Psychophysical Assessments

The most widely used paradigm to assess central hypersensitivity consists in the application of a standardized stimulus to a peripheral tissue and the recording of a subject’s response. These measurements are known as quantitative sensory tests (QSTs). However, if such tests stand alone it is difficult to conclude if a detected hypersensitivity is caused by a peripheral mechanism, central mechanisms, or a combination of the two. Examples of responses to the stimulus are pain intensity and pain threshold. The paradigm relies on the assumption that a nonpainful stimulus, when applied to a noninjured tissue, can evoke pain only if central nociceptive pathways are hypersensitive. Similarly, a low-input noxious stimulus applied to a noninjured tissue would produce exaggerated pain in the presence of hypersensitive central pain pathways (hyperalgesia and/or allodynia).

Objective Assessments

The aforementioned methods rely on subjective pain reports. Different objective electrophysiologic parameters of central hypersensitivity have been investigated. Among them, the nociceptive withdrawal reflex (NWR) has potential for clinical applications, because it consumes little time and requires relatively simple equipment. The NWR is typically measured by applying percutaneously an electrical stimulus to a sensory nerve of the lower extremity, such as the sural nerve at the foot, and registering the electromyography response during leg withdrawal. It is considered as a measure of sensitivity of spinal nociceptive pathways. For instance, if the reflex is elicited with low stimulus intensities, it can be inferred that these spinal pathways are sensitized. The NWR has also been used as response for the assessment of endogenous pain modulation and temporal summation (discussed later).

Spread of Pain and Referred Pain Areas

Spread of pain areas from the site of primary injury to noninjured body areas is a commonly observed clinical phenomenon. Extensive basic research has revealed structural changes in the central nervous system that account for spread of pain areas. Several experimental studies have described this phenomenon in humans. For instance, pain after the injection of hypertonic saline into a muscle is perceived at a much wider body area in chronic pain patients, compared with pain-free subjects. The combined findings of basic and clinical research indicate that neuroplastic changes at least contribute to, and are likely the main determinants for, spreading of pain areas outside the primary site of injury.

Therefore, measuring the area of pain in general and referred pain in particular may be simple methods to assess central hypersensitivity. For this parameter, there are no normative values or guidelines. Nevertheless, whenever the pain area significantly spreads outside the primary site of lesion, the clinician may suspect that sensitization processes are relevant components of the patient’s complaints.

Measurement of Endogenous Modulation

Central hypersensitivity may also be the result of inefficient endogenous pain modulation (balance between descending inhibition and facilitation). In humans, the most established paradigm to measure this phenomenon is conditioned pain modulation, which mirrors diffuse noxious inhibitory control tested in animal models. Conditioned pain modulation assessment consists of applying two concomitant painful stimuli at two distant body areas: a conditioning and a test stimulus. If endogenous pain modulation is efficient, the conditioning stimulus is expected to attenuate the pain (increase the pain threshold or decrease the pain intensity to a given stimulus) elicited by the test stimulus. For instance, pressure pain threshold to stimulation of the lower extremity increases when immersing the hand into ice water (cold pressor test).

Measurement of Temporal Summation

Temporal summation occurs when a stimulus, delivered at constant intensity, evokes increased pain reactions. For instance, repeating an electrical stimulus every 0.5 seconds leads to increased pain sensation during the stimulation, although the intensity of the stimuli is constant. This phenomenon can also be analyzed by electromyography, whereby a withdrawal spinal reflex is evoked only by the last one or two of a train of five stimuli.

Temporal summation is thought to reflect facilitatory mechanisms within the central nervous system. Several investigations have found a facilitation of temporal summation in pain patients.

Evidence for Central Hypersensitivity in Musculoskeletal Pain

The issue of central hypersensitivity has been extensively investigated in humans. The typical study design has involved the comparison between a group of patients and a control group of pain-free subjects. Different QST methods have been used to profile the two groups. These studies have applied stimuli of different nature to different body areas. Despite heterogeneity, they have consistently shown enhanced pain responses in patients compared with control subjects. Hypersensitivity has been found in a variety of pain conditions, including low back, neck, temporomandibular joint, elbow, and knee pain ; tension-type headache ; endometriosis ; and fibromyalgia. More details are available in previous reviews. A first epidemiologic study on a large patient population in a university pain clinic used pressure algometry at the toe to determine the prevalence of widespread hypersensitivity; depending on the percentile of normative values considered, generalized central hypersensitivity affected 17.5% to 35.3% of patients.

These studies have provided a better understanding of phenomena that had been poorly recognized. Exaggerated pain and expansion of pain areas can occur after limited tissue damage. This can be the result of enhanced nociceptive processes within the central nervous system. Clinicians should be aware of this pathophysiology and should explain it to their patients as one of the possible determinants of their symptoms. Noticeably, one of the primary expectations of patients in a pain clinic is to gain a better understanding of their problems.

Limitations

Several limitations apply to these kinds of studies. QST is a measure of pain sensitivity and does not allow conclusions on the causes and mechanisms underlying hypersensitivity. When the stimulus is applied to tissues that cannot be a primary source of nociception, such as the lower limb in neck pain, it is reasonable to infer that some site of the central nervous system is sensitized. Unfortunately, we cannot say much more. For instance, clinicians are still unable to say whether or to what extent pain hypersensitivity is the result of psychosocial factors. Cognitive and affective components may be determinants of hypersensitivity at brain level, or at the level of descending pathways that facilitate pain transmission at the spinal cord.

Some models may allow more mechanistic inferences. For instance, facilitated NWR is likely the result of sensitization of spinal pathways. Enlargement of nociceptive reflex receptive fields after electrical stimulation is suggestive for the presence of sensitization of spinal neurons not directly connected to the site of primary stimulation.

Importantly, QST may allow detection of only widespread central hypersensitivity. The sensitivity of central nociceptive pathways connected to the site of primary nociceptive input cannot be tested with QST, because these tests do not differentiate central from peripheral sensitization. For example, a low pain threshold detected with pressure stimulation of the knee in patients with knee pain does not tell whether sensitization of injured tissues of the knee, sensitization of central pathways, or both are being detected.

Reliability

One of the requirements for clinical applicability of any test is reliability. It can be defined as the degree to which a test measures the same way each time it is used under the same condition with the same subjects. Ideally, the results of a test on a subject should be the same even if different examiners are involved and the assessment is made at different time-points.

Several studies have investigated the reliability of QSTs. Collectively, the data show that the reliability of most QSTs is at least acceptable. Most studies have been conducted on pain-free subjects, but studies on patients have been recently published. Most of them have confirmed the encouraging results obtained with healthy volunteers.

Reference Values

For research purposes, mean scores of patient groups are typically compared with those of groups of pain-free subjects. This approach is obviously not applicable if abnormalities are to be detected in individual patients. For this purpose, the availability of reference values in the pain-free population is essential. In recent years, some large studies that aimed to determine reference values of QSTs have been published. Some of them are suitable for profiling patients with neuropathic pain ; others have been designed for painful musculoskeletal conditions.

Such studies are demanding because they require large sample sizes and the control of several factors that may influence the measurements, such as demographic or psychosocial variables. The available investigations are important steps toward the clinical application of QSTs. However, they require replication to ensure generalizability. Furthermore, the reference values depend on the body site where the stimuli are applied. Thus, the available data have to be taken with caution when applied to clinical practice.

Validity

The use of QSTs for the assessment of central hypersensitivity is largely based on face validity. Face validity can be defined as the extent to which a test seems “on its face” to sample what it was intended to measure. For example, an innocuous stimulus applied to the lower extremity may be perceived as painful by patients with neck pain. Because there is no pathology at the lower extremity, it can be inferred that the pain evoked in these patients is the result of sensitization processes in the central nervous system. This assumption, although reasonable, does not provide firm evidence for central hypersensitivity. Ideally, clinicians need to ensure that tests are measuring the construct they claim to be measuring (ie, they need to demonstrate construct validity). In the case of central hypersensitivity, construct validity could be studied by comparing the results of a QST with the results of a reference method (gold standard) that directly measures sensitization of central nociceptive pathways. This would allow the calculation of sensitivity, specificity, and likelihood ratios of the QST, and ultimately the estimation of their diagnostic value. Because of the lack of an established gold standard for central hypersensitivity in humans, construct validity of QST cannot be assessed.

Therapeutic Options

Central hypersensitivity is at least initiated by a nociceptive input arising from the injured area. It is therefore possible that any treatment that reduces or abolishes the transmission of the nociceptive flow to the spinal cord can attenuate or reverse central hypersensitivity.

A recent study in an animal model of cervical facet joint injury found that intra-articular bupivacaine, administered immediately after injury, significantly attenuated hyperalgesia, neuronal hyperexcitability, and dysregulation of excitatory signaling proteins; however, bupivacaine at Day 4 had no effect on these outcomes. These data suggest that reversion of central plasticity changes may not be achieved by late interventions at the primary site of injury.

However, clinical studies have shown that after hip or knee replacement pain relief was associated with attenuation of central hypersensitivity. Similarly, a study in chronic neck pain after whiplash injury found that medial branch radiofrequency neurotomy reduced indicators of augmented central pain processing. These findings suggest that central augmentation processes are maintained by peripheral nociceptive input and can therefore be reversed also in the chronic phase. The issue of peripheral modulation of central hypersensitivity remains an important field of future research.

Virtually all centrally acting medications have the potential to provide pain relief by acting, at least partly, on central hypersensitivity. Some of them can do so nonspecifically by reducing the amount of nociceptive flow within the central nervous system. Others can more specifically target mechanisms of neuronal hyperexcitability. N -methyl- d -aspartate antagonists (NMDA) are among medications that act specifically on central hypersensitivity, given the involvement of the N -methyl- d -aspartate receptor in the generation of neuronal hyperexcitability. Ketamine is the most effective drug of this class, but its use in chronic pain is limited by an unfavorable side effect profile, concerns regarding potential drug abuse, lack of long-term clinical data, and limited bioavailability after oral administration.

Inflammatory and neuropathic conditions are associated with a reduction in inhibitory glycinergic and GABAergic control of dorsal horn neurons; this reduction in γ-aminobutyric acid (GABA)–mediated endogenous inhibitory control leads to exaggerated pain and hyperalgesia. Potentiation of GABA _A receptor–mediated synaptic inhibition by benzodiazepines reverses hypersensitivity in animal studies. A preliminary experimental human study suggested that benzodiazepines may be antihyperalgesic. However, these compounds produce sedation and tolerance, which severely limit dose escalation and clinical applicability. Subtype-selective compounds targeting the α ₂ and/or α ₃ subunit of the GABA _A receptor produce antihyperalgesia in mice and rats, without causing sedation or tolerance. These findings, if confirmed in human studies, may open new perspectives for a more selective targeting of hypersensitivity with GABAergic drugs. Gabapentin and pregabalin may attenuate central sensitization presumably by acting at the calcium channel alpha-2-delta-1 subunit that is upregulated in hypersensitivity states.

Pharmacologic inhibition of central sensitization can be achieved by opioids and antidepressants, via descending opioidergic, serotoninergic, and noradrenergic pathways. However, opioids can also enhance pain sensitivity and severe concerns regarding their use in chronic pain have been raised in recent years.

Inhibitors of the 5-HT3 receptors, such as tropisetron and ondansetron, may attenuate generalized central hypersensitivity because of the role of mechanisms involving these receptors in hypersensitivity states. Clinical studies have shown an effect of inhibitors of the 5-HT3 receptors on fibromyalgia and neuropathic pain. However, a more recent study on low back pain yielded negative results on intensity of low back pain and parameters of central hypersensitivity.

It is conceivable that psychological treatments attenuate central hypersensitivity by acting at modulatory mechanisms within the brain and descending inhibitory pathways. The authors are not aware of investigations that have addressed this issue.