Evaluation of the Patient with Chronic Pain
The evaluation of any patient with pain should include several key components. Informationabout location, onset, and quality of pain, as well as alleviating and exacerbating factors, should be obtained, along with a pain history that includes pre-vious therapies and changes in symptoms over time. In addition to physical symptoms, chronic pain usu-ally involves a psychological component that should be addressed as well. Questionnaires, diagrams, and pain scales are useful tools in helping patients ade-quately describe the characteristics of their pain and how it affects their quality of life. Information gath-ered during the physical examination can help dis-tinguish pain location, type, and systemic sequelae, if any. Imaging studies such as plain radiographs, computed tomography (CT), magnetic resonance imaging (MRI), and bone scans can often suggest physiological causes. All components are necessary for a comprehensive evaluation of the pain patient prior to determining appropriate treatment options.
Reliable quantitation of pain severity helps deter-mine therapeutic interventions and evaluate the efficacy of treatments. This is a challenge, however, because pain is a subjective experience that is influ-enced by psychological, cultural, and other variables. Clear definitions are necessary, because pain may be described in terms of tissue destruction or bodily or emotional reaction.
The numerical rating scale, Wong-Baker FACES rating scale, visual analog scale (VAS), and McGill Pain Questionnaire (MPQ) are most commonly used. In the numerical scale, 0 corresponds to no pain and 10 is intended to reflect the worst possible pain. The Wong-Baker FACES pain scale, designed for children 3 years of age and older, is useful in patients with whom communication may be difficult. The patient is asked to point to various facial expressions rang-ing from a smiling face (no pain) to an extremely unhappy one that expresses the worst possible pain. The VAS is a 10-cm horizontal line labeled “no pain” at one end and “worst pain imaginable” on the other end. The patient is asked to mark on this line where the intensity of the pain lies. The distance from “no pain” to the patient’s mark numerically quantifies the pain. The VAS is a simple and efficient method that correlates well with other reliable methods.
The MPQ is a checklist of words describ-ing symptoms. Unlike other pain rating methods that assume pain is one-dimensional and describe intensity but not quality, the MPQ attempts to define the pain in three major dimensions:
(1) sensory–discriminative (nociceptive pathways), (2) motivational–affective (reticular and limbic structures), and (3) cognitive–evaluative (cerebral cortex). It contains 20 sets of descriptive words that are divided into four major groups: 10 sensory, 5 affective, 1 evaluative, and 4 miscellaneous. The patient selects the sets that apply to his or her pain and circles the words in each set that best describe the pain. The words in each class are given rank according to severity of pain. A pain rating index is derived based on the words chosen.
Psychological evaluation is useful whenever medical evaluation fails to reveal an apparentcause for pain, when pain intensity, characteristics, or duration are disproportionate to disease or injury, or when depression or other psychological issues are apparent. These types of evaluations help define the role of psychological or behavioral factors. The most commonly used tests are the Minnesota Multiphasic Personality Inventory (MMPI) and Beck Depression Inventory.
The MMPI is a 566-item true–false question-naire that attempts to define the patient’s personality on 10 clinical scales. Three validity scales serve to identify patients deliberately trying to hide traits or alter the results. Cultural differences can affect scores.Moreover, the test is lengthy and some patients find its questions insulting. The MMPI is used primar-ily to confirm clinical impressions about the role of psychological factors; it cannot reliably distinguish between “organic” and “functional” pain.
Depression is very common in patients with chronic pain. It is often difficult to determine the relative contribution of depression to the suffering associated with pain. The Beck Depression Inven-tory is a useful test for identifying patients with major depression.
Several tests have been developed to assess functional limitations or impairment (disability). These include the Multidimensional Pain Inventory (MPI), Medical Outcomes Survey 36-Item Short Form (SF-36), Pain Disability Index (PDI), and Oswestry Disability Index (ODI).
Emotional disorders are commonly associated with complaints of chronic pain, and chronic pain often results in varying degrees of psychological dis-tress. Determination of which came first is often dif-ficult. In either case, both the pain and emotional distress need to be treated. Table 47–7 lists emo-tional disorders in which treatment should be pri-marily directed at the emotional disorder.
Electromyography and nerve conduction studies complement one another and are useful for con-firming the diagnosis of entrapment syndromes, radicular syndromes, neural trauma, and poly-neuropathies. They can often distinguish between neurogenic and myogenic disorders. Patterns of abnormalities can localize a lesion to the spinal cord, nerve root, limb plexus, or peripheral nerve. In addi-tion, they may also be useful in excluding “organic” disorders when psychogenic pain or a “functional” syndrome is suspected.
Electromyography employs needle electrodes to record potentials in individual muscles. Mus-cle potentials are recorded first while the muscle is at rest and then as the patient is asked to move the muscle. A triphasic motor unit action poten-tial is normally seen as the patient voluntarily moves the muscle. Abnormal findings suggestive of denervation include persistent insertion poten-tials, the presence of positive sharp waves, fibrillary activity, or fasciculation potentials. Abnormalities in muscles produce changes in amplitude and duration as well as polyphasic action potentials.
Peripheral nerve conduction studies employ supramaximal stimulations of motor or mixed sen-sorimotor nerve, whereas muscle potentials are recorded over the appropriate muscle. The time between the onset of the stimulation and the onset of the muscle potential (latency) is a measurement of the fastest conducting motor fibers in the nerve. The amplitude of the recorded potential indicates the number of functional motor units, whereas its duration reflects the range of conduction velocities in the nerve. Conduction velocity can be obtained by stimulating the nerve from two points and com-paring the latencies. When a pure sensory nerve is evaluated, the nerve is stimulated while action potentials are recorded either proximally or distally (antidromic conduction).Nerve conduction studies distinguish between mononeuropathies (due to trauma, compression, or entrapment) and polyneuropathies. The latter include systemic disorders that may produce abnor-malities that are widespread and symmetrical or that are random (eg, mononeuropathy multiplex).