This is one of the more common hereditary disorders of muscle function, and is sometimes called a form of muscular dystrophy. It causes widespread disease in other organs however. Myotonia is the name applied to a condition in which a muscle is slow to relax after contraction.
There is considerable clinical variability in the signs and symptoms of this form of myotonic dystrophy. Symptoms may not appear until the third or fourth decades or later. There is no childhood form of this disease. Weakness in neck and finger muscles may be the first symptom but eventually weakness is apparent in the arms and legs. Cataracts are found in the majority of older patients but no hearing deficits occur in this form. Arrhythmia of the heart beat occurs in a minority of individuals.
Myotonia makes fine movement difficult. It is often noticeable when a patient has difficulty relaxing the hand after a handshake. It is also apparent to patients when using their tongue. Muscle pain can be severe and debilitating. There is little mental deterioration with age. Insensitivity to insulin is found in about half of patients. A significant number of males have low testosterone levels and frontal balding.
Myotonic dystrophy has an unusual genetic etiology. Instead of a change (mutation) in a specific gene, the one responsible for this disease is elongated with duplication of DNA in a certain pattern. The number of DNA molecules duplicated is variable and does not seem to correlate with severity of symptoms. Genetic counseling, therefore, requires individual DNA studies and even then the risk of recurrence falls into a broad range. The DNA duplication follows an autosomal dominant transmission pattern but somewhat unpredictably.
Neurologists and ophthalmologists usually collaborate on the diagnosis. The prognosis is highly variable due as the severity of disease varies among individuals. Longevity is not impacted. No effective treatment is available for the muscle weakness but those with muscle pain can benefit from pain medications. Physical therapy may be helpful as well. Cataracts should be removed when they interfere significantly with vision.