Paramyotonia congenita (PMC) is an autosomal dominant myotonic disorder characterized by cold-induced prolonged localized muscle contraction and weakness. Patients may experience episodes of generalized weakness (periodic paralysis) unassociated with cold exposure (summary by Ptacek et al., 1992).

Myotonia congenita is characterized by muscle stiffness present from childhood; all striated muscle groups including the extrinsic eye muscles, facial muscles, and tongue may be involved. Stiffness is relieved by repeated contractions of the muscle (the "warm-up" phenomenon). Muscles are usually hypertrophic. Whereas autosomal recessive (AR) myotonia congenita is often associated with more severe manifestations (such as progressive minor distal weakness and attacks of transient weakness brought on by movement after rest), autosomal dominant (AD) myotonia congenita is not. The age of onset varies: in AD myotonia congenita onset is usually in infancy or early childhood; in AR myotonia congenita the average age of onset is slightly older. In both AR and AD myotonia congenita onset may be as late as the third or fourth decade of life.

Brody disease (BROD) is an autosomal recessive skeletal muscle disorder characterized by exercise-induced muscle stiffness and cramps primarily affecting the arms, legs, and eyelids, although more generalized muscle involvement may also occur. Symptom onset is most often in the first decade, but many patients present and are diagnosed later in life. Skeletal muscle biopsy typically shows variation in fiber size, increased internal nuclei, and atrophy of type II muscle fibers. Rare patients have been reported to develop malignant hyperthermia after administration of anesthesia, suggesting that patients with the disorder should be tested. The disorder results from defective relaxation of fast-twitch (type II) skeletal muscle fibers due to defects in calcium homeostasis and reuptake in the muscle fiber (summary by Odermatt et al., 2000 and Molenaar et al., 2020).

In a report on the 37th ENMC Workshop, Rudel and Lehmann-Horn (1997) stated that the sodium channelopathies can be divided into 3 different forms: paramyotonia, potassium-aggravated myotonia, and periodic paralysis. Potassium-aggravated myotonia includes mild myotonia fluctuans, severe myotonia permanens, and acetazolamide-responsive myotonia.

Myotonia congenita is characterized by muscle stiffness present from childhood; all striated muscle groups including the extrinsic eye muscles, facial muscles, and tongue may be involved. Stiffness is relieved by repeated contractions of the muscle (the "warm-up" phenomenon). Muscles are usually hypertrophic. Whereas autosomal recessive (AR) myotonia congenita is often associated with more severe manifestations (such as progressive minor distal weakness and attacks of transient weakness brought on by movement after rest), autosomal dominant (AD) myotonia congenita is not. The age of onset varies: in AD myotonia congenita onset is usually in infancy or early childhood; in AR myotonia congenita the average age of onset is slightly older. In both AR and AD myotonia congenita onset may be as late as the third or fourth decade of life.

Schwartz-Jampel syndrome type 1 (SJS1) is a rare autosomal recessive disorder characterized by muscle stiffness (myotonia) and chondrodysplasia. Affected individuals usually present in childhood with permanent muscle stiffness or bone deformities. Common clinical features include mask-like facies (narrow palpebral fissures, blepharospasm, and pursed lips); permanent muscle stiffness with continuous skeletal muscle activity recorded on electromyography; dwarfism; pectus carinatum; kyphoscoliosis; bowing of long bones; and epiphyseal, metaphyseal, and hip dysplasia. The disorder is slowly progressive but does not appear to alter life span (summary by Stum et al., 2006).

Myofibrillar myopathy-10 (MFM10) is an autosomal recessive structural muscle disorder characterized by onset of muscle pain, cramping, and exercise fatigue in the first or second decades of life. Some patients have mild contractures of the large joints apparent in early childhood. Affected individuals have a characteristic appearance of a thick neck and prominent shoulder girdle with anteverted shoulders and a tendency toward kyphosis. There is no apparent muscle weakness, but some affected individuals show progressive muscle rigidity leading to limited mobility. There is variable cardiac involvement, ranging from chest pain with left ventricular hypertrophy to subclinical signs such as abnormal EKG or elevated cardiac enzymes. Skeletal muscle biopsy shows structural abnormalities with myofibrillar disorganization and accumulation of autophagocytic vacuoles (summary by Hedberg-Oldfors et al., 2020). For a general phenotypic description and a discussion of genetic heterogeneity of myofibrillar myopathy, see MFM1 (601419).

Mitochondrial complex IV deficiency nuclear type 23 (MC4DN23) is an autosomal recessive disorder characterized by infantile-onset encephalopathy (Rius et al., 2022). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.