These are all relatively uncommon diseases. DMD has the highest prevalence of the myopathies.
Clinical features of myopathy
Clinical features of myopathy:
- The hallmark symptom of myopathy (and neuromuscular disease) is weakness
- Weakness predominantly affecting proximal muscle groups (shoulder and limb girdles) is typical
manifests itself in
different ways at
- Decreased foetal movements movements in utero
- Floppy infant neonatally
- Motor delay in the toddler years
- Reduced muscle strength and power in older children and adults.
- Myalgia may occur in inflammatory myopathies
- Muscle-stretch reflexes are preserved
- Somatosensory reflexes are preserved
- Variation of
- Fluctuating muscle power suggests metabolic myopathy (for example McArdle disease)
- Fatigability (or progressive weakness with exertion relieved by rest) is a feature of myasthenia gravis where the defect is in neuromuscular transmission.
- Common symptoms:
- Symmetrical proximal muscle weakness
- Absence of sensory symptoms (paraesthesia)
- Atrophy of muscles (and reduced reflexes) occurs late with myopathies (early with neuropathy)
- Waddling gait of DMD at age 3-6 years is typical.
- How acute are
- Weakness over hours suggests toxic cause or episodic paralysis
- Weakness developing over days- consider dermatomyositis or rhabdomyolysis
- Weakness over weeks suggests polymyositis, steroid myopathy, endocrine myopathy.
- Pain and tenderness without weakness- consider other causes
- Which muscle
groups are affected?
- Proximal muscle groups- difficulty rising from chair, climbing stairs, shaving, hair combing
- Distal muscles- difficulty walking (flaping gait), grasping, handwriting.
- Difficulty with exercise
- Cramps and myalgia with exercise (early with glycogen storage disorders and after prolonged exercise with lipid storage disorders)
- Progressive muscle weakness in some metabolic myopathies.
- Past medical
- Systemic lupus erythematosus
- Rheumatoid arthritis
- Polyarteritis nodosa.
- Endocrine disease
- Renal disease
- Autoimmune disease:
- Family history:
- Muscular dystrophy
- Other relevant conditions or myopathies.
- Lipid lowering drugs
- Pottery industry-glazing salts can cause hypokalaemic paralysis.
- Symmetrical proximal muscle weakness
- Muscle tenderness very rare with myopathy
- Fever with inflammatory causes
- There is usually no wasting but there may be hypertrophy of muscle (atrophy is a late sign)
- Reflexes and sensation usually normal
- Hypotonia is common in some myopathies (for example congenital myopathies)
- There may be helpful additional signs such as the skin changes of dermatomyositis
- Urine should be examined- myoglobinuria in acute alcoholic myopathy can cause renal tubular necrosis.
This list includes other conditions causing weakness:
- Guillain-Barré Syndrome
- Eaton-Lambert Myasthenic Syndrome
- Myasthenia Gravis
- Cerebral Palsy
- Spinal Muscular Atrophy
- Congenital hypomyelinating neuropathies.
- It may be
clinical features of
- Myopathy where distal muscle groups are affected (myotonic dystrophy, myopathy of Welander)
- Peripheral neuropathIies which affect proximal muscles (diabetic amyotrophy, motor neurone disease).
- Reduced muscle- stretch reflexes
- Somatosensory abnormalities.
- Weakness affects distal muscles- although there are exceptions:
- Some complex
cases may have both
which can lead to
- Diabetes mellitus can cause both neuropathy and inflammatory myopathy
- Cancer can cause dermatomyositis and chemotherapy peripheral neuropathy in the same patient
- Radiculopathy (from degenerative disc disease) can occur in patients with myopathy.
Blood and urine tests
These together with ECG examination are most useful in acute situations.
- Creatinine kinase (with isoenzymes)
- Electrolytes including calcium and magnesium
- Serum myoglobin
- Urea and serum creatinine
- Urinalysis and urine microscopy- myoglobinuria inferred by positive urinalysis with few red cells at microscopy
- Full blood count
- Thyroid function tests
- Antinuclear antibodies.
- Changes of hypokalaemia- increased P-R interval, U waves, wide QRS and nonspecific ST-T changes
- Sinus arrhthmias, deep Q waves and elevated R waves precordially (for example in DMD).
Muscle biopsy is important in diagnosis but findings under the microscope are rarely pathognomonic. Interpretation requires close consideration of the clinical history in conjunction with the microscopic featues to make a diagnosis.
- Excludes primarily neurogenic processes (for example spinal muscular atropy)
- Proximal muscles of lower extremeties often exhibit most prominent features
- Often helps to confirm diagnosis but not in itself diagnostic.
- May help exclude neurological disease
- May help in assessing complications (musculoskeletal or involving other organs).
The genetic basis of the primary myopathies means that genetic testing can be essential to the specific diagnosis.
This depends on the diagnosis as well as the severity and extent of disease.
Myopathy can, rarely, present acutely or with acute complications. Examples include:
- Respiratory failure can occur in a number of the myopathies
- Aspiration pneumonia may be associated with this
- Cardiac complications may be associated including cardiomyopathy and conduction defects.
- Some metabolic
- Oral supplements
- Cautious use of IV potassium
- Prophylactic drugs (spironolactone and acetazolamide).
- Carbohydrate loading (for example early in attacks with hyperkalaemic periodic paralysis)
- Glucose and insulin.
- Causes life threatening renal complications and associated metabolic problems (hyperkalaemia)
- Usually requires intensive care management.
- Treatment with corticosteroids
- Be aware of associated temporal arteritis.
Long term care
- Monitor pulmonary function (early restrictive pattern may occur before onset of symptoms)
- Beware of symptoms of nocturnal hypoxia (poor sleep, nightmares, headaches)
- May require tracheostomy and permanent ventilation.
- Specific medication may be useful in particular situations for particular myopathies
- Genetic counselling
- Tendon release surgery for example to prolong ability to walk.
- Physical aids:
- Walking aids
- Adaptive devices.
- Family support
- Dietary advice
- General- for example to prevent obesity
- Respiratory failure
- Aspirartion pneumonia
- Many associated deformities can occur
- Joint contractures
- Chest deformities
- Spinal deformities including scoliosis.
- Malignant hyperthermia can occur with central core disease.
This depends on the specific diagnosis. The primary disorders are incurable conditions with varied prognosis. Secondary myopathy may be corrected by treating the underlying cause.
Genetic counselling is, in some of the most common myopathies such as DMD, the only intervention that can prevent disease. In general:
- Give genetic counseling early
- Test early for carrier status where appropriate
- Consider prenatal diagnostic testing where appropriate
- Advances in molecular genetics may help in the future.