What is inclusion body myopathy, and how does it differ from ALS?

Inclusion body myositis ( IBM) is a rare muscle disorder that—similar to ALS—leads to muscle weakness (paresis) and muscle wasting (myopathy). IBM is also a slowly progressive disease that typically affects middle-aged individuals and is associated with progressive motor impairment. In some cases, it can be difficult to distinguish between IBM and progressive muscular atrophy (PMA). The physical findings for IBM and PMA may show similarities. Even in an EMG examination, IBM (although it is a muscle disorder) can “mimic” a neurological disorder.

An important distinguishing feature is the location and severity of the paralytic symptoms: While PMA primarily affects the extensor muscles of the hands and flexor function remains intact for a long time, the symptoms of IBM are the opposite: weakness of the flexor muscles of the hands predominates, while the function of the extensors is relatively well preserved. If it is not possible to differentiate between IBM and PMA based on the results of a neurological examination or an EMG, a muscle biopsy may be considered, as it usually allows for a distinction between the two conditions.