Deep Somatic Pain
Deep somatic pain (otherwise known as musculoskeletal pain) is defined as pain originating from structures derived from the embryonic somites, excluding the skin โ namely, bone, periosteum, ligaments, tendons, joint capsules, and deep fascia. Noxious stimulation of any of these tissues can potentially generate pain. When this pain is perceived locally at the site of stimulation, it is termed local deep somatic pain (or, in the context of the spine, often referred to as nociceptive back pain).
Sclerotomal pain, as conceptualized clinically, represents the referred component of this deep somatic pain. It arises when the nociceptive input from the deep somatic structure is sufficiently intense or prolonged to activate the central mechanisms of convergence-projection and potentially central sensitization, leading to the perception of pain in segmentally related areas distant from the actual source
Deep somatic pain sits in contrast to the less common cutaneous and neuropathic pain. The neurophysiology of deep somatic pain is likely to be different to cutaneous and neuropathic pain.
Frameworks like that proposed by Bogduk explicitly categorize pain originating from spinal structures into local nociceptive pain and somatic referred pain, distinguishing both from radicular pain (nerve root origin) and radiculopathy (nerve conduction block).
Somatic referred pain, a type of referred pain that largely corresponds to the patterns historically described as sclerotomal, stands in distinction to visceral referred pain, and indicates that the somatic tissues are the source of pain referral. The pain can be referred to remote sites. The pattern is different to that found in dermatome maps, and has been termed the sclerotome.
The sensitivity of deep structures in decreasing order is periosteum, ligament, joint capsule, tendon, fascia, muscle. Pain from muscles tend to refer to the joint on which it acts. Spinal structures refer in a somewhat segmental fashion in the thoracic levels, and into the respective limbs in the cervical and lumbar levels. There are large degrees of overlap.
The neurophysiological underpinnings of local deep somatic pain and its referred (sclerotomal) component are largely shared. Both rely on the activation of nociceptors within deep tissues and the transmission of signals primarily via Aฮด and C-fiber afferents. The mechanisms responsible for pain referral, particularly convergence-projection and central sensitization, are general processes applicable to noxious input from any deep somatic structure capable of producing sufficiently strong signals, not just those originally mapped by Inman and Saunders. The characteristic quality of the pain โ deep, dull, aching, poorly localized โ is also common to both local deep somatic pain and its referred counterpart, reflecting the nature of C-fiber mediated nociception. The transition from purely local deep pain to local pain accompanied by referred (sclerotomal) pain likely depends on factors such as the intensity and duration of the stimulus, the specific structures involved, and the state of excitability within the central nervous system.
Resources
References
