Fig. 1.1
(a, b) Nociceptive versus inflammatory pain signal stimuli and conduction pathways. (c, d) Dysfunctional versus neuropathic pain signal stimuli and conduction pathways
Nociceptive Versus Neuropathic Pain
Nociceptive Pain
A nociceptor is a nerve fiber preferentially sensitive to a noxious stimulus or to a stimulus that would become noxious if prolonged. Nociceptive pain is the perception of nociceptive input, usually due to tissue damage (e.g., postoperative pain). Nociceptive pain is further subdivided into somatic and visceral pain. Somatic pain arises from injury to body tissues. It is well localized but variable in description and experience. Visceral pain is pain arising from the viscera mediated by stretch receptors. It is poorly localized, deep, dull, and cramping (i.e., pain associated with appendicitis, cholecystitis, or pleurisy).
One classification system of pain further subdivides nociceptive pain as musculoskeletal pain, inflammatory pain (e.g., inflammatory arthropathies, postoperative pain, tissue injury, infection), or mechanical/compressive pain (e.g., low back pain, neck pain, visceral pain from expanding tumor masses) [12].
Neuropathic Pain
Neuropathic pain arises from abnormal neural activity secondary to disease, injury, or dysfunction of the nervous system. It commonly persists without ongoing disease (e.g., diabetic neuropathy, trigeminal neuralgia, or thalamic pain syndrome). Neuropathic pain is further subdivided as follows:
Sympathetically mediated pain (SMP) is pain arising from a peripheral nerve lesion and associated with autonomic changes (e.g., complex regional pain syndrome I and II, formerly known as reflex sympathetic dystrophy and causalgia) [13, 14].
Peripheral neuropathic pain is due to damage to a peripheral nerve without autonomic change (e.g., postherpetic neuralgia, neuroma formation).
Central pain arises from abnormal central nervous system (CNS) activity (e.g., phantom limb pain, pain from spinal cord injuries, and poststroke pain).
Neuropathy is also described as mononeuropathy if one nerve is affected, mononeuropathy multiplex if several nerves in different areas of the body are involved, and polyneuropathy if symptoms are diffuse and bilateral.
Pathogenesis and Transmission of the Pain Signal
Pain sensation begins in the periphery of the nervous system. Pain stimuli are sensed by specialized nociceptors that are the nerve terminals of the primary afferent fibers. The pain signal is then transmitted to the dorsal horn of the spinal column and transmitted through the CNS where it is processed and interpreted in the somatosensory cerebral cortex (Fig. 1.2).
Fig. 1.2
This is the major route by which pain and temperature information ascend to the cerebral cortex (Reproduced with permission from: Bear MF, Connors BW, and Parasido, MA. Neuroscience – Exploring the Brain, 2nd ed. Philadelphia: Lippincott Williams & Wilkins, 2001. ©2001 Lippincott Williams & Wilkins)
Multiple ascending pathways may be involved in relaying nociceptive information to the brain, including spinocervical, spinobulbar, spinopontine, spinomesencephalic, spino-diencephalic (containing spinothalamic tracts), and spinotelencephalic pathways. The majority of the wide dynamic range and nociceptive-specific neurons project contralaterally within the spinal cord and ascend within the anterolateral quadrant, forming the spinothalamic tract, which synapses in the thalamus (Fig. 1.3). Neurons from the thalamus project to multiple brain areas in the primary and secondary somatosensory cortex, cingulate cortex, prefrontal cortex, insular cortex, amygdala, and the cerebellum.
Fig. 1.3
Convergence of sensory nerves from the viscera and superficial areas onto the same neurons in spinal cord (Reproduced with permission from: Bear M, Conner B, Paradiso M. Neuroscience, Exploring the Brain, 2nd Ed. Baltimore: Lippincott Williams & Wilkins, 2000. ©2000 Lippincott Williams & Wilkins)
Nerves involved (pain pathway/central perception center—spinothalamic) and the pain signal (biochemistry/receptors):
Once exposed to noxious stimuli, nociceptors initiate the pain signal via myelinated A-delta or unmyelinated C-fibers. The signal is transmitted via chemical transmission (either peptidergic cells (substance P) or non-peptidergic cells (purinergic ligand-gated ion channel)). The signal ascends from the viscera or periphery to the dorsal root ganglia of the spinal cord. Once received, the signal then ascends via the spinothalamic tract to the thalamus and is then transmitted to the sensory cortex for interpretation.
Four physiologic processes are associated with pain: transduction, transmission, modulation, and perception as follows:
Transduction refers to the conversion of a noxious stimulus (thermal, mechanical, or chemical) into electrical activity in the peripheral terminals of nociceptor sensory fibers.
Transmission refers to the passage of action potentials from the peripheral terminal along axons to the central terminal of nociceptors in the central nervous system. Conduction is the synaptic transfer of input from one neuron to another.
Modulation refers to the alteration (e.g., augmentation or suppression) of sensory input.
Perception refers to the “decoding”/interpretation of afferent input in the brain that gives rise to the individual’s specific sensory experience.
Anatomy
Pelvis
The pelvis consists of the greater and lesser pelvis. Contained within, the pelvic girdle consists of the inferior-most portion of the abdominal cavity. The pelvis is part of the appendicular skeleton, providing structure for the lower limbs. The pelvis provides attachment for the muscles of the lower limbs and provides load bearing for the axial skeleton while sitting or standing. The pelvis is of critical importance to identifying landmarks of reference for many of the clinical procedures used to treat pain or anesthetize various nerve distributions. The pelvic girdle is formed by the right and left pelvic bones (consisting of the ilium, ischium, and pubis) and the sacrum medially and posteriorly. The landmarks of the ilium include the anterior superior iliac spine (ASIS) and the posterior superior iliac spine (PSIS). Each ischium contains an ischial tuberosity which is of clinical importance for procedures later discussed. The ischial tuberosities are the “sitting bones” which contact the sitting surface though protected by muscle, fascia, and connective tissue (Fig. 1.4).
Fig. 1.4
Pelvic gridle. (a, b). Features of the pelvic gridle demonstrated anatomically (a) and radiographically (b). The pelvic gridle is formed by the two hip bones (of the inferior axial skeleton) anteriorly ad laterally and the sacrum (of the axial skeleton) posteriorily. (c). The hip bone is in the anatomical position when the anterior superior iliac spine (ASIS) and the anterior aspect of the pubis lie in the same vertical plane. The preadolescent hip bone is composed of three bones-ilium, ischium, and pubis – that meet in the cup – shaped acetabulum. Prior to their fusion, the bones are united by a triradiate cartilage along a Y-shaped line (blue). (d). An adult’s right hip bone in the anatomical position shows the bones when fused. (b courtesy of Dr. E. L. Lansdown, Professor of Medical Imaging, University of Toronto, Toronto, ON, Canada.) (Moore – Clinically Oriented Anatomy 7th Edition pg. 329, 340)
Pelvic Floor
The pelvic floor is a conical structure consisting of the coccygeus and levator ani muscles and their accompanying the fascias (Figs. 1.5a, 1.6c and 1.7; Table 1.1.). The pelvic floor delineates the pelvic cavity and contents from the superficial perineum inferior to the pelvic floor structures. The lateral border is formed by the obturator internus muscle, innervated by the obturator nerves which are located medial to the obturator internus and are supplied by L5, S1, and S2. The obturator nerve is at risk for damage via compression during childbirth, causing painful stimuli to the medial thigh as well as cause spasms of the adductors. The posterosuperior wall is contained by the piriformis which is innervated by S1 and S2. The inferior border consists of the coccygeus (ischiococcygeus) and Levator ani group (puborectalis, pubococcygeus, and iliococcygeus) all of which are primarily supplied by branches of S4 (and to a lesser degree by S5 in regard to the coccygeus).
