Quantitative Sensory Testing (QST) is the most widely used paradigm to assess central hypersensitivity, it consists of the application of a standardized stimulus to a peripheral (nerve) tissue and recording the patient’s response.¹  The paradigm relies on the assumption that a non-painful stimulus, when applied to a non-injured tissue, will evoke pain only if central nociceptive pathways are hypersensitive.  Determining if the hypersensitivity (neuropathic pain) is caused by a peripheral mechanism, central mechanisms, or a combination of the two, can be difficult clinically due to their interdependence.

Neuropathic pain is defined as “Pain caused by a lesion or disease of the somatosensory nervous system”. ²  The somatosensory nervous system provides information about the body including skin, musculoskeletal, and visceral organs, which has clinical relevance for the clinicians providing neuromusculoskeletal treatment, specifically dry needling treatment. Neuropathic pain may be spontaneous or evoked, as an increased response to a painful stimulus (hyperalgesia) or a painful response to a normally nonpainful stimulus (allodynia). ³  This definition provides a broad concept and shares common features of chronic musculoskeletal pain with other pain conditions e.g. neuropathic, and visceral.

It is an ongoing clinical challenge to make a prognosis on the potential outcome of treatment and certainly the time required to achieve a treatment result.  QST has shown some promise in assisting clinicians, specifically neuromusculoskeletal practitioners, in estimating these difficult prognoses. ⁴ In this paper an overview of a novel bedside (IDN) QST exam is provided, and how it can be used as a part of comprehensive evaluative plan in estimating if dry needling would be of clinical value as a part of the treatment plan.

Application: The stimulus applied in QST can be chemical, thermal, electrical, vibrational or physical (pressure) to evoke a response.  In the IDN bedside QST exam the application of a  physical pressure is the most readily available stimulus that can be trained and practiced achieving an acceptable level of intra-tester reliability.¹ The IDN bedside QST exam involves applying a consistent pressure to 16 standardized assessment points in two peripheral nerve pathways, one in the upper extremity and one in the lower extremity. In the IDN system, homeostatic neuro-trigger points are defined as areas of increased sensitivity that are universal in all humans, and are proposed to be of central origin based on meeting the following criteria:

1. Homeostatic neuro-trigger point sensitivity must be present bilaterally and symmetrically
2. Homeostatic neuro-trigger points develop in predictable locations and in a predictable sequence in all humans. The sequence of peripheral sensitization is the result of anatomical, physiological, biomechanical and behavioral factors
3. Homeostatic neuro-trigger points develop slowly over time based on intrinsic and extrinsic factors, which may represent the patient’s health condition and healing potential
4. Homeostatic neuro-trigger point sensitization occurs linearly along nerve trunks in the limbs and occurs as an area or patch on the torso and face.
5. The sensitivity and specificity (size/area) of homeostatic neuro-trigger points are highly dynamic.

The Radial and the Saphenous nerves were chosen to be assessed because they are the first peripheral nerves to become sensitized in the upper and lower extremities respectively.⁵   Once the proper application of the stimulus is taught⁶, the examiner applies the stimulus (pressure) down the standardized nerve pathways (homeostatic points) in all four extremities for a total of 16 possible positive findings.

Because the IDN bedside QST exam is both an objective tactile interpretation by the examiner, and a subjective report of discomfort by the patient, a positive finding requires both.  All positive findings are calculated with the range being from 0 to 16. The greater the number of positive findings the greater the correlation to the sensitization being centrally mediated.^5,7-9  

To make the IDN bedside QST exam of clinical utility the following was considered:

1. Applicable to every patient
2. Reproducible with any patient for any condition
3. Able to be administered by all clinicians who are properly trained
4. Testing is of short duration
5. Quantitative results that can assist in clinical decision making
6. Efficacy of dry needling treatment

The difficulty in assessing pain, and certainly the measurement of it, relates to its complexity and subjective nature.  When we measure pain clinically, we invariably target subjective pain because it is perceivable and easy to find.  Objective pain is not perceivable, patients that have objective pain will not know they have it unless it is triggered.  Objective pain is, by definition, a passive neuro-trigger point sensitivity that is only perceivable when palpated.  It is objective pain that is assessed in the IDN bedside QST exam because it is relatively stable in intensity and location (radial and saphenous nerve pathways). For this discussion objective pain is directly related to the level of  sensitivity being maintained within the homeostatic points via central processes.⁷  In other words, homeostatic points are objective pain, which are centrally mediated.  The greater the number of sensitized homeostatic points a patient carries is directly related to their degree of central sensitization, which in turn influences the manageability of their symptoms and efficacy of the dry needling treatment.^5,7,9

Interpretation:

As is the case in many bedside clinical tests, it is not possible to make definitive statements, this certainly applies to making estimates on the presence and degree of central sensitivity.  That said, central sensitivity has been extensively investigated in humans.⁴  When exaggerated pain responses and expansion of pain areas occur after limited tissue damage it is reasonable to infer that it is the result of enhanced nociceptive processes within the central nervous system^1,7,10,11 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.

QST is primarily a measure of pain sensitivity and does not allow conclusions to be drawn on the causes, mechanisms or location (peripheral or central) of the underlying hypersensitivity. For example, if a patient with shoulder pain did a vigorous run the morning of the IDN bedside QST exam they would experience exercised induced inflammation, which sensitizes the peripheral nervous system leading to false positive QST findings.  Recognizing the limitations, and understanding the intent is to only discover centrally mediated findings, the IDN bedside QST exam was modified as follows:

• Patients should only be assessed when at a resting baseline of physical activity to minimize acute inflammation influencing the findings.
• Pain medications and chemicals affecting consciousness, such as alcohol or marijuana, may influence the accuracy of the patient’s response.
• The assessment of both upper and lower extremities provides a better systemic view of patient condition.
• Assessment of multiple points linearly down the nerve path gains insight into the degree of sensitivity.

The interpretation of the IDN bedside QST exam is empirically based on tens of thousands of patient examinations and treatments dating back from the 1970’s to today.^5,7,8  Founded in the premise that when the human body is under stress from injury, disease, surgery or other physiological events, inflammation increases causing peripheral sensitization and likely central effects.  In patients presenting with chronic pain, this peripheral sensitization profoundly spreads, represented clinically as inflamed sensory nerves (homeostatic points). ^6,7 The number of positive findings (peripheral sensitization) discovered during the IDN bedside QST exam is an indicator of the severity of inflammation in the body.  The greater the number of positive findings is an empirical estimate of how many dry needling treatment sessions are needed to reduce the systemic inflammation and how long the pain relief can last.

Research

QST has demonstrated potential benefits when compared with traditional neurological diagnostic tools. For example, around 80% of the peripheral nervous system consists of small nerve fibers¹² however, traditional diagnostic methods (e.g. electromyography, nerve conduction velocity, and evoked potential), primarily focus on the large nerve fibers. ^11,12 Deep tissue pain sensation transmits through small caliber A-delta (group III), and C fibers (group IV). ¹³ QST can target these fibers by using frequencies that target small fibers (e.g., current perception threshold and vibratory perception threshold) or sensory stimuli (e.g., pain and temperature) that are preferential to these fibers. This lends itself to bedside assessment using pain, generated by the examiners pressure, as the measure.  Potential disadvantages are that the specificity of these responses has not been adequately demonstrated, and this testing is not completely objective because pain is subjective by nature requiring the patient to provide a voluntary response.  Considering the currently used methods of visual analog scales and pain questionnaires, the IDN bedside QST exam may provide a more objective measurement of neural sensitivity.

In the literature there is strong evidence for central hypersensitivity (abnormal pain response) being a prognostic factor for poor outcomes in chronic musculoskeletal pain.^4,14   The evolution of pain theory and evidence of a central component of post-injury pain hypersensitivity implicate central sensitivity in musculoskeletal pain mechanisms.¹⁰  Involvement of the central nervous system in musculoskeletal pain mechanisms (specifically in chronic or maladaptive pain) is emerging as a new target area for treatment. Interestingly, acute and chronic pain have similar mechanisms and the temporal relationship may be more related to central mechanisms in making the transition from acute to chronic pain. This suggests that early detection might allow clinicians to make a more accurate prognosis for their patients.

Clinically, a patient who has been injured in a motor vehicle accident (MVA) is a good example of persistent pain complaints lasting well past the timeframe that tissue healing is expected to be complete.  There is research to show this persistent pain, specifically in post MVA’s, to be a function of neurologic sensitivity being maintained within the nerve structure secondary to the early vascular changes resulting from the injury.^11,15  Patients present with cervical and upper extremity symptoms that are not localized to specific dermatomes or myotomes. A likely explanation for these diffuse symptoms is the development of central sensitivity driven by the persistent nociceptive barrage into the dorsal horn. ^14,16Treatment that focuses solely on the peripheral driver misses the complexity of the systemic involvement.

A systematic review and meta-analysis⁴ demonstrated a predictive relationship between baseline QST, a measure of pain hypersensitivity, and musculoskeletal pain and disability at follow-up. These studies showed this predictive relationship across multiple musculoskeletal conditions (OA, LBP, WAD, post-operative pain) affecting different anatomical sites (knee, hip, low back, neck, shoulder), and across different QST modalities and study contexts (cohort studies and RCTs). This review demonstrated that pain hypersensitivity predicts prognosis and that QST might help identify patients who could most benefit from interventions aiming to improve pain and disability. Clinically identifying which patients might be at particular risk of a poor outcome is important in order to identify those who are most likely to benefit from treatment.  At a minimum this allows the clinician to have an informed discussion with the patient about the expected prognosis and potential treatment outcome prior to initiation of care.

Clinical Relevance:

QST is a safe, simple and useful bedside tool to determine the potential benefit of engaging in a treatment plan and the potential treatment outcome.  The IDN bedside QST exam is not intended to differentiate diseases or to investigate the mechanism of diseases.  The goal of the IDN bedside QST exam is to provide the information necessary for evaluating the self-healing potential of a patient and the projected efficacy of dry needling treatments.  IDN bedside QST exam accompanied by a full patient history, orthopedic examination, diagnostic testing, and the clinician’s experience and intuition provides a more inclusive view of patient condition and prognosis.

Future studies are needed to determine the reliability of specific QST approaches and establish clinically meaningful thresholds in specific pathologies in order to validate QST as a clinical decision aid for neuromusculoskeletal conditions.

Dr. Frank Gargano, PT, DPT, CIDN, MCTA, CMP

Dr. Gargano is President & Founder of the Integrative Dry Needling Institute (IDN), a continuing education provider for medical professionals that treat neuromusculoskeletal pain and dysfunction. 

References

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