What are the dermatomes of the lower extremities in order?

Dermatomes of the Lower Extremities

The dermatomes of the lower extremities are arranged as serial semicircles, not as bands extending spirally from the low back down to the lower extremity. 1

Dermatome Distribution by Spinal Level

• L1: Inguinal region and upper portion of the buttocks 1
• L2: Anterior and mid-thigh regions 1, 2
• L3: Distal anterior thigh, knee, and medial leg 1, 2
• L4: Medial aspect of the leg, medial malleolus, and medial foot 2
• L5: Lateral aspect of the leg, dorsum of the foot, and web space between first and second toes 2, 3
• S1: Posterior aspect of the buttock, thigh, leg, lateral foot, and plantar surface 2, 3
• S2: Posterior thigh and leg, with an interposed pattern within the S1 dermatome 1
• S3-S5: Perianal and genital regions 1

Clinical Significance of Dermatome Knowledge

• Accurate knowledge of dermatomes is essential for diagnosing radiculopathies, as sensory changes often follow dermatomal patterns 2
• L5 dermatome stimulation sites are typically located at the web space between the first and second toes 4, 3
• S1 dermatome stimulation sites are typically located over the dorsum of the foot at the distal fifth metatarsal 4, 3

Diagnostic Applications

• Dermatomal somatosensory evoked potentials (DSEPs) can be used to assess radiculopathies, though they show less accuracy and sensitivity compared to EMG 4
• The P40 latency in DSEP testing correlates with height and can be predicted using specific formulas:
  • L5 dermatome: P40 latency (ms) = height (m) × 23.7 + 8.6 3
  • S1 dermatome: P40 latency (ms) = height (m) × 24.5 + 8.7 3

Anatomical Considerations

• The arrangement of dermatomes differs from previous descriptions, particularly in the S1-S2 region where S2 is "interposed" within the S1 dermatome 1
• Afferents from reference points located on the ventral median line of the hindlimb project to two separate fields in the spinal cord 5
• Central projection fields of dermatomes are evenly shaped and located within corresponding spinal cord segments 5

Common Pitfalls in Dermatome Assessment

• Variation exists between individuals, making strict adherence to dermatome maps potentially misleading in clinical practice 1, 2
• Overlapping innervation between adjacent dermatomes can mask subtle sensory deficits 2
• Using dermatomal somatosensory evoked potentials alone for diagnosing radiculopathy may lead to false negatives (27%) and false positives (9%) 4