A dermatome is defined as an area of skin innervated by a single dorsal nerve root. ¹ Knowledge of dermatomes is derived from the work of Sir Henry Head, Otfrid Foester, ² Jay Keegan, Frederic Garrett, and others. ³ Many different methods were used by these authors and may have contributed to the variable findings and representations of what is understood as “the precise boundaries” of dermatomes. The differences could have also arise because several of these methods were based on physiology rather than anatomy.⁶ The ability of the central nervous system to suppress, facilitate, and reorganize the activities of primary sensory neurons may also account for differences in mapping. ⁴ Additionally, sensory neurons with a ganglion cell at one level through intersegmental anastomoses among posterior spinal rootlets are allowed to enter the spinal cord at another level, ^{4,​ 5,​ 6,​ 7,​ 8} resulting in differences in skin supply by the dorsal roots, dorsal root ganglia, and spinal nerves. ⁴ In this chapter, the history of the discovery of dermatomes is explored.

In 1893, William Thorburn, a surgical registrar in the Manchester Royal Infirmary, after observing patients with spinal cord lesions published detailed maps of the lumbar and sacral dermatomes (▶ Fig. 20.1). ^{4,​ 9,​ 10,​ 11,​ 12} Thorburn speculated that the dermatomes existed because of contributions from “certain serial sections of the nervous system.” He went on to state that “it remains to be proved that these ‘sections’ are spinal segments, nerve roots, or other serial arrangements.” ^{4,​ 12}

In 1892, maps of the dermatomes was also constructed by Professor M. Allen Starr of the College of Physicians and Surgeons in New York after examining patients with cauda equina syndrome (▶ Fig. 20.2). ^{4,​ 13}

From identifying the locations of afflicted skin in herpes zoster patients and from monitoring patients with visceral nonneurological disorders and spinal cord injuries, Sir Henry Head also constructed dermatomal maps (▶ Fig. 20.3). ⁴ His initial work on the association between cutaneous tenderness and visceral disease began in 1893, and he created charts outlining what others refer to as “Head’s zones,” which show the distribution of cutaneous tenderness in many diseases. ¹⁴ Through examining cases of herpes zoster, Head observed that the areas of herpetic eruptions matched the “area of tenderness” he had described. ^{4,​ 14} Consequently, he made the deduction that the “areas of tenderness” corresponded to spinal cord segments. ⁴ By observing a case of sensory loss due to a “fracture of the 1st and 2nd lumbar vertebrae” L1 was determined. ¹⁴ Head found that the upper border of L1 matched the upper border of the sensory loss and that this area coincided with the upper border of the “gluteocrural area,” one of his proposed “areas of tenderness.” He inferred that these areas were the same and that the lower border of L1 must be the lower border of the “gluteocrural area.” After deriving such conclusions at L1 and later S1 to S5, Head noticed that the lateral area of the leg had no designated dermatome. He deduced that this area must be L5 because it was located next to the sacral skin segments. Using a case of presumed spinal cord injury, L4 was determined and a case of herpes zoster rash determined L3. The three dermatomes L3–L5 were proposed by Head to be involved after observing the pattern of the rash. As a result, L3 was assigned to the area outside the already determined areas, L4 and L5. Later, he speculated that L2 must be between L1 and L3. In 1900, Head and Campbell studied 500 cases of shingles and sketched a map showing the distribution of cutaneous lesions. ¹⁵ They also found that there were overlaps between adjacent nerve territories and considered that body shape influences differences in the shape of the affected skin. ¹⁶ The roots involved in most cases of herpes zoster could not be identified; however, they were able to identify them in 16 autopsy cases. Eight segments in those 16 cases (between T1 and L1) were represented. Head had no confirmation for C5 to C8 or areas below L1, and therefore there were uncertainties when it came to mapping the arm and leg. ^{4,​ 15}

Using the “method of remaining sensibility,” Sir Charles Scott Sherrington studied the dermatomes in monkeys and produced dermatomal maps in 1893 and 1898. ⁴ Sherrington found that after many roots above and below a given nerve root had been sectioned the remaining areas of sensation in the skin indicated input from the unsectioned root. ^{4,​ 17,​ 18} Sherrington also found evidence to support Herringham’s ventral axial line. ¹⁹ He found gaps where contiguous dermatomes were missing in the proximal portions of the dorsal and ventral parts of both the upper and the lower limbs. According to Sherrington, “the gap” formed an axial line that coursed downwards from the midline at the level of the sternal angle to the forearm. ^{16,​ 17}

Otfrid Foester, a German neurologist and neurosurgeon, adopted Sherrington’s approach but applied it to humans. He severed multiple nerve roots and electrically stimulated the distal end of the divided root that resulted in vasodilatation in the dermatomal area. ¹ One of the shortcomings of Foester was his lack of awareness that removing a nerve most likely resulted in pain due to input to the central nervous system, probably resulting in death in his patients. ¹ In the words of Dr. Robert Wartenburg, “he helped his patients, but they had to pay the price by being subjected to physiological experimentation.” ²⁰ Foester found from his research that removing a single root did not result in loss of sensibility, so he made the deduction that dermatomes in humans overlap. He also observed that only one rootlet from the entire posterior root was needed for sensibility within a dermatome and identified different dermatomes for different sensory modalities. ⁴ He found that the areas of vasodilatation matched the dermatomes determined by anesthesia. This finding presented similarities in distribution to the map drawn by Head from his studies of shingles. ¹⁶

Following the association made between disc herniation and back and leg pain in 1934, ²¹ Keegan and Garrett contradicted Sherrington and Foester’s hypothesis about nerve roots and cutaneous sensibility. They proposed that decreased sensibility of the skin resulted from disc compression of a single nerve root. ^{22,​ 23,​ 24,​ 25,​ 26,​ 27} They observed patients with different herniations of the cervical and lumbar areas and made maps reflecting the areas of decreased sensibility on the limbs. From the upper limbs studied in 165 cases, 47 cases showed that a single root was affected. Note that 707 cases showed that a single nerve root was affected out of 1,264 cases of the lower limb studied. They also used Novacain injection to test a single lower cervical nerve root in 10 medical students volunteers. ¹⁶ The maps constructed by Keegan and Garrett, like Head’s maps, showed no overlap of dermatomes (▶ Fig. 20.4). ⁴ Keegan and Garrett dismissed Foester’s claim that removing a single nerve root caused no sensory loss. ^{16,​ 22} They also disagreed with Sherrington and concluded that “dermatomic loops” and “dorsal axial lines do not exist.” ²² Last ²⁸ analyzed Keegan and Garrett’s publications and found the following limitations in their findings:

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