Anatomy of Pain

(ĐTĐ-Anatomy of Pain) – Knowledge of the peripheral anatomy of the human body is essential in evaluating the complex problems found in a patient with pain. The clinical problem is often reduced to the simple question, “Is the pain in an area supplied by a single nerve root, a single peripheral nerve, or a branch of a peripheral nerve?” A physical examination to evaluate pain, weakness, and their distribution often leads the clinician to better localize the nerves involved in the patient’s report of pain.

FIGURE 49-3. The connections and interchanges of the funiculi in the brachial plexus.

Somatic Innervation

A difficulty encountered in the diagnosis of pain is secondary to the overlap of cutaneous fields of segmental and peripheral nerves, as well as the overlapping innervation of muscle and bone. Single cutaneous nerves innervate sharply defined regions with little overlap, but these fibers regroup in the peripheral nerve and are again redistributed in the brachial or lumbosacral plexus. This makes it impossible to follow individual fibers from the dorsal roots to the areas innervated by the individual cutaneous nerve. Adjacent cutaneous nerves may be supplied by fibers from more than one spinal nerve (Fig. 49-3) (95). Anatomists and physicians have attempted to define areas of the skin, muscle, and bone that are of the exclusive domain of a single spinal cord root, as well as the areas of overlap. The size of these areas varies from nerve to nerve and from individual to individual. As with all knowledge of the human body, anatomy of sensation and pain rests on a foundation of original work performed by investigators too numerous to list. The figures in this chapter have taken some of the best work in the field and compiled it into a comprehensive review (Figs. 49-4 through 49-17). In delving into the literature in this area of anatomy, one cannot help but feel that he or she is standing on the shoulders of giants without whose work the figures in this chapter would be impossible.

FIGURE-4. Dermatome, myotome, and sclerotome distribution for C3. Dermatome: neck. Myotome: paraspinals, trapezius, and diaphragm. Sclerotome: bones–vertebra and periosteum; joints–facet; ligaments–longitudinal, ligamentum flavum, and interspinous.

FIGURE-5. Dermatome, myotome,and sclerotome distribution for C4. Dermatome: shoulder. Myotome:paraspinals, trapezius, diaphragm, scapular abductors. Sclerotome: bones–vertebra, periosteum, and clavical; joints–facet; ligaments–longitudinal, ligamentum flavum, and interspinous.

FIGURE-6. Dermatome, myotome, and sclerotome distribution for C5. Dermatome: lateral shoulder and lateral arm. Myotome: paraspinals, scapular abductors, scapular elevators, shoulder extensors, shoulder rotators, and elbow flexors. Sclerotome: bones–vertebra and periosteum, scapula, and humerus; joints–facet; ligaments–rotator cuff, longitudinal, ligamentum flavum, and interspinous.

FIGURE-7. Dermatome, myotome, and sclerotome distribution for C6. Dermatome: lateral arm, lateral forearm, and lateral hand. Myotome: paraspinals, shoulder adductors, elbow flexors, forearm pronators, forearm supinators, and wrist flexors. Sclerotome: bones–scapula, humerus, radius, and lateral fingers; joints–facet, shoulder, and elbow; ligaments–longitudinal, ligamentum flavum, and interspinous.

FIGURE-8. Dermatome, myotome, and sclerotome distribution for C7. Dermatome: midhand and middle finger. Myotome: paraspinals, elbow extensors, forearm pronators, and wrist extensors. Sclerotome: bones– scapula, humerus, radius, ulna, and middle fingers; joints–facet and wrist; ligaments–longitudinal, ligamentum flavum, and interspinous.

FIGURE-9. Dermatome, myotome, and sclerotome distribution for C8. Dermatome: medial forearm and medial hand. Myotome: paraspinals, elbow extensors, wrist flexors, grip, finger abduction, finger flexion, finger adduction, finger opposition, and finger extension. Sclerotome: bones– vertebra and periosteum, ulna, and medial fingers; joints–facet and wrist; ligaments–longitudinal, ligamentum flavum, and interspinous.

FIGURE-10. Dermatome, myotome, and sclerotome distribution for T1. Dermatome: medial arm and medial forearm. Myotome: paraspinals, finger adduction, finger flexion, finger abduction, finger opposition, and finger extension. Sclerotome: bones–vertebra and periosteum, ulna, and medial fingers; joints–facet; ligaments–longitudinal, ligamentum flavum, and interspinous.

FIGURE-11. Dermatome, myotome, and sclerotome distribution for L1. Dermatome: groin and flank. Myotome: paraspinals, hip flexors, spine extensors, and spine rotators. Sclerotome: bones–vertebra and periosteum; joints–facet; ligaments– longitudinal, ligamentum flavum, interspinous. Serosal surface: abdominal wall. Viscera: large intestine, kidney, ureter, suprarenal, prostate, and uterus.

FIGURE-12. Dermatome, myotome, and sclerotome distribution for L2. Dermatome: thigh and upper buttock. Myotome: paraspinals, hip flexors, and hip adductors. Sclerotome: bones–vertebra and periosteum, iliac crest, and medial femur; joints–facet; ligaments–longitudinal, ligamentum flavum, and interspinous. Serosal surface: posterior abdominal wall, descending large intestine, ureter, bladder, and abdominal aorta.

FIGURE-13. Dermatome, myotome, and sclerotome distribution for L3. Dermatome: upper buttock, medial thigh, knee, and medial calf. Myotome: paraspinals, hip flexors, hip adductors, and knee extensors. Sclerotome: bones–iliac crest, ischium, femur, patella, and proximal tibia; joints–facet, hip, and knee; ligaments–longitudinal, ligamentum flavum, and interspinous. Serosal surface: posterior abdominal wall. Viscera: abdominal aorta.

FIGURE-14. Dermatome, myotome, and sclerotome distribution for L4. Dermatome: knee, anterior lower leg, and medial foot. Myotome: hip adductors, hip extensors, knee extensors, ankle dorsiflexors, and ankle invertors. Sclerotome: bones– vertebra and periosteum, iliac wing, femur, tibia, and medial foot; joints–facet, hip, and knee; ligaments–longitudinal, ligamentum flavum, and interspinous. Serosal surface: posterior abdominal wall. Viscera: abdominal aorta.

FIGURE-15. Dermatome, myotome, and sclerotome distribution for L5. Dermatome: lateral lower leg, medial foot. Myotome: paraspinals, hip extension, knee flexion, ankle eversion, ankle inversion, and big toe extension. Sclerotome: bones– vertebra and periosteum, iliac wing, femur, tibia, proximal fibula, and medial foot; joints–facet, sacroiliac, hip, knee, ankle, and large toe; ligaments–longitudinal, ligamentum flavum, and interspinous.

FIGURE-16. Dermatome, myotome, and sclerotome distribution for S1. Dermatome: posterior lower leg, lateral lower leg, and lateral foot. Myotome: hip extensors, hip abductors, knee flexors, ankle evertors, ankle plantar flexors, and toe dorsiflexors. Sclerotome: bones–vertebra and periosteum, sacrum, ischium, femur, tibia, and mid-foot; joints– sacroiliac, hip, knee, ankle, and large toe; ligaments–longitudinal, ligamentum flavum, and interspinous.

FIGURE-17. Dermatome, myotome, and sclerotome distribution for S2. Dermatome: posterior upper leg and posterior lower leg. Myotome: knee flexors, ankle plantar flexors, toe dorsiflexors, toe abduction, and toe adduction. Sclerotome: bones– sacrum, coccyx, distal fibula and lateral foot; joints–sacroiliac, ankle, and toes; ligaments–longitudinal, ligamentum flavum, and interspinous.

Dermatomes

Different techniques were used in determining the site and extent of innervation. Foerster’s (96) data were based on remaining sensibility of the skin innervated by an intact posterior root isolated by severing several nerve roots above and below. In determining the field of unaltered sensation, Foerster demonstrated considerable overlap of contiguous nerve roots. Similar techniques were used by Bing (97), the Armed Forces Institute of Pathology, and Haymaker and Woodhall (98). Keagan and Garrett (99) used hyposensitivity to pin scratch in cases of herniated intervertebral discs, which resulted in the most extensive dermatome map. Keagan and Garrett contended that no sensory overlap exists between dermatomes, which is contrary to most investigators’ experiences. Richter and Woodruff used the electrical skin resistance method (100,101) over sympathectomized areas of the skin. It is noted that dermatomes of almost all cutaneous nerves are beyond the anatomic boundaries noted on gross dissection. Clinical data suggest that areas of sensory deficit extend further proximally than mapped by Foerster, but not as far proximally as suggested by Keagan and Garrett. For this reason, the dermatomes used in the figures are derived from an interpolation of areas determined by personal experience and multiple other authors (96–104).

Myotomes

The determination of muscular innervation is the result of the analysis by many observers of traumatic and surgical outcomes evaluated during the postoperative and recovery periods (98,104,105). The results of these analyses are provided in detail in Tables 49-1 and 49-2. The myotomes used in these drawings are derived from the experience of multiple clinicians.

TABLE 1 Upper Extremity Muscle Innervation

Spinal segments in parentheses signify those that are occasionally present; those in boldface are the major segments supplying the muscle designated. Segments not in boldface are usually present but to a minor degree. a The small branch from the radial nerve also innervates the brachialis muscle.

Sclerotomes

Areas of segmental innervation of bone (sclerotomes) are closely linked with muscle innervation. Injury to bone, ligament, tendon, fascia, and other mesodermal structures of the body may result in pain referred in a sclerotomal distribution (98,106). Notable references for peripheral nerve innervation of the skeleton include Haymaker and Woodhall (98), Dejerine (107), Foerster (108), and Brash (109). The peripheral nervous system of the skeleton is closely linked to muscle innervation. Most of the bones of the skeleton receive their innervation from nerve twigs of the attached muscles. Some parts of the skeleton, especially the joints, receive branches directly from nerve trunks (99).

TABLE 2 Lower Extremity Muscle Innervation

Spinal segments in parentheses signify those that are occasionally present; those in boldface are the major segments supplying the muscle designated. Segments not in boldface are usually present but to a minor degree.

Autonomic Innervation

The peripheral component of the autonomic nervous system is concerned with innervation of visceral glands, blood vessels, and nonstriated muscle. The relationship to pain has been confined to the visceral components in this section (104,105,110–112).

REFERENCES

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