Congenital Myopathies
The ‘floppy infant’ group of muscular disorders hypotonic, delayed motor activity and delayed development
Central Core Disease:
Autosomal dominant. Abnormal structure and muscle enzymes in central area of muscle fibre. Some are susceptible to malignant hyperthermia (genes located next o each other on chromosome 19); decreased foetal movements; breech presentation; hypotonic at birth; delay in reaching motor milestones; extremity weakness (legs > arms); also higher prevalence of congenial hip dislocation, scoliosis, pes cavus or clubfeet. Most are non-progressive
Nemaline Myopathy:
Uncommon disorder due to mutation in genes (5 have so far been recognised) – characterised by thread/rod like structures found in muscle fibres. Most have muscle weakness and hypotonia from infancy, but childhood and adult onset forms do occur. Respiratory involvement results in poorer outcomes. Most are in a wheelchair or have difficulty ambulating and have contractures Associated with scoliosis, pes cavus and/or varus deformities of foot
Myotubular myopathy/Centronuclear myopathy:
Muscle fibres have a tubular like structure similar to foetal muscle; three variants (neonatal, late infancy-early childhood and late childhood forms)
Periodic Paralysis:
Rare autosomal dominant – affect 4-5/100 000; characterised by attacks of flaccid weakness with variations in potassium levels.
3 classical symptoms – transient attacks of weakness; myotonia; weakness between attacks (may be progressive)
1) Hypokalaemic periodic paralysis
• onset at puberty – may be triggered by excessive carbohydrate intake, insulin injection or adrenaline; M>F; rare
• potassium content <3.0mEq/l
• attacks usually begin after period of rest (eg night) and last for up to 24-48 hours
• varies from slight leg weakness to complete paralysis
2) Hyperkalaemic periodic paralysis
• onset in early childhood; M=F
• attacks induced by period of immobility or fasting – carbohydrate ingestion can reverse attack rather than cause; usually occur during daytime
• attacks milder and shorter than hypokalaemic attacks
• weakness is in proximal limb muscle, though distal can be affected
Treatment:
Correction of potassium level abnormalities
Acetazolamide (carbonic anhydrase inhibitor)
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