Foot drop

Figure 1: Course of the common, deep (blue) and superficial peroneal nerve, sites of stimulation for motor nerve conduction studies are shown by green discs. Figure 1 Textbooks of neurology are just like cookery or gardening books. It is all so easy on paper. With a good working knowledge of neurological anatomy, it seems, almost any problem can be precisely localised. A couple of of hours yomping through a busy out patients leads to a rapid re-evaluation of that view. Foot drop, however, is a common neurological problem that is particularly amenable to an anatomical approach, so this month I will briefly outline the anatomy of peroneal nerve and discuss the clinical and neurophysiological approach to foot drop. The common peroneal nerve is a branch of the sciatic nerve. The sciatic nerve is formed in the pelvis by fibres from the lumbosacral trunk (L4,5) and by fibres from S1,2,3. The nerve immediately leaves the pelvis through the greater sciatic notch, below the piriformis muscle. The nerve may divide immediately, or may pass either above or through the piriformis. In the gluteal region the nerve lies deep to gluteus maximus, between the greater trochanter and the ischial tuberosity. The nerve then passes down the back of the thigh to the apex of the popliteal fossa. In the thigh the nerve divides into lateral common peroneal and medial tibial divisions. The common peroneal division supplies fibres to the short head of biceps femoris. The common peroneal nerve leaves the popliteal fossa between the tendon of biceps femoris and the lateral head of gastrocnemius. It crosses behind the head of the fibula and passes laterally around the neck of the fibula, where it is particularly vulnerable to compression or blunt trauma. The nerve gives off the sural communicating branch to the sural nerve, and the lateral cutaneous nerve of the calf. The nerve pierces the peroneus longus muscle to divide into deep and superficial branches. The deep peroneal nerve supplies the muscles of the anterior compartment (table 1). The superficial peroneal nerve supplies the muscles in the lateral compartment (table 1) and the skin over the anterior lower leg and dorsum of the foot.