Somatic Referred Pain
Somatic referred pain is a specific type of referred pain where the pain originates from deep somatic structures (such as muscles, fascia, ligaments, joint capsules, or bones) but is perceived in other somatic tissues, often at a distance from the primary site of pathology. In other words, it is pain perceived in a region innervated by nerves coming from somatic structures other than those that innervate the source of pain. The mechanism is through convergence. These areas of referral typically share the same segmental sensory innervation as the originating structure.
Somatic referred pain has a characteristic quality. It is deep, achy, expanding pressure, and is felt in a broad area. The location remains consistent. Patients can clearly identify the centre of pain, but find it hard to define the boundaries.
In contrast, radicular pain is shooting or lancinating, and extends along a narrow band. Neuropathic pain is burning and has sensory abnormalities. Somatic referred pain has no neurological deficit.
Inman and Saunders (1944) were the first to describe sclerotomes. They envisaged sclerotomes representing sensory innervation of skeletal tissues. They were distinct from myotomes and dermatomes. Unfortunately no anatomical basis has been found to corroborate the sclerotome. Dermatomes were originally mapped from patients with herpes zoster, and then confirmed through dorsal rhizotomy. Myotomes were mapped from patients with spinal cord injuries, and then confirmed through EMG studies. We don't know if "sclerotome patterns" are a result of central connections or peripheral segmental innervation.
The interspinous ligaments were the first to be tested by injecting hypertonic saline. Then from the 1970s other structures were investigated. The cervical facet joints are the only pain patterns that have been shown to be fairly consistent between individuals. The source of pain from other regions cannot be easily inferred from pain maps due to high variability.
Clinical examples of somatic referred pain are common in musculoskeletal medicine:
- Pain from lumbar facet joints or sacroiliac joints frequently refers to the buttock, groin, or posterior thigh. It can even occasionally refer below the knee.
- Trigger points in muscles (as seen in myofascial pain syndrome) are well-known sources of somatic referred pain, with each muscle having characteristic referral patterns. For example, trigger points in the infraspinatus muscle can refer pain to the anterior shoulder and down the arm.
- Pain originating from cervical spine structures (e.g., facet joints, intervertebral discs) can refer to the head (cervicogenic headache), shoulder blade area, or upper arm.
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