Fascicular Syndrome
A fascicular lesion of the brachial plexus may present with signs restricted to one muscle or to a few muscles. The clinical manifestations of fascicular brachial plexus lesions may be indistinguishable from those of peripheral nerve branchlesions. The diagnosis of a fascicular brachial plexus lesion is established by the combination of weakness in a muscle innervated by a distal or intermediate nerve of the brachial plexus and (1) clinical or electrophysiological abnormality in a muscle innervated from the same spinal segment but through a proximal brachial plexus nerve; or (2) a Horner syndrome (Figure 222.1). The combination of weakness of a muscle innervated by distal brachial plexus nerves and Horner syndrome imply C8 or T1 ventral root or spinal nerve damage, or both.

A	B

Figure 222.1.— [A] Inability to extend the distal phalange of the right thumb. [B] Right-sided Horner syndrome. This patient had a brachial nerve lesion confirmed by EMG findings of proximal brachial plexus nerve involvement.

Peripheral Nerve Injury
Peripheral nerve lesions always produce segmental flaccid monoparesis. Segmental limb weakness due to peripheral nerve injury may occur with lesions that damage a nerve, a major branch, or just a few fascicles within a nerve or a branch. Injury to the whole nerve or a major branch produces characteristic clinical findings that differentiate these injuries from brachial plexus injury and other peripheral nerve injuries. Involvement of a secondary branch or a fascicle within a nerve or a branch produces a pattern of weakness that can seldom be clinically differentiated from a fascicular brachial plexus lesion.