Quantitative Sensory Testing Overview

Overview

This segment covers quantitative sensory testing (QST) as a key part of the neurologic exam for quantitative pain assessment. QST helps clinicians select appropriate assessments to match various patient needs, understand pain mechanisms, guide treatment, and track changes over time.

Quantitative Sensory Testing (QST): Purpose and Scope

• QST is integrated into in-depth neurologic exams, evaluating both peripheral and central nervous system involvement in pain processing.
• Provides a systematic and objective way to assess pain states, distinguishing between sensory loss (hypoalgesia) and sensory gain (hyperalgesia, allodynia).
• QST enables clinicians to set a baseline and monitor progress or setbacks throughout treatment, offering more nuanced insight than pain ratings alone.

Detection Threshold Testing

• This approach identifies the exact moment a person detects a specific sensation, giving insight into peripheral nerve health and function.
• Common testing modalities include:
  • Mechanical thresholds: Semmes-Weinstein monofilament testing, typically used for detecting touch in patients with neuropathy or sensory deficits, but also useful for assessing pain-related changes in the nervous system.
  • Vibration sense and light touch: Standard neurological checks that reveal both hypersensitivity and hyposensitivity.
  • Pain detection via pinprick.
  • Dynamic light touch using a paintbrush, which can uncover hyperalgesia (increased pain from painful stimuli) or allodynia (pain from non-painful stimuli).
• Changes in detection thresholds can reflect altered nervous system processing, such as peripheral or central sensitization, particularly in those with persistent pain.

Temperature Testing and Pain Thresholds

• Simple temperature sensitivity tests offer practical, low-cost insights into pain processing at the bedside or clinic.
• Coin testing: Uses a mildly warmed or chilled coin to assess warmth and cold sensitivity.
  • Sensitivity to cold often suggests central sensitization.
  • Sensitivity to warmth may indicate peripheral sensitization.
• The ice cube test:
  • Place an ice cube on an affected area (e.g., hand) for 10 seconds, repeating the procedure on adjacent and contralateral regions (e.g., forearm, opposite hand).
  • Record how long before the sensation becomes painful and compare responses between areas to differentiate localized versus widespread sensitivity.
  • This method helps clinicians track changes over time—even if standard pain ratings are unchanged—by monitoring how long a patient can tolerate cold stimuli.

Pain Pressure Threshold (PPT) with Algometry

• Uses an algometer to quantify the minimum pressure that becomes painful or uncomfortable for the patient.
• Procedure:
  • Clearly instruct the patient: “Let me know when the pressure changes to pain or discomfort.”
  • Apply pressure slowly to the test site.
  • Perform three measurements per site, with about 15 seconds between repeats, and use the average for accuracy.
  • Test the painful area as well as surrounding regions (above, below, or contralateral) to determine the extent or localization of sensitivity.
• PPT is valuable for objectively tracking treatment impacts and identifying patterns of regional or localized sensitization.

Temporal Summation (Wind-up) vs Conditioned Pain Modulation

• Temporal summation:
  • Occurs when repeated, identical mechanical or temperature stimuli lead to increasing pain intensity (wind-up phenomenon).
  • Indicates abnormal central pain processing and is characteristic of sensitized nervous systems.
• Conditioned pain modulation:
  • In non-sensitized systems, repeated exposure to a stimulus results in reduced pain (adaptation or habituation).
  • Example: The decreasing discomfort experienced as one slowly settles into a cold pool.
  • Persistent pain often disrupts this adaptive response, resulting in heightened or sustained pain with ongoing repetitive stimuli.
• Differentiating between these phenomena informs clinicians about underlying central nervous system changes in people with persistent pain.

Tactile Acuity Testing

• Assesses the clarity of brain-body sensory mapping and somatosensory representation.
• Two-point discrimination test:
  • Evaluates the minimal distance at which two points are felt as distinct; reflects the sharpness of the somatosensory homunculus.
  • Persistent pain can cause “homuncular smudging,” reducing tactile acuity and potentially increasing the area the brain feels needs protection.
• Localization grid technique:
  • Draw a grid on the patient's back or show them a diagram, then ask them to indicate where they are touched.
  • This process not only assesses mapping clarity but also doubles as a treatment technique to enhance mind-body awareness and sensory representation.
• Joint Position Error (JPE):
  • Most often used in the neck, testing proprioception and awareness of head position.
  • Relevant in conditions such as concussion or whiplash, where there is impaired position sense.

Knowledge Check: Answers and Clarifications

• The best tactile acuity test among options: Two-point discrimination.
• Temporal summation refers to the progressive increase in pain with repeated identical stimuli (wind-up).
• Pain pressure threshold measures pain onset via gradual pressure using an algometer.
• Semmes-Weinstein monofilaments evaluate mechanical detection thresholds, not tactile acuity.

Structured Summary of Tests and Targets

Key Terms & Definitions

• Quantitative Sensory Testing (QST): Objective tests measuring sensation and pain thresholds.
• Allodynia: Pain from typically non-painful stimuli (e.g., light touch).
• Hyperalgesia: Heightened pain from painful stimuli.
• Hypoalgesia: Reduced response to painful stimuli; indicating sensory loss.
• Temporal Summation (Wind-up): Increased pain with repeated identical input.
• Conditioned Pain Modulation: Decrease in pain with repetitive stimulus; indicates healthy nervous system modulation.
• Somatosensory Homunculus: Brain’s body map for sensory inputs.
• Homuncular Smudging: Blurring of this sensory map caused by persistent pain.
• Pain Pressure Threshold (PPT): Minimum applied pressure perceived as painful.

Action Items / Next Steps

• Integrate QST into initial and follow-up assessments for patients with persistent pain.
• Employ low-tech thermal tools (coins, ice cubes) to identify patterns of sensitization.
• Use consistent algometry procedures with standardized instructions and repeated measures.
• Regularly assess tactile acuity with two-point discrimination and localization grid methods; consider as both assessment and therapeutic interventions.
• Distinguish between temporal summation and conditioned pain modulation to better understand central pain processing status.
• Continuously track and document QST metrics to monitor progress and inform treatment beyond patient pain ratings.