Mechanical Thresholds

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Some background:

Mechanical nociceptive threshold (MNT) testing is used for both research and the clinical evaluation of pain. It is widely used in animal studies to evaluate treatments which either increase the threshold (analgesics) or which decrease it (irritants, trauma, disease). Analgesics now generally require testing for efficacy, dose and comparison against a gold standard under laboratory conditions prior to drug registration. In these studies the animal’s aversive response is taken as the endpoint and called the threshold. A similar approach is used in basic research where the mechanical threshold is commonly used in rodents to determine the presence or absence of pain and its exacerbation or alleviation.┬áStudies in human subjects enable the distinction between a whole range of painful and non-painful sensations to be made. In animals, however, the sensation experienced is unknown and we assume that a stimulus that is painful to us and which produces an appropriate response in the animal is the threshold for pain.

Many methods for MNT testing are described in the literature, making it difficult to compare data from different studies. Results are affected by the following:

  • probe size and shape
  • ┬áthe rate and method of force application (hand-held or fixed on the subject)
  • the interval between tests
  • The characteristics of the tissue that is stimulated
  • the testing environment and level of distraction
The size of the probe tip also influences the type of tissue most affected by the stimulus. Larger probes (probably larger than 2mm in man) elicit deep muscle pain while smaller probes elicit cutaneous pain. Studies have shown that the application of local anaesthetic cream to the skin increases MNT very little with the larger probes, showing that the pain is largely from deep muscle.
Tissue types are not equally compressible and MNT is affected by their physical properties. Nociceptor stimulation is related to tissue distortion and smaller probes (2mm versus 11.3mm diameter) have proved better for evaluating bone (periosteal) pain as they produce more compression here at lower applied force.
Tissue distortion is decreased when muscles are hard (under tension) as transmission of the force to the deeper tissues is reduced, leading to higher MNT.
Conversely, adipose tissue is easily compressed and, although a normal amount of subcutaneous fat does not affect MNT significantly, the probe simply has to travel further before the pressure rises. In really obese subjects, the MNT may be increased as the forces are not transmitted to the deeper tissues.