What is the most likely symptom of a disc lesion between L4 (lumbar 4) and L5 (lumbar 5)?

Symptoms of L4-L5 Disc Lesion

A disc lesion between L4 and L5 will most likely lead to weakness of foot dorsiflexion (option c). 1, 2

Neurological Manifestations of L4-L5 Disc Lesions

L4-L5 disc lesions typically affect the L5 nerve root as it exits the spinal canal, resulting in a characteristic pattern of neurological deficits:

• Motor deficits:

  • Weakness of foot dorsiflexion (inability to lift the foot upward) 2
  • Weakness of great toe extension 1
  • Potential weakness of ankle inversion 2

• Sensory deficits:

  • Numbness and decreased sensation along the anterolateral calf 2
  • Sensory changes on the dorsum of the foot 2
  • The small toe (5th toe) is typically affected by S1 nerve root compression, not L5 2

• Reflex changes:

  • Knee jerk (patellar reflex) is primarily mediated by L4 nerve root 2
  • Ankle jerk (Achilles reflex) is primarily mediated by S1 nerve root 2
  • L4-L5 disc lesions typically do not significantly affect either of these reflexes 2

Anatomical Basis

The L4-L5 disc is one of the most common sites for disc herniation in the lumbar spine 3. When a disc herniates at this level, it typically affects the L5 nerve root which exits below the L5 pedicle. The L5 nerve root innervates the muscles responsible for foot dorsiflexion, particularly the anterior tibialis muscle 2.

The American College of Radiology notes that L4-L5 disc herniations can cause significant neurological symptoms that may require intervention if conservative management fails 4. According to Praxis Medical Insights, the clinical presentation of disc herniation at this level includes weakness in the distribution of the affected nerve root, which in this case would be the L5 nerve root causing foot dorsiflexion weakness 1.

Clinical Significance

When examining a patient with suspected L4-L5 disc lesion, testing foot dorsiflexion strength is a critical component of the neurological examination. The patient should be asked to dorsiflex the foot against resistance, and any weakness compared to the contralateral side suggests L5 nerve root involvement 1, 2.

It's important to note that severe L4-L5 disc herniations can sometimes progress to cauda equina syndrome, which represents a surgical emergency. The most common cause of cauda equina syndrome is lumbar disc herniation at the L4-L5 and L5-S1 levels 4.

Common Pitfalls

• Confusing L5 and S1 radiculopathies: S1 radiculopathy typically affects ankle plantar flexion and the ankle jerk reflex, while L5 radiculopathy affects foot dorsiflexion 2
• Failing to test specific muscle groups: Testing isolated muscle groups like the extensor hallucis longus (great toe extension) can help differentiate L5 from other nerve root lesions 1
• Overlooking intradural disc herniations: In rare cases, disc material can herniate through the dura, causing more severe neurological deficits 5

In conclusion, weakness of foot dorsiflexion is the most characteristic finding of an L4-L5 disc lesion, making option (c) the correct answer.