Onset in males at 3–5 years Mainly affects axial and proximal muscles (difficulty in walking with waddling gait, Gower’s manoeuvre^a and pseudo-hypertrophy of gastrocnemius) Low IQ in up to 30% Negative dystrophin immunostain and fibre degeneration on muscle biopsy; elevated CK and polyphasic potentials on EMG Survival into mid-20s (death from respiratory complications) Physiotherapy to prevent contractures and respiratory infections Becker muscular dystrophy Onset at 5–15 years Dystrophin present, but abnormal Milder course with calf pseudo-hypertrophy; mental retardation is uncommon Elevated CK Survival into 30–40s Myotonic dystrophy Autosomal dominant disorder Caused by a trinucleotide repeat sequence on chromosome 19 A multi-system disorder with frontal balding, bilateral ptosis, myopathic facies, muscle thinning and wasting, cataracts, testicular atrophy and mild mental retardation Myotonia is observed following voluntary contraction, for example clenching the fist followed by attempted rapid opening Percussion myotonia and tongue myotonia may also be seen ECG abnormalities are found in the majority and patients can suffer sudden cardiac death. Associated problems include diabetes which should be looked for. General anaesthetics should be administered with care a Gower’s manoeuvre refers to the act of use of hands to push off the thighs while rising from the floor.

Approach to Diagnosis

Distinguishing myopathies from peripheral neuropathies, anterior horn cell diseases (e.g. motor neurone disease) and neuromuscular junction disorders (e.g. myasthenic syndromes) requires careful clinical evaluation supplemented by investigations including neurophysiological testing, imaging, muscle biopsy and genetic exploration.

Determining aetiology of a myopathy depends upon a careful history and examination to elicit distinguishing features, including family history, age of onset and the rate of progression (including eliciting if symptoms are persistent or episodic), and the presence of additional features such as muscle aching and pain (myalgia) or urine turning black (myoglobinuria). Any provoking or relieving factors should be noted. From examination, the pattern of muscle involvement (facial, bulbar and, if involving limbs, whether proximal or distal and symmetric or asymmetric) will give further clues as will additional examination findings such as wasting, pseudo-hypertrophy, delayed relaxation after voluntary contraction (myotonia) and involuntary, spontaneous quivering of muscle bundles (myokymia).

Many of the above-mentioned muscle diseases are outside the scope of this book. The important acquired muscle diseases, however, include the inflammatory myopathies which are treatable and are covered in more detail in the following section. Remember neurologists are often asked to assess patients with weakness, where this is found to be secondary to ageing, immobility, critical illness or cancer.

The Treatable Inflammatory Mmyopathies