Myotonic Dystrophy Type 2 (DM2), also known as Proximal Myotonic Myopathy (PROMM), is an inherited muscular disorder primarily affecting skeletal muscles. DM2 typically manifests in adulthood, leading to progressive muscle weakness and wasting over time. It is a multisystemic disorder, meaning it can affect various bodily functions, including the heart and eyes.
Genetic Origins of Muscular Dystrophy Type 2
Muscular Dystrophy Type 2 is caused by a specific genetic mutation involving a tetranucleotide repeat expansion. This expansion occurs within the CNBP gene, also known as zinc finger protein 9 (ZNF9), located on chromosome 3q21. In individuals without DM2, a specific CCTG sequence repeats fewer than 75 times. However, in those with DM2, this CCTG sequence is abnormally expanded, often repeating 75 to over 11,000 times. This excessive repetition disrupts how the cells function, ultimately leading to the symptoms observed in the condition.
DM2 is inherited in an autosomal dominant pattern. This means that a person only needs to inherit one copy of the altered gene from an affected parent to develop the disorder. Each child of an affected parent has an approximate 50% chance of inheriting the expanded gene and developing DM2.
Key Symptoms and Manifestations
Symptoms of Muscular Dystrophy Type 2 typically begin in adulthood. A distinguishing feature of DM2 is proximal muscle weakness, affecting muscles closer to the body’s center, such as those in the thighs, hips, and upper arms. This weakness can make everyday activities challenging, including climbing stairs, rising from a seated position, or lifting objects. Weakness in the hips and thighs can also impact walking and balance.
Myotonia, difficulty relaxing muscles after they have contracted, is another common symptom. This can manifest as a prolonged grip after shaking hands or stiffness in the jaw. Myotonia often improves with repeated muscle activation (“warm-up”). Many individuals with DM2 also experience significant muscle pain, which can present as aching or cramping in the neck, back, shoulders, hips, and upper legs.
Beyond muscular symptoms, DM2 can affect other body systems. Early-onset cataracts are frequently observed. Cardiac conduction abnormalities, affecting the heart’s electrical system, can lead to irregular heartbeats. Insulin resistance can also occur.
The Diagnostic Process
Diagnosing Muscular Dystrophy Type 2 begins with a detailed clinical evaluation and a thorough review of the patient’s medical history. A healthcare provider will inquire about general health, past illnesses, and specific symptoms, especially muscle weakness, stiffness, and pain patterns. This initial assessment helps in understanding the type of muscular dystrophy suspected.
Following the clinical evaluation, electromyography (EMG) is often performed. During an EMG, a small needle electrode is inserted into a muscle to measure its electrical activity both at rest and during gentle contraction. This test can detect myotonia, characterized by prolonged electrical discharges after muscle activation, and other myopathic changes indicative of muscle disease. EMG helps distinguish muscle disorders from nerve-related conditions.
The definitive diagnosis of DM2 is confirmed through genetic testing, typically using a blood sample. This test specifically looks for the abnormal CCTG repeat expansion in the CNBP gene. The test can confirm the presence of the expansion, leading to a molecularly confirmed diagnosis.
Approaches to Management and Care
Currently, there is no cure for Muscular Dystrophy Type 2, so management focuses on alleviating symptoms and improving quality of life. A multidisciplinary team approach is generally employed, involving various specialists to address the wide range of symptoms. Physical therapy plays a significant role in maintaining muscle strength, flexibility, and overall function. Therapists guide patients through exercises to help prevent muscle atrophy and maintain joint mobility.
Occupational therapy assists individuals in adapting to daily living activities, providing strategies and adaptive equipment to enhance independence as weakness progresses. Pain management is also a considerable aspect of care. Medications may be prescribed, and physical therapy techniques can help reduce discomfort.
Assistive devices such as canes, walkers, and orthoses can provide support, improve balance, and aid mobility. Regular monitoring for associated conditions is also an ongoing part of care. This includes periodic heart check-ups with a cardiologist and regular screenings for cataracts by ophthalmologists.