Myotonic disorders are a group of genetic muscle diseases characterized by prolonged muscle contractions (delayed relaxation) after voluntary muscle contraction or after being stretched. This is also known as myotonia. The term "myotonic" comes from the Greek words "mys," meaning muscle, and "tonos," meaning tone or tension.

There are several types of myotonic disorders, with the two most common being myotonic dystrophy type 1 (DM1) and myotonic dystrophy type 2 (DM2), also known as proximal myotonic myopathy (PROMM). These conditions are caused by mutations in different genes but share similar symptoms, including muscle stiffness, weakness, and wasting.

Myotonia can be experienced as muscle stiffness or cramping, particularly after periods of rest or when attempting to relax the muscle after a contraction. For example, individuals with myotonic disorders may have difficulty releasing their grip on an object or letting go of a handshake. Myotonia can also affect the muscles used for speaking and swallowing, leading to speech and swallowing difficulties.

In addition to myotonia, myotonic disorders can cause other symptoms such as:

* Muscle weakness and wasting, which can progress over time and affect various muscle groups, including those in the face, neck, hands, forearms, and legs.
* Fatigue and decreased endurance due to muscle weakness and wasting.
* Cardiac abnormalities, such as arrhythmias and conduction defects, which can increase the risk of sudden cardiac death in some cases.
* Respiratory problems, including weakened respiratory muscles and reduced lung capacity, which can lead to breathing difficulties and an increased risk of respiratory infections.
* Cataracts, which are common in individuals with myotonic dystrophy type 1.
* Hormonal imbalances, such as insulin resistance and testicular atrophy.
* Central nervous system involvement, including cognitive impairment, mood disorders, and sleep disturbances.

Myotonic disorders are typically inherited in an autosomal dominant manner, meaning that a child has a 50% chance of inheriting the condition if one parent is affected. However, some cases may arise due to new mutations in the affected gene. Currently, there is no cure for myotonic disorders, but various treatments can help manage symptoms and improve quality of life. These may include physical therapy, speech therapy, respiratory support, medications to treat cardiac abnormalities or hormonal imbalances, and assistive devices to aid in daily activities.