Neurological Effects of L5-S1 Disc Lesion
An L5-S1 disc lesion will most likely result in lost sensation in the big toe (option d). 1
Neuroanatomical Basis of L5-S1 Disc Lesions
- L5-S1 disc lesions typically affect the L5 nerve root as it exits the neural foramen, causing compression that leads to specific neurological deficits 1
- The L5 nerve root innervates the dorsal aspect of the foot, including the big toe, making sensory loss in this area a characteristic finding of L5 nerve root compression 1
- The extraforaminal space at L5-S1 has unique anatomical features compared with upper lumbar levels, which can contribute to nerve compression in this region 2
Clinical Manifestations of L5-S1 Disc Lesion
- Lost sensation in the big toe and dorsal aspect of the foot is a hallmark finding of L5 nerve root compression 1
- L5 nerve root compression typically affects foot dorsiflexion and great toe extension rather than plantar flexion 1
- Plantar flexion is primarily controlled by the S1 nerve root, not L5, and would not be the primary deficit in an L5-S1 disc lesion 1
- The knee jerk reflex is primarily mediated by the L3-L4 nerve roots, not L5-S1, making option (c) incorrect 1, 3
- Depression or absence of the patellar tendon reflex is common with L3-L4 disc herniation, not L5-S1 3
Differential Diagnosis and Common Pitfalls
- It's important not to confuse L5-S1 disc lesions with L3-L4 disc herniation, which typically presents with knee reflex abnormalities and anterior/medial thigh pain 3
- Multiple factors can contribute to L5 nerve root compression at the L5-S1 level, including disc herniations, osteophytes, ligamentous compression, and disc space collapse 2
- The widespread distribution of L5 and S1 nerve roots must be considered during clinical evaluation, as electrophysiological studies have shown that effects can extend beyond the expected anatomical distribution 4
Diagnostic Considerations
- MRI is the mainstay of imaging for lumbosacral plexus and nerve root pathology, providing superior definition of intraneural anatomy and pathologic lesions 1
- Clinical diagnosis should be confirmed by electrodiagnostic studies to differentiate radiculopathy from plexopathy 1
- Selective nerve root blocks can be helpful in confirming the specific nerve root involved in cases where imaging findings are ambiguous 5