Myotonic Dystrophy (MD) is an inherited form of muscular dystrophy causing progressive muscle degeneration and weakness. This genetic condition affects skeletal muscles and impacts multiple body systems, including the heart, eyes, and endocrine system. The severity and combination of symptoms vary widely, depending on the specific genetic type and the age of onset.
The Hallmark Muscle Symptoms
The defining feature of Myotonic Dystrophy is myotonia, the delayed relaxation of a muscle after voluntary contraction. This manifests as difficulty releasing a handshake or temporary stiffness in the face or jaw after chewing. Myotonia is often worsened by cold temperatures or rest, but it typically improves after the muscle is repeatedly used.
Progressive muscle weakness and atrophy are central components of the condition. This weakness commonly affects the facial muscles, resulting in a slack expression and drooping eyelids (ptosis). Wasting of the neck muscles, particularly the sternocleidomastoid, is also frequently observed.
Weakness also impacts the lower limbs, often causing foot drop due to weakened muscles that lift the front of the foot. Deterioration of muscle strength in the hands makes fine motor tasks increasingly difficult. As the disease progresses, men may also experience early frontal balding.
Systemic Symptoms Affecting Other Body Systems
MD is multi-systemic, affecting several internal organs and processes. One of the most serious effects is on the cardiac system, where patients frequently develop heart conduction defects or heart block. These electrical abnormalities can lead to serious complications, including sudden death.
The eyes are commonly affected, with cataracts representing one of the most frequent ocular symptoms. These clouding of the lens often develop at a much younger age than is typical. Other eye-related issues include misalignment of the eyes, known as strabismus.
Endocrine and metabolic systems show significant involvement, often leading to insulin resistance and increasing the risk of Type 2 diabetes. Some people also experience thyroid dysfunction or hypogonadism, which may contribute to infertility.
The smooth muscles of the gastrointestinal (GI) tract are impacted, leading to various motility issues. Common GI symptoms include chronic constipation, abdominal pain, and bloating. Weakness of the pharyngeal muscles can cause difficulty swallowing (dysphagia), increasing the risk of accidentally inhaling food or liquid into the lungs.
Distinguishing Symptoms of Myotonic Dystrophy Type 1 and Type 2
Myotonic Dystrophy Type 1 (DM1) and Type 2 (DM2) share myotonia and muscle weakness but exhibit distinct patterns of presentation. DM1, often called Steinert’s disease, typically involves more pronounced weakness in the distal muscles, such as the hands and feet.
DM1 is associated with more severe systemic involvement, including a higher risk of serious cardiac defects and significant endocrine issues. The disease course of DM1 is generally more severe, often having an earlier age of onset. The facial and neck muscles are also more consistently and severely affected in DM1.
In contrast, Myotonic Dystrophy Type 2 (DM2), also known as Proximal Myotonic Myopathy (PROMM), primarily presents with weakness in the proximal muscles, such as the shoulders and hips. This proximal pattern often makes standing up from a chair or climbing stairs difficult. Individuals with DM2 often experience significant muscle pain (myalgia) and stiffness.
Systemic involvement in DM2 is generally milder than in DM1, though cataracts remain common. While myotonia is a core feature of DM2, it can be less prominent. DM2 typically presents later in adulthood, with a slower progression.
Unique Symptoms of Congenital and Childhood Onset
Congenital Myotonic Dystrophy (CDM) occurs when DM1 presents at birth, resulting in a distinct and severe symptom profile. Infants with CDM are born with profound generalized muscle weakness and lack of muscle tone (hypotonia). Myotonia, the muscle stiffness typical of adult forms, is absent at this stage.
These newborns frequently face life-threatening complications, most commonly respiratory failure. Feeding difficulties are also a significant challenge due to facial muscle weakness, often requiring a feeding tube. Other physical symptoms may include club foot.
For children who survive the neonatal period, or those with later childhood-onset DM1, cognitive symptoms become a primary concern. These children frequently experience developmental delays, learning disabilities, and intellectual impairment. Cognitive issues often involve problems with attention and executive function.
While muscle weakness and facial involvement are present in childhood-onset DM1, the cognitive and behavioral challenges tend to be the most pronounced initial features. Unlike the adult form, severe myotonia is not common in young children but typically begins to manifest as they approach adolescence.