What is the most likely result of an L5-S1 (Lumbar 5-Sacral 1) disc lesion?

L5-S1 Disc Lesion Neurological Effects

An L5-S1 disc lesion will most likely result in lost sensation in the big toe (option d) due to compression of the L5 nerve root, which innervates the dorsal aspect of the foot including the big toe.

Neuroanatomical Basis of L5-S1 Disc Lesion

• 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 1
• Extraforaminal compression of the L5 nerve at the lumbosacral junction can result from disc herniations, osteophytes, or ligamentous compression 2

Clinical Manifestations of L5-S1 Disc Lesion

• Sensory deficits:

  • Lost sensation in the big toe and dorsal aspect of the foot is a hallmark finding of L5 nerve root compression 1
  • Patients may experience dysesthesias along the L5 dermatome distribution 2

• Motor deficits:

  • L5 nerve root compression typically affects foot dorsiflexion and great toe extension rather than plantar flexion 1
  • Plantar flexion (option a) is primarily controlled by the S1 nerve root, not L5, and would not be the primary deficit in an L5-S1 disc lesion 1
  • Weak dorsiflexion (option b) is associated with L4-L5 disc herniation affecting the L5 nerve root, not specifically L5-S1 disc lesion 1

• Reflex changes:

  • The knee jerk reflex (option c) is primarily mediated by the L3-L4 nerve roots, not L5-S1 1
  • L5-S1 disc lesions typically affect the ankle jerk reflex (S1) rather than the knee jerk reflex 1

Anatomical Considerations

• The L5 nerve root exits below the L5 pedicle and travels through the L5-S1 foramen, making it vulnerable to compression from L5-S1 disc pathology 2
• The extraforaminal space at L5-S1 has unique anatomical features compared with upper lumbar levels, contributing to specific patterns of nerve root compression 2
• Electrophysiological studies have shown that in single nerve root lesions of L5 or S1, pathological findings can extend beyond the anatomical distribution of the affected nerve 3

Diagnostic Considerations

• MRI is the mainstay of imaging for lumbosacral plexus and nerve root pathology, providing superior definition of intraneural anatomy and pathologic lesions 4
• Extraforaminal L5 nerve root compression can be caused by various pathologies including disc herniation, osteophytes, and ligamentous compression 5
• Clinical diagnosis should be confirmed by electrodiagnostic studies to differentiate radiculopathy from plexopathy 4

Clinical Implications

• Early recognition of the specific neurological deficits associated with L5-S1 disc lesions is crucial for appropriate management 2
• Surgical decompression may be necessary in cases of significant neurological deficit or when conservative management fails 2
• Minimally invasive approaches to L5-S1 disc lesions can provide good to excellent clinical results in properly selected cases 2