[Unique Hand Muscle Atrophy: The Split Hand Symptom].

When examining a patient with upper limb atrophy, amyotrophic lateral sclerosis (ALS) should be considered an important disease, because ALS is a devastating disease, and entirely different from cervical spondylotic amyotrophy. The split hand symptom is a very important indicator for diagnosis, because this symptom has been recognized as being specific to ALS. In the split hand symptom, the thenar and first dorsal interosseous (FDI) muscles are preferentially affected. In contrast, the hypothenar muscle (abductor digiti minimi, ADM) is relatively preserved. FDI and ADM muscles are innervated by the same C8/Th1 spinal segments and the same ulnar nerve; however, in this symptom, these muscles are affected differentially, indicating a potentially specific pathological mechanism. In this article, an ALS patient with split hand has been presented. In addition, recent advances in ALS research, the utility of the split hand symptom for diagnosing ALS, and the pathological mechanism of the split hand symptom, have been introduced.