Variations in the Pattern of Muscle Innervation by the L5 and S1 Nerve Roots

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Abstract
Evidence based on electrical studies is presented for the standard and anomalous patterns of innervation of msucles supplied by the 5th lumbar and 1st sacral nerve roots. Although considerable controversy still exists, previous work in this field suggests that the L5 nerve root supplies tibialis anterior, extensor hallucis longus, extensor digitorum brevis and the lateral head of gastrocnemius, while the S1 nerve root innervates the medial head of gastrocnemius, soleus and abductor hallucis. To confirm the reliability of these data, the L5 and S1 nerve roots of 50 patients were electrically stimulated during surgery, and distally evoked responses in the relevant muscles were recorded, using surface electrodes. The essential reliability of the proposed table of segmental innervation and that most muscles have a dual innervation, with 1 nerve root being dominant, was confirmed. Eight patients (16%) exhibited a marked departure from the normal pattern. On occasion the extensor digitorum brevis and the lateral head of gastrocnemius can be supplied by S1 and the soleus and medial head of gastrocnemius can be supplied by L5. In a prospective study of 100 patients presenting with clinical evidence of lumbosacral nerve root entrapment, the level of nerve root involvement, as predicted by electromyography, was compared with the operative findings. Correct preoperative nerve root localization was achieved in 84%. At least half the failures in prediction are thought to be due to anomalies of innervation. A further study of 12 patients with disorders of bony segmentation revealed anomalous muscle innervation in 7, so that diagnostic errors may frequently be expected in this group. In clinical practice, whenever anomalous bony segmentation is encountered, the likelihood of variable nerve root innervation should be appreciated, and both nerve roots in question should be explored at operation.

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