In the picture above, you can see a depression in the first web space (between thumb and index finger). This is suggestive of wasting of first dorsal interosseous (FDI) muscles. This patient had bilateral C8-T1 lesion.
Small muscle wasting can have varied localizations: Lower cervical cord lesion, anterior horn cell disease, C8-T1 root lesion, lower trunk plexopathy and ulnar nerve lesion. Careful clinical examination is required, before investigations such as MRI cervical spine, nerve conduction studies and EMG can be ordered for.
Split hand sign: It is specific for amyotrophic lateral sclerosis (ALS). In ALS, there is wasting of FDI muscles, associated with thenar muscle wasting. However, hypothenar muscles are relatively spared. See the picture below (source: Med J DY Patil Univ)