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Introduction
In medical school, we learned that myopathies present with proximal and axial weakness, and often a raised serum creatine kinase. Many muscle diseases do indeed present in just this fashion, and figure 1 highlights some of these. However, after practising neurology for some time, we encounter exceptions to this rule. The distribution of weakness in some myopathies may be distal-a pattern more suggesting neuropathy-and some patients have proximal weakness not caused by a myopathy.
Muscle disease generally presents with weakness but sometimes first manifests with cardiac or respiratory problems, muscle pain (myalgia) or stiffness; myotonic dystrophy type 1 (DM1) shows still more diverse presentations. 1
Inevitably, since muscle diseases cause weakness, one is easily mistaken for another. Often this does not matter greatly, but distinguishing genetic from acquired myopathy is important to avoid potential iatrogenic harm. In my experience, certain signs and symptoms may prompt rather perfunctory conclusions; for example, scapular winging can too easily suggest facioscapulohumeral dystrophy (FSHD), or recurrent rhabdomyolysis can lead only to a metabolic work-up. I will highlight some useful rules of thumb to prevent us falling into such traps.
There is a great wealth of potentially expensive and time-consuming tests, many of which are recherché fields in their own right. I have been on many a 'wild goose chase' with pages of negative results, a red face and a frustrated patient frowning at me. The essential first step is, therefore, still the clinical assessment.
Sometimes muscle disease may be the unexpected culprit, sometimes it is wrongly implicated, and sometimes we need a little nudge to get the diagnosis spot on ( figure 2 ). Remember not to assume that you and the patient mean the same thing when using words like weakness, numbness and fatigue. 2
Chameleon presentations
Distal weakness that is actually myopathic
Our neurological instincts tell us that distal weakness is neurogenic (a neuropathy or motor neurone disease). However, some myopathies cause distal weakness in the forearm and tibial muscles (especially anterior but less commonly posterior); anterior tibial weakness causes the common problem of foot drop, and gastrocnemius weakness causes difficulties with climbing stairs and with the 'push-off' stage of walking. A powerful clinical clue is that the small muscles of the hands and feet may...