Neurological Disorders: Myotonic Dystrophy

Introduction

Myotonic Dystrophy (DM or dystrophia-myotonica) is a type of muscular dystrophy characterized by the presence of myotonia. Muscular dystrophy is the term used for a myopathy (muscle diseases) that is hereditary in nature (inherited or genetically mediated).

It is one of the commonest muscular dystrophies in the adults.

Myotonia is an abnormal muscle condition where the muscle has difficulty in relaxing after it has contracted.

What causes this disease?

It is a genetic disease. It is inherited in an autosomal dominant pattern. More specifically speaking it belongs to trinucleotide repeat disorders where 3 DNA bases repeat abnormally again and again within a gene resulting in gene mutation.

What are the types of MD?

The two main types are; DM1 & DM2. In the DM1 there are two types again; the adult type, and the more severe form congenital myotonic dystrophy. Please note that myotonia congenital (or congenital myotonia) is a related (due to the presence of myotonia) but a different disease.

Clinical Manifestations

The following manifestations may be seen in different combinations;

• Myotonia – difficulty in relaxing the muscles. For example after shaking hand with somebody the patient may not be able to let the grip go
• Muscle stiffness
• Generally more difficulty in initiating a motor activity
• Muscle weakness – predominantly distal muscles
• Muscle thinning (atrophy) – predominantly distal muscles
• Weakness and thinning of facial muscles giving a characteristic facial appearance
• Swallowing and breathing difficulties (with more advanced disease)

DM is a systemic disease as it involves many other organs other than the skeletal muscles so there could be additional clinical manifestations. These organs are usually involved;

• Heart
• Endocrine (diabetes)
• Testicular atrophy
• Eyes (cataract) etc

Patients may also manifest with excessive day time sleepiness, some cognitive abnormalities etc.

Diagnosis

The diagnosis of DM requires first of all a strong clinical suspicion since many symptoms are vague and nonspecific. The following investigations may be employed;

• EMG/NCS (electromyography/nerve conduction study)
• Muscle biopsy
• Genetic (DNA) test etc

Treatment

There is no cure for this progressive neurological disorder. Management accordingly is focused on supportive and symptomatic aspects.

Myotonia may be treated with medications like mexelitine, phenytoin etc

Physical and occupational therapy are other measures that might help.

A neurologist is frequently involved in the care of patients with Myotonic Dystrophy.



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