Co-occurrence of ipsilateral partial Horner’s syndrome in a patient with monomelic amyotrophy

Monomelic amyotrophy (MMA) is a lower motor neuron predominant disorder affecting an upper limb, which can mimic amyotrophic lateral sclerosis (ALS). It often presents with unilateral, distal upper limb weakness and atrophy, whose trajectory is one of an initial period of progression followed by a prolonged plateau, as opposed to the typically relentless progression as is seen in ALS. This case report describes a novel observation of a patient with MMA with an unexplained ipsilateral partial Horner's syndrome (miosis and ptosis). Horner's syndrome is known to result from sympathetic dysfunction from lesions from the hypothalamus to the cervical/upper thoracic spine and can be seen with brachial plexopathies, but has never been, to our knowledge, described in MMA. This finding is of interest because it may facilitate earlier diagnosis of MMA in isolated upper extremity, lower motor neuron-predominant syndromes, as Horner's syndrome is not known to complicate ALS.