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 myotonic dystrophy. The congenital form is the most severe. Infants have poor muscle tone, psychomotor delays, generalized weakness, and often have difficulty breathing. Symptoms may not appear in other patients until the second decade or later. Usually, weakness is apparent in the arms and legs first but eventually in facial muscles as well, causing difficulty swallowing and slurred speech. The lids appear to droop and individuals cannot close their eyes tightly. Some patients have difficulty moving their eyes. Cataracts are found in the majority of older patients. Arrhythmia of the heart beat occurs and in some cases is fatal. Hearing loss has been found in some patients.
Deep muscle pain is common and may be severe. Myotonia makes fine movement difficult. It is often noticeable when a patient has difficulty relaxing the hand after a handshake. It is also often apparent to patients when using their tongue. The weakness of the facial muscles contributes to a rather bland expression, sometimes called a myotonic facies. Some older patients become disabled and have difficulty breathing. Some mental deterioration occurs with age in the more severely affected individuals.
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 the DNA in a certain pattern. The number of DNA molecules duplicated is variable and in general patients with the most duplications have the more serious disease. Genetic counseling, therefore, requires individual DNA studies and even then the risk of recurrence falls into a broad range.
Neurologists and ophthalmologists usually collaborate on the diagnosis. The prognosis is highly variable due as the severity of disease varies among individuals. Young children with the most severe disease may live only a few years, adults with mild disease may have a normal life span, while other adults with more serious symptoms often have reduced longevity. No effective treatment is available 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.