Myotonic Dystrophy (MD) is a chronic, inherited disorder causing progressive muscle loss and weakness. It is a complex, multi-system condition, affecting skeletal muscles, the heart, eyes, endocrine system, and brain. Symptoms are highly variable, ranging from mild, late-onset issues to severe, life-threatening complications present from birth.
Core Musculoskeletal Symptoms
The defining characteristics of Myotonic Dystrophy are myotonia and progressive muscle weakness. Myotonia is the delayed relaxation of a muscle after a voluntary contraction, a hallmark feature of the condition. This manifests as difficulty releasing a handshake, opening the hand after gripping an object, or problems with facial muscles after speaking or chewing. This delayed relaxation is often worsened by cold temperatures or stress but tends to improve slightly with repeated exercise.
The second main feature is progressive muscle weakness and wasting, which defines the “dystrophy” component. Weakness typically starts in the muscles farthest from the center of the body, such as the face, neck, hands, and lower legs. This pattern of muscle atrophy leads to a characteristic facial appearance, often described as a “hatchet face,” with drooping eyelids (ptosis). Weakness in the lower legs can cause foot drop, contributing to gait disturbances and an increased risk of falls.
Systemic Manifestations Beyond Muscle
Myotonic Dystrophy is a systemic disorder affecting numerous organs and tissues outside of skeletal muscles. The heart is a major site of involvement, and cardiac conduction defects are among the most serious complications. These abnormalities affect the electrical signals regulating the heartbeat, leading to various arrhythmias, including atrial fibrillation and flutter. Conduction issues may progress to severe atrioventricular block, posing a risk of fainting, dizziness, and sudden death.
Ocular manifestations are common, with most individuals developing posterior subcapsular cataracts. These cataracts often have a characteristic iridescent appearance and can develop at an early age. The endocrine system is frequently affected, often resulting in insulin resistance that can progress to type 2 diabetes. Other hormonal issues include thyroid dysfunction and hypogonadism, which may affect fertility.
Within the Central Nervous System (CNS), many people experience hypersomnia, or excessive daytime sleepiness, which impacts daily function. Cognitive impairment, particularly affecting executive functions, attention, and memory, is also a reported symptom. Gastrointestinal issues like constipation, difficulty swallowing (dysphagia), and gallbladder problems are common due to smooth muscle involvement.
Symptom Variation Based on Type and Onset
The presentation and severity of Myotonic Dystrophy symptoms vary significantly based on Type 1 (DM1) or Type 2 (DM2) and the age of onset. DM1, caused by a mutation in the DMPK gene, has the widest clinical spectrum, including congenital, childhood, and adult-onset forms. Congenital DM1 is the most severe, presenting at birth with profound hypotonia, generalized weakness, and often life-threatening respiratory failure. Children who survive this form frequently experience developmental delays and intellectual disability.
The classic adult-onset DM1 typically presents between the second and fourth decades of life, starting with pronounced myotonia and distal muscle weakness in the face and hands. This form is strongly associated with severe systemic issues, including the highest risk for cardiac conduction abnormalities and significant cognitive changes.
Myotonic Dystrophy Type 2 (DM2), caused by a mutation in the CNBP gene, generally has a later onset in adulthood and is considered milder than classic DM1. Unlike DM1, DM2 does not have a congenital form. The pattern of muscle weakness differs, primarily affecting the proximal muscles closer to the body’s center, such as the neck, shoulders, and hips.
Myotonia in DM2 is typically less severe than in DM1, although debilitating muscle pain (myalgia) is much more common. While DM2 still involves systemic features like cataracts and insulin resistance, the cardiac and cognitive involvement tends to be less frequent or less severe compared to classic DM1.