffness gives way to flaccid weakness or even to paralysis on intensive exercise and cooling (Fig. 3.7; Haass et al., 1981). Some, but not all, families with PC also have attacks of generalized hyperkalemic periodic paralysis for an hour or less (see below), provoked by rest after strong exercise or by potassium ingestion. In contrast, the cold-induced weakness usually lasts for several hours even when the muscles are promptly rewarmed. During a severe paralytic attack, the muscle stretch reflexes are diminished or absent.

Paramyotonia mutations are situated either in the inac-tivation gate, the intracellular loop connecting domains

III and IV (T1313M: McClatchey et al., 1992; T1313A: Bouhours et al., 2004), in the voltage sensor of repeat

IV (R1448H/C/S/P: Ptacek et al., 1992; Chahine et al., 1994; Lerche et al., 1996; Bendahhou et al., 1999) or in the intracellular S4-S5 loops (F1473S: Lerche et al., 1997; A1152D: Bouhours et al., 2005). Paramyotonia families with R1448 substitutions (Fig. 3.5) also have

Paramyotonia Congenita Pregnancy

Fig. 3.7. Contractions of a paramyotonia patient at different temperatures. Periods of voluntary isometric muscle contractions (in Newton) and the corresponding surface EMG activity underneath (modified from Haass et al., 1981). The patient was asked to maximally contract his muscles for about 3-5 s and then to relax. The upper two traces show the warm-up phenomenon at 37°C, the lower two traces the paradoxical myotonia, i.e., slowed relaxation during exercise after 30-min cooling of the forearm in water of 15°C. Note the reduced muscle strength after cooling.

Fig. 3.7. Contractions of a paramyotonia patient at different temperatures. Periods of voluntary isometric muscle contractions (in Newton) and the corresponding surface EMG activity underneath (modified from Haass et al., 1981). The patient was asked to maximally contract his muscles for about 3-5 s and then to relax. The upper two traces show the warm-up phenomenon at 37°C, the lower two traces the paradoxical myotonia, i.e., slowed relaxation during exercise after 30-min cooling of the forearm in water of 15°C. Note the reduced muscle strength after cooling.

attacks of generalized hyperkalemic periodic paralysis, provoked by rest or ingestion of potassium, lasting for an hour or less. In contrast to the short-lasting spontaneous weakness in hyperkalemimc periodic paralysis and paramyotonia, the cold-induced paramyotonic weakness usually lasts several hours even when the muscles are immediately rewarmed. Also, carriers of other mutations show overlapping features: in a Japanese pedigree, the mutation M1370V resulted in paramyotonia in one family member and in hyperkalemic periodic paralysis in others (Okuda et al., 2001). Also, with hyperkalemic periodic paralysis mutations such as M1360V, T704M and M1592V (Fig. 3.5), paramyotonic signs have been reported in single families (Kelly et al., 1997; Wagner et al., 1997; Kim et al., 2001; Brancati et al., 2003). I693T has been published as a paramyotonia mutation (Plassart et al., 1996) although it causes weakness in the absence of stiffness and would therefore be compatible with a hyperkalemic periodic paralysis mutation.

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