The motor band sign is a radiological sign described in amyotrophic lateral sclerosis (ALS).

It refers to the appearance of the primary motor cortex on axial GRE or SWI MRI sequences in patients with amyotrophic lateral sclerosis and some of its clinical variants ^1,2,6. On these sequences, and in the axial plane, curvilinear bands of low signal may be appreciated within the cortical gray matter of the primary motor cortex (i.e. 'motor bands' of low signal) ^1,2,6. Occasionally, T2/FLAIR sequences may also demonstrate the same low signal ^1-4.

While this signal abnormality is usually seen along the whole primary motor cortex, in the progressive bulbar palsy variant, the signal change may only be seen laterally (i.e. in the faciobulbar region; see homunculus) ⁶. Furthermore, although this pattern of signal change is usually seen bilaterally, it has been reported to also occur unilaterally ².

This sign is produced because of iron accumulation within microglia in the motor cortex ¹. The mechanisms for this have not been fully elucidated, however it has been postulated that perhaps microglia accumulate this iron as they phagocytose degenerating neurones from the primary motor cortex ¹. The neurones most affected are those in the middle layer of the primary motor cortex ⁶.

It is important to note that this radiological sign is not completely specific for amyotrophic lateral sclerosis ^1-4. Indeed, low T2/FLAIR/GRE/SWI signal in the precentral gyrus has been documented to also occur in patients with other neurological disorders (e.g. Parkinson disease, Alzheimer disease, spinocerebellar ataxia type 17) and even in many normal adults as an age-related phenomenon ^1-5. This might be due to the fact that the primary motor cortex has the highest cortical concentration of iron ¹.