Dermatomes and the Lumbar Plexus

There are various proposed models that mark the distributions for these dermatomes. As such, significant variations exist in the literature. In this chapter, we will briefly describe the general map of each dermatome and their clinical significance. We will discuss further three dermatome maps that have been regarded as the primary sources for the various dermatome maps in circulation today: Foerster’s, Head and Campbell’s, and Keegan and Garrett’s maps. Dermatome maps in general are inconsistent. The segmental pattern typically presented leads to areas of overlap between consecutive spinal nerves. More recently, Lee et al conducted a literature review comparing various dermatome maps and proposed an evidence-based dermatome map that is discussed below. ¹

21.2 Lower Extremity and Genitalia

The skin distribution over the lower extremity and the genitalia are covered by the lumbar and sacral dermatomes (▶ Fig. 21.1, ▶ Fig. 21.2, ▶ Fig. 21.3, ▶ Fig. 21.4). L1 covers the upper part of the lower extremity, hip girdle, and the skin overlaying the back over the L1 vertebrae. L2 covers the anterior and medial aspect of the thigh. L3 represents the anterolateral part of the thigh, down to the medial part of the knee, and the medial aspect of the posterior part of the lower leg. L4 covers the posterolateral thigh, down to the lateral part of the knee and the anterior aspect of the anterior part of the lower leg. L4 also covers the plantar part of the foot and the 2nd, 3rd, and 4th digits. S1 is distributed over the lateral aspect of the thigh down to the posterior part of the lower leg, heel, and lateral aspect of the foot. S2 covers the posterior thigh and the medial aspect of the posterior leg. The skin overlying the perineum is covered by S2 to S5. S2 and S3 cover the skin over the penis and the scrotum. S3 covers parts of the penis and scrotum and the perianal region. S4 and S5 cover the perianal region with S5 covering the skin adjacent to the anus. ²

Fig. 21.1 A typical dermatome pattern illustration L1–L4 distribution via branches of the lumbar plexus.

(Reproduced with permission from Gilroy AM, MacPherson BR, Ross LM, Schuenke M, Schulte E, Schumacher U. Atlas of Anatomy. 2nd ed. New York, NY: Thieme Medical Publishers; 2012. Illustration by Karl Wesker.)

Fig. 21.2 Individual nerve branches of the lumbar plexus and their contributions to regional dermatomes of the upper anterior thigh.

Fig. 21.3 Although the perineum is largely supplied by the pubdental nerve in regard to cutaneous innervation, note the significant aspects of it innervated by branches of the lumbar plexus here illustrating the ilioinguinal nerve and genitofemoral branches.

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