The fourth dorsal interosseus pedis muscle: a useful muscle in routine electromyography

The aim of the study was to assess the utility of needle examination of the fourth dorsal interosseus pedis (FDIP) muscle in routine EMG. FDIP is an easily accessible muscle and may be less prone to blunt trauma as compared with the other foot muscles but despite its potential utility in electrophysiology, little if any data are available on the FDIP muscle. Detailed neurologic examination, nerve conduction, and needle EMG studies were performed in 118 individuals referred to the EMG laboratory of a tertiary referral center. The FDIP was studied by inserting a needle obliquely at an angle of about 30 degrees immediately proximal to the fourth and fifth metatarsal heads. In 44 subjects (patients/volunteers) with normal studies, <5% had increased insertional activity (IA) in the FDIP, but none had sustained spontaneous activity (SA). Among 32 patients with mixed sensorimotor polyneuropathy, FDIP showed abnormal SA in >90%. In patients with S1 root compromise, 66% had sustained SA in FDIP but none with involvement of L3, L4, or L5 nerve root alone. Two of the four patients with tibial and one of five with peroneal neuropathies had increased SA in this muscle; none with other lower limb focal neuropathies. In patients with lumbosacral plexopathy, FDIP showed denervation potentials with diffuse plexus involvement but not with upper plexopathy. Almost all patients tolerated the FDIP needle examination well. End plates were frequently encountered in the FDIP muscle. We concluded that in normal feet, the FDIP muscle has a low incidence of sustained SA. Abnormal SA in FDIP correlates well with the overall neurologic condition, and it may be a useful muscle to include in routine electrodiagnostic evaluation.