Review Question - QID 219596

At the L4-L5 level, the L5 neural elements are located lateral and ventral relative to the other neural elements. Injury to the L5 neural elements would be expected to manifest with weakness to extensor hallicus longus (EHL) (Answer 1).

Much research has been done into the anatomy of the lumbar and sacral nerve roots within the spinal cord itself, and this anatomy can be reliably observed with CT myelography. Neuroanatomy is helpful not only in patient evaluation but also intraoperatively, especially in the case of intradural lesions. Overall, the more cranial a nerve root, the more ventral its location within the thecal sac. Furthermore, motor bundles are typically located more ventral than the associated sensory bundles.

Arslan et al. performed a cadaveric investigation to describe the anatomy of the lumbar and sacral nerve roots within the thecal sac. Their study included 10 formalin-fixed male cadavers. Overall, the nerve root diameter increases from L1 to S1, and then decreases from S1 to S5. Furthermore, the average diameter of the lateral recess decreased from L1 to L4, and then increased at the L5 level. The authors conclude that intimate knowledge of intrathecal neuroanatomy is valuable in understanding the compressive effects that mass-occupying lesions or anatomy may have with respect to intradural spinal procedures.

Cohen et al. performed a radiographic study to describe the anatomy of the cauda equina on CT scan and MRI. Their study included 10 human cadaveric specimens that underwent advanced imaging and subsequent dissection. Overall, the authors found a strong correlation between advanced imaging and gross anatomic findings, and they provide a detailed level-by-level outline of the neural element orientation at a variety of lumbar and spinal disc levels.

Figure A is a contrast-enhanced CT scan at the L4-L5 disc level. The L5 nerve roots are located most ventral, the S2-S5 nerve roots most dorsal and the S1 elements are located in the mid portion of the thecal sac. Sensory elements within each grouping are located more dorsally than their motor counterparts.

Incorrect Answer
Answer 2: Iliopsoas weakness would be expected with injury to the L2 nerve root, which has already exited the thecal sac.
Answer 3: Peroneal weakness would be expected with injury to the S1 nerve root, which is located dorsal to the indicated neural bundle.
Answer 4: Tibialis Anterior weakness would be expected with injury to the L4 nerve root, which has already exited the thecal sac.
Answer 5: Quadriceps weakness would be expected with injury to the L3 nerve root, which has already exited the thecal sac.