What is the possible cause of numbness on the plantar aspect of the first 2-3 toes in a patient with flat foot?

Medial Plantar Nerve Entrapment (Tarsal Tunnel Syndrome)

The most likely diagnosis is medial plantar nerve entrapment, a variant of tarsal tunnel syndrome, which commonly causes numbness on the plantar aspect of the first 2-3 toes in patients with flat foot deformity.

Pathophysiology and Clinical Correlation

The medial plantar nerve, a terminal branch of the tibial nerve, provides sensory innervation to the plantar surface of the first three toes and the medial aspect of the fourth toe 1. In patients with pes planus (flat foot), the altered biomechanics create chronic compression of the medial plantar nerve as it passes through the tarsal tunnel beneath the flexor retinaculum 1.

Flat foot deformity increases tension and compression on the medial plantar nerve through several mechanisms:

• The collapsed medial longitudinal arch causes increased valgus positioning of the hindfoot, stretching neurovascular structures 2, 3
• Abnormal pronation during weight-bearing activities creates repetitive traction on the nerve 1
• The altered talocalcaneal angle (typically >30 degrees in severe pes planus versus 24 degrees in normal adults) directly impacts the tarsal tunnel space 3

Differential Considerations

While the presentation strongly suggests medial plantar nerve entrapment, other conditions affecting this anatomic distribution should be considered:

Morton's neuroma typically affects the third webspace (less commonly the second) and causes burning pain extending to the toes, but this occurs on the dorsal aspect between toes, not the plantar surface of individual toes 4, 5. The location described in this case (plantar aspect of first 2-3 toes) is inconsistent with Morton's neuroma.

Diabetic peripheral neuropathy can cause plantar numbness but typically presents with symmetric, stocking-glove distribution affecting all toes bilaterally, not isolated to the medial plantar nerve distribution 1. Assessment for diabetes should still be performed given the high prevalence of concurrent conditions 1.

Peripheral arterial disease may cause foot numbness but is associated with other signs including absent pulses, dependent rubor, and tissue loss 1. The tibial artery occlusive disease can produce foot pain and numbness, but this would not be limited to the first 2-3 toes 1.

Diagnostic Approach

Initial clinical examination should focus on:

• Palpation along the course of the tibial nerve posterior to the medial malleolus, assessing for Tinel's sign (percussion-induced paresthesias radiating into the plantar foot) 1
• Assessment of the medial longitudinal arch with weight-bearing examination to document the degree of pes planus 1
• Monofilament testing to quantify loss of protective sensation in the affected distribution 1, 6
• Evaluation of dorsalis pedis and posterior tibial pulses to exclude vascular insufficiency 1, 6

Imaging studies should proceed as follows:

• Weight-bearing radiographs of the foot (anteroposterior, lateral, and oblique views) to assess the talocalcaneal angle and identify tarsal coalition or other bony abnormalities 1
• Ultrasound may identify nerve compression or swelling within the tarsal tunnel, though this is operator-dependent 1
• MRI is the most sensitive modality for visualizing soft tissue structures, nerve edema, and space-occupying lesions within the tarsal tunnel, but is typically unnecessary unless conservative management fails 1

Management Strategy

Conservative treatment should be initiated first:

• Custom orthotic devices with medial longitudinal arch support to reduce pronation and decompress the tarsal tunnel 1
• Footwear modification with proper arch support and avoidance of shoes that increase pronation 1
• Physical therapy focusing on posterior tibial tendon strengthening to improve arch support 1

If symptoms persist beyond 3 months despite conservative measures:

• Ultrasound-guided corticosteroid injection around (not into) the tibial nerve may provide temporary relief, though evidence is limited for tarsal tunnel syndrome specifically 1
• Referral to a foot and ankle specialist for consideration of tarsal tunnel release surgery if conservative measures fail and symptoms significantly impair function 1

Critical Pitfalls to Avoid

Do not assume the absence of diabetes excludes neuropathy - tarsal tunnel syndrome is a compressive neuropathy that occurs independently of metabolic causes 1. However, screening for diabetes is still warranted given the high prevalence of concurrent conditions 1.

Do not overlook vascular assessment - while the presentation suggests nerve entrapment, absent pedal pulses or an ankle-brachial index <0.9 would indicate peripheral arterial disease requiring different management 1, 6.

Do not delay imaging if red flags are present - rapidly progressive symptoms, severe pain out of proportion to findings, or signs of infection require urgent evaluation with MRI and possible surgical consultation 1, 6, 7.