Contents
Introduction
The main feature of muscle disorders is weakness without reflex loss. This helps us to differentiate these disorders from the neurological causes of weakness:
- UMN lesion – weakness with increased reflexes
- LMN lesion – weakness with decreased reflexes
Investigating Muscle Disorders
Distribution pattern of:
- weakness
- Wasting
- Hypertrophy
- Consistency
Serum muscle enzymes – serum creatinine phosphokinase is the most prominent of these. Levels of this will be raised in dystrophies, and inflammatory conditions, but will be normal in myasthenia gravis, LEMS, myotonias, and other innervations problems.
Genetic testing of family members can be useful for some inherited conditions
Electromyography – in weakness, the amplitude is likely to be reduced. There are also characteristic signs for other conditions:
- Myopathy – short duration, polyphasic action potentials. Sometimes there may also be spontaneous fibrillation
- Denervation –fibrillation
- Myotonic discharge –high frequency ‘whine’
- Myasthenia gravis – gradually reducing amplitude – known as decrement
- LEMS –gradually increasing amplitude – known as increment
Muscle biopsy – again, this can reveal specific abnormalities for the different disorders:
- Denervation – atrophy/hypertrophy of particular fibre groups
- Dystrophy and myositis – diffuse abnormal cells – nuclei become central, there may be necrosis, and there is invasion of the tissue by inflammatory cells
MRI – may show changes in some cases of myositis
Examples of Muscle Disorders
- Myasthenia Gravis
- Muscular Dystrophy
- Duchenne’s / Becker’s Muscular Dystrophy
- Myopathy
- Myositis
- Myotonia
