Neurologic Approach to Diagnosis of Low Back Pain




Neurologic Approach to Diagnosis of Low Back Pain

NEUROLOGY IN PRACTICE



Neurologic Approach to Diagnosis of Low Back Pain
James R. Lehrich, MD
Harvard Medical School


Key words: back pain, pain, lumbar spine, lumbardisks
Introduction
Low back pain is extremely common and has major economic significancein industrial societies. It is reported to occur in 26% of the working populationeach year and occurs to a disabling degree in 2 to 8%. Eighty percent ofthe population have at least one episode of low back pain in their lifetimes.It is the fifth leading reason for medical office visits in the United States.Low-back injury compensation accounts for 33% of all workers' compensationcosts (1/3 for medical treatment, 2/3 for indemnity). Seventy-five percentof compensation payments go to back patients, although they constitute only3% of total compensation patients (Klein et al, 1984; Hart et al, 1995).

Low back pain occurs most frequently between the ages of twenty and fortyand is more severe in older patients. There is no strong association basedon sex, height, body weight, or physical fitness. High-risk occupationsinclude miscellaneous labor, garbage collection, warehouse work, and nursing,all of which are usually associated with lifting, twisting, bending, andreaching.

Prognosis

The typical attack involves 35 days (median) of pain and 9 to 21 daysout of work. Those who are out of work longer than 6 months have only a50% likelihood of returning. This drops to 25% after more than 1 year outand to nil after 2 years. After an initial episode, the probability of recurrenceis increased fourfold. Treatment tends to be less successful in workers'compensation cases. The average cost for care is 4.5 times greater if thepatient is represented by an attorney.

Classification
Low back pain is an important part of neurologic and general medicalpractice. Of all office visits for back pain, 56% are to family practitionersand internists, 25% are to orthopedic surgeons, 7% are to neurosurgeons,and 4% are to neurologists. These patients make up 10% of the average neurologist'scaseload (Hart et al, 1995). The neurologist is usually called upon to evaluateand treat the patient with acute or subacute pain and symptoms and signsof nerve-root irritation-radiating pain, weakness, numbness, and bladderor bowel symptoms. Such patients make up a small minority of those withlow back pain. In many of these cases, the neurologist is also asked toassess the presence and degree of impairment (physical defect) and disability(what the patient can or can't do as a result of this defect) (AMA, 1993).

There are a multitude of causes of low back pain. Even when there isevidence of nerve-root involvement (radiculopathy), not every patient hasa herniated lumbar disc. The classifications outlined inTable 1 may be helpful in the differential diagnosis.

Clinical Evaluation
As for any neurologic patient, the general physical and neurologic examinationsand history are essential. This discussion will emphasize areas of specialconcern in the evaluation of back pain.

History

Particular note should be made of any preceding trauma, prior attacksof pain, prior evaluations, prior or current treatment, and the durationand progression of symptoms. The patient should be asked about weakness;numbness; dysesthesias and paresthesias; bladder, bowel, and sexual dysfunction;and any accompanying abdominal or flank pain.

Pain

Inquire as to the quality, location, and radiation of pain, and any exacerbatingor relieving factors and activities. Severe, constant back pain persistingat night suggests the presence of neoplasm, infection, or lateral recessnerve-root compression. Pain fibers are present in the annulus surroundingthe disc (in the spinal ligaments, facets, and joint capsules) but not inthe intervertebral disc itself. Nerve-root pain is usually brief, sharp,and shooting, is often increased by coughing, straining, standing, or sitting,and is usually relieved by lying down.

Peripheral-nerve or plexus pain is usually described as burning, tingling(pins and needles), or "asleep" or numb in quality; it is usuallyworse when the patient is lying down at night. (SeeTable 2.) In painful radiculopathies and mononeuropathies, the areaof pain and sensory abnormality may extend beyond the known sensory distributionof the affected peripheral nerve or beyond the dermatome of the affected-rootor dorsal-root ganglion, as in postherpetic neuralgia. This phenomenon hasbeen attributed to central nervous system plasticity. In most nerve-rootsyndromes, however, a precise description of radiating pain will help localizeto a nerve-root level.

Physical Examination

The general physical examination can be as important as the neurologicexamination and should include the vasculature (especially the pedal pulses),abdomen, inguinal areas, and rectum (especially if a cauda equina syndromeis suspected). The patient should be undressed.

Watch how the patient moves, sits, and stands. Look for atrophy (measurethe calf and thigh circumferences for asymmetry), fasciculations, pelvictilt ("bad" side is down), involuntary knee flexion (to guardagainst root traction), scoliosis, and cafe au lait spots (they might indicateneurofibromatosis). Gait testing should include walking on heels and ontoes.

Palpate the lower spine, paraspinal muscles, sciatic notches, and sciaticnerve looking for tenderness, muscle spasms, and radiating pain. Muscletenderness may be associated with nerve-root irritation (calf muscles withS1, anterior tibial muscles with L5, and quadriceps with L4).

Nerve Root Stretching
Roots may be impinged upon or tethered by herniated discs or other lesions,so that stretching the root causes pain. This should be tested by havingthe patient bend forward or by straight-leg raising (SLR). SLR is performedby raising the extended leg of a supine patient to determine whether thisaction elicits pain in the leg, buttock, or back, and, if so, at what anglefrom the horizontal the pain occurs. The pain is usually worsened by dorsiflexionat the ankle and relieved by flexion of the knee and hip. Positive SLR resultsusually indicate S1 or L5 root irritation. Pain occuring in the contralateral,symptomatic leg when the asymptomatic leg is raised is considered a positivecrossed SLR test, which usually indicates the presence of a disc herniationmedial to the nerve root, often with an extruded disc fragment. ReverseSLR tests detect L3 or L4 root irritation. The patient lies prone or onhis side, and the thigh is extended at the hip joint. If the patient haship or groin pain, the examiner should rotate the hip; pain on hip rotationsuggests hip disease rather than radiculopathy.

Motor Examination
It is helpful to bear in mind certain features of the motor examinationof the lower extremities in patients with low back pain. Since it is difficultto detect proximal leg weakness when the patient is lying down, it may benecessary to ask her to attempt to rise from a squatting position. Similarly,gastrocnemius weakness is easiest to detect with repeated rising up on thetoes. Toe flexors and extensors usually become weak before the foot musclesdo. If the gluteus maximus (supplied by S1) is weak, one buttock may sag;gluteus medius (L5) weakness may cause a lurching or waddling (Trendelenburg)gait. In a patient with root pain, do not test the dorsiflexors of the footwith the knee extended, since this may stretch the S1 or L5 root and increasesciatic pain. For the same reason, quadriceps strength should be testedwith the patient prone. Muscles are innervated by more than one nerve root,so total paralysis implies a lesion of multiple roots or of peripheral nerves.Even if a single root has been severed, there is little weakness. Atrophyis rarely seen unless symptoms have been present for more than three weeks.Severe atrophy should raise the suspicion of an extradural neoplasm.

Sensory Examination
A dermatomal distribution of loss of pinprick and touch sensation indicatesand localizes root involvement







(Figure1).



Dermatomal and peripheral nerve sensory distributions (from Keegan,JJ, Garrett, FD. Anat. Rec. 102:411, 1948; and Haymaker, W, Woodhall,B. Peripheral Nerve Injuries. 2d ed. Philadelphia. Saunders. 1953).







Because there is a wide overlap of root distributions, a single rootlesion usually causes mild hypalgesia. The examiner may not be able todetect any sensory deficit, even though the patient has sensorysymptoms.

Tendon reflexes
Asymmetry of the ankle and knee jerks can be helpful in identifying theaffected nerve root.

Nerve-Root Syndromes

In S1 nerve-root syndromes (see Table 3),leg pain is often worse than low back pain. Pain and paresthesias are feltin the buttock, posterior thigh, posterolateral calf and heel, and sometimesin the lateral foot and last two toes. Numbness and pinprick hypalgesiamay be in the fifth toe and lateral foot, and, to a lesser degree, in theposterolateral calf and posterolateral thigh. There may be weakness of thetoe flexors, the gastrocnemius, and (rarely) the hamstrings as well as toeadbuction and eversion of the foot. The ankle jerk is often diminished orabsent.

In L5 nerve-root syndrome, low back pain is often worse than leg pain.Pain and paresthesias radiate to the posterolateral thigh, groin, lateralcalf, dorsomedial foot, and first two toes. Numbness and hypalgesia maybe found in the great toe and medial foot, and, to a lesser extent, theanterolateral calf. Weakness may be noted in the extensor hallucis longus(EHL), the tibialis anterior (TA), and peroneal muscles, causing a footdrop. There is usually no reflex loss.

In L4 and L3 nerve-root syndromes, low back pain is worse than leg pain.There may be some anterior thigh pain. Numbness and hypalgesia may be presentover the anteromedial thigh and knee. Weakness may be detected in the quadricepsand iliopsoas muscles. The knee jerk is often diminished or absent.

Inconsistencies

Since some patients with back pain may exaggerate their symptoms, especiallyin medicolegal or workers' compensation cases, it is important to detectinconsistencies in the clinical presentation (Macnab and McCullock, 1990;Waddell et al, 1980). The history may indicate that the patient is ableto engage in activities inconsistent with the severity of his complaints.Symptoms may be described in an exaggerated or histrionic manner. Duringthe examination, tenderness may be elicited by minimal pressure or overareas where pain would not be expected. Axial loading, by pressing downon top of the head, or rotating the body at the hips or shoulders shouldnot elicit low back pain. It may be helpful to distract the patient, tosee whether there is more mobility in spontaneous activities than elicitedduring the formal examination. The patient may be observed during dressing(especially shoes and socks) and getting on and off the examination table.Straight-leg raising (SLR) should be tested when she is in a sitting position(e.g., while testing the plantar responses or measuring the calf circumferences)as well as when supine. There should be no significant difference in thedegree of hip flexion with sitting or supine SLR or when the patient bendsforward while standing. Nondermatomal or otherwise nonanatomic distributionsof sensory loss should also be noted. In the motor examination, sudden givingway, jerky movements, or weakness appearing to involve many muscle groupsshould raise suspicion. Note whether the patient appears to overreact duringthe examination, including what appears to be a disproportionate verbalizationof pain on minimal provocation, dramatic facial expressions, muscle tensionand tremors, collapsing when asked to bear weight, and requiring a companionfor dressing and undressing. These observations must be considered in thecontext of the overall examination. Severe pain can cause extreme reactionsin some patients.

Laboratory Evaluation
Laboratory tests are not necessary in every patient. They are importantto verify the diagnosis if surgery is contemplated and are sometimes necessaryto clarify the differential diagnosis. Since X rays, CT and MRI scans, myelograms,and other tests may be abnormal in asymptomatic patients, the results mustbe interpreted with the entire clinical presentation in mind. Some guidelinesare outlined in Table 4. (For a detailed discussion,see "Laboratory Evaluation of Low Back Pain" in this issue.)

Management
Most patients with back pain will respond to conservative treatment.Guidelines appropriate to the primary care physician as well as to the neurologistare outlined in Table5 . (For a more detaileddiscussion, see "Management of Low Back Pain" in this issue.)

References
Klein BP, Jensen RC, Sanderson LM: Assessment of workers'compensationclaims for back strains/sprains. J Occup Med 26:443, 1984.

Hart LG, Deyo RA, Cherkin DC: Physician office visits for low back pain.Frequency, clinical evaluation, and treatment patterns from a US NationalSurvey. Spine 20:11, 1995.

American Medical Association. Guides to the evaluation of permanent impairment,4th ed. Chicago: AMA, 1993.

Macnab I, McCullock J: Backache, 2nd ed. Baltimore: Williams and Wilkins,1990.

Waddell G, McCulloch JA, Kumel EG, et al: Nonorganic signs in low backpain. Spine 5:117, 1980.

Miller GM, Forbes GS, Onofrio BM. Magnetic resonance imaging of the spine.Mayo Clin Proc 64:986, 1989.


Copyright (C) 1996 Massachusetts Institute of Technology








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