What are the dermatomes of the lower limb?

Dermatomes of the Lower Limb

The dermatomes of the lower limb follow a specific pattern with L4 located in the medial aspect of the leg, L5 in the lateral aspect of the leg and foot dorsus, and S1 in the posterior aspect of the backside, thigh, leg, and plantar foot skin. 1

Anatomical Distribution of Lower Limb Dermatomes

L1-L3 Dermatomes

• L1: Covers the inguinal region and upper anterior thigh
• L2: Covers the anterior and mid-thigh region
• L3: Covers the anterior and medial thigh, extending to the knee

L4-S1 Dermatomes (Primary Lower Limb Dermatomes)

• L4 Dermatome:

  • Medial aspect of the leg
  • Medial malleolus
  • Medial foot to the great toe

• L5 Dermatome:

  • Lateral aspect of the leg
  • Dorsum of the foot
  • Dorsal surface of toes 1-4
  • Web space between first and second toes

• S1 Dermatome:

  • Posterior aspect of the buttock
  • Posterior thigh
  • Posterior calf
  • Lateral foot and sole
  • Lateral and plantar aspects of the foot
  • Small toe

S2-S4 Dermatomes

• S2: Posterior thigh and calf (overlapping with S1)
• S3-S4: Perianal region and genitalia

Clinical Significance

Understanding dermatome distributions is crucial for:

1. Diagnosing Radiculopathy: Radicular pain and sensory changes follow dermatomal patterns, helping differentiate from peripheral neuropathy 2

   • Radiculopathy: Unilateral symptoms in a specific dermatomal distribution
   • Peripheral neuropathy: Typically bilateral and symmetrical with "stocking-glove" distribution

2. Localizing Spinal Nerve Compression: Accurate knowledge of dermatomes helps identify the specific nerve root affected in conditions like disc herniation 2

3. Surgical Planning: Dermatome mapping aids neurosurgeons in identifying the appropriate level for intervention 1

Important Considerations

• Dermatome Overlap: There is significant overlap between adjacent dermatomes, with most skin areas receiving innervation from at least two spinal levels 3

• Individual Variation: Substantial variation exists between individuals in the exact boundaries of dermatomes 3, 4

• Evidence-Based Mapping: Traditional dermatome maps have shown inconsistencies, with more recent evidence suggesting arrangements that differ from classical descriptions 3

• Animal Model Insights: Research using animal models suggests that lower limb dermatomes are arranged as serial semicircles rather than spiral bands 5, 6

Clinical Assessment of Dermatomes

When assessing dermatomes clinically:

• Test light touch and pain sensation along the key areas of each dermatome
• Compare sides for asymmetry
• Document areas of sensory loss or altered sensation
• Correlate with motor findings and reflex changes:
  • L4: Knee extension, patellar reflex
  • L5: Great toe and foot dorsiflexion, no specific reflex
  • S1: Foot plantar flexion, ankle reflex

Understanding these dermatome patterns is essential for accurate diagnosis and management of conditions affecting the lower limb, particularly in distinguishing between radiculopathy and peripheral neuropathy.