Which vertebrae and nerves are associated with pain in the left ankle and top of the foot?

Neuroanatomical Origin of Left Ankle and Top of Foot Pain

Pain in the left ankle and top of the foot is primarily caused by L5 nerve root compression, typically from an L4-L5 or L5-S1 disc lesion affecting the L5 nerve root as it exits the neural foramen. 1

Specific Vertebral Levels and Nerve Roots

L5 Nerve Root (Primary Source)

• The L5 nerve root innervates the dorsal aspect of the foot, including the big toe, making it the primary source of top-of-foot pain 1
• L5-S1 disc lesions typically compress the L5 nerve root as it exits, causing characteristic sensory loss in the big toe and dorsal foot 1
• L5 nerve root compression affects foot dorsiflexion and great toe extension rather than plantar flexion 1
• Most sensory innervation to the foot derives from L5 and S1 dermatomes, with significant overlap between these structures 2

S1 Nerve Root (Secondary Consideration)

• The S1 nerve root produces lateral foot and plantar flexion deficits, not primarily top-of-foot pain 3
• S1 involvement would manifest more with lateral foot symptoms and ankle jerk reflex changes rather than dorsal foot pain 1

Disc Levels Responsible

L5-S1 Disc Lesion

• L5-S1 disc lesions are the most common cause, as more than 90% of symptomatic lumbar disc herniations occur at L4/L5 and L5/S1 levels 4
• This level affects the L5 nerve root specifically, producing the characteristic dorsal foot sensory loss 1
• The ankle jerk reflex (not knee jerk) is typically affected at this level 1

L4-L5 Disc Lesion

• L4-L5 disc herniations can also compress the L5 nerve root, producing identical dorsal foot symptoms 4
• This represents the other major site where L5 nerve root compression occurs 4

Diagnostic Approach

Clinical Examination Findings

• Lost sensation in the big toe and dorsal aspect of the foot is the hallmark finding of L5 nerve root compression 1
• Weakness of foot dorsiflexion and great toe extension confirms L5 involvement 1
• The straight-leg-raise test has high sensitivity (91%) for diagnosing herniated discs affecting these nerve roots 4

Imaging Recommendations

• MRI is the mainstay imaging modality for lumbosacral nerve root pathology, providing superior definition of intraneural anatomy 1
• The American College of Radiology recommends MRI of the lumbosacral plexus as the preferred imaging modality with a rating of 9/9 3, 4
• Peripheral nerve-related symptoms can be evaluated with ultrasound or MRI, though ultrasound has higher resolution for focal nerve evaluation 5

Electrodiagnostic Studies

• Clinical diagnosis should be confirmed by electrodiagnostic studies to differentiate radiculopathy from plexopathy 1
• These studies help distinguish between nerve root compression and peripheral nerve lesions 3

Common Pitfalls to Avoid

• Do not confuse L5-S1 disc lesion symptoms (affecting foot dorsiflexion and great toe extension) with S1 nerve root symptoms (affecting primarily plantar flexion) 4
• The knee jerk reflex is mediated by L3-L4 nerve roots, not L5-S1, so its presence does not rule out L5 pathology 1
• Neurologic heel pain can occur from more proximal nerve impingement syndromes, requiring referral for problems originating in the lumbar area 5
• When peripheral nerve symptoms are suspected, subspecialist referral for evaluation including EMG and nerve conduction studies should be considered 5