Infantile myofibromatosis: a nontraumatic cause of neonatal brachial plexus palsy.

Most injuries to the neonatal brachial plexus occur acutely at birth, and are iatrogenic in origin. However, when weakness is accompanied by atrophy, nontraumatic etiologies should be considered. The differential diagnosis of chronic congenital brachial plexopathy includes cervical bone malformations, humeral osteomyelitis, varicella, and compression from various types of infantile tumors. An illustrative male infant delivered at 37 weeks of gestation with wasted musculature of the left upper arm, ipsilateral Horner's syndrome, and a hemidiaphragm is presented. On further examination, this patient manifested an underlying cervical tumor compressing the brachial plexus. Diagnostic steps leading to the pathologic identification of a solitary cervical myofibroma included physical examination, electromyography, radiographic imaging, and open biopsy. This report emphasizes the importance of differentiating acute from chronic congenital plexus palsy and of recognizing the possibility that infection or neoplasm may underlie the latter.