Radicular or “root” pain has some of the characteristics of referred pain but differs in its greater intensity, distal radiation along the course of the nerve containing the affected root, and factors that excite it. The mechanism is stretching, irritation, or compression of a spinal root at any site within or central to the intervertebral foramen. The pain is sharp, often intense, and usually superimposed on the dull ache of referred pain; it nearly always radiates from a paracentral position near the spine to some part of the lower limb. Coughing, sneezing, and straining characteristically evoke this sharp radiating pain, although each of these actions may also jar or move the spine and enhance local pain. Any maneuver that stretches the nerve root—for example, “straight-leg raising” or bending at the waist in cases of sciatica—evokes radicular pain. The patterns of radicular pain are described in the sections on prolapsed discs further on in the chapter, and the distribution of cutaneous innervation of the spinal roots is shown in Figs. 8-2 and 8-3. The most common pattern is sciatica, pain that originates in the buttock and is projected along the posterior or posterolateral thigh. It results from irritation of the L5 or S1 nerve root as summarized in the review by Ropper and Zafonte. Paresthesia or superficial sensory loss, soreness of the skin, tenderness in certain regions along the nerve and loss of a tendon reflex usually accompany radicular pain. If the anterior roots are involved as well, there is weakness, atrophy, or muscular twitching.
In patients with severe circumferential constriction of the cauda equina due to spondylosis (lumbar spinal stenosis), sensorimotor impairment and referred pain are elicited by standing and walking. The symptoms are projected to the calves and the backs of the thighs thereby simulating the exercise-induced symptoms of iliofemoral vascular insufficiency—hence the term spinal claudication has been applied to the activity-induced symptoms of lumbar stenosis [see “Lumbar Stenosis (Spondylotic Caudal Radiculopathy)” later in this chapter].
Referred pain from structures of the lower back (sometimes called pseudoradicular) does not, as a rule, project below the knees and is not accompanied by neurologic changes other than sometimes a vague numbness without demonstrable sensory impairment. This is in contrast to the pain of root compression. Pain resulting from muscular spasm usually occurs in relation to local spinal irritation and may be thought of as a nocifensive reflex for the protection of the diseased parts against injurious motion. Chronic muscular contraction may give rise to a dull, sometimes cramping ache. One can sometimes feel the tautness of the sacrospinalis and gluteal muscles and demonstrate by palpation that the pain is localized to them. However, except for the most severe degrees of acute injuries of the back, the spasms are difficult to detect and their contribution to back pain has appeared to us to be relatively small.
In addition to assessing the character and location of the pain, one should determine the factors that aggravate and relieve it, its constancy, and its relationship to activity and to rest, posture, forward bending, coughing, sneezing, and straining. Frequently, the most important lead comes from knowledge of the mode of onset and the circumstances that initiated the pain. Inasmuch as many painful conditions of the back are the result of injuries incurred during work or in accidents, the possibility of exaggeration or prolongation of pain for purposes of compensation must be kept in mind.
Examination of the Lower Back
The main goals of the examination of the back are to differentiate pain that is caused by nerve root compression from musculoskeletal strains, metastatic spinal tumor, and infectious and inflammatory diseases of the spine and hips.