|Principles and rules of somatosensory examination
Providing the stimulus
The three factors that need to be consistent when testing specific sensory modalities are:
- The precision of the stimulus
- The intensity of the stimulus
- The sequencing and rhythm of the stimuli
Specificity of the stimulus
The stimulus needs to be specific to the sensory modality that is being examined. For example, if touch localization is being examined, a small area of skin needs to be touched, and the contact pressure needs to be sufficiently light in order to prevent stimulation of deep pressure receptors. A common error made by clinicians is simultaneously providing the proper stimulus while erroneously contacting the area being examined with a finger or sleeve of a shirt. Conversely, when testing pain (i.e., sharp/dull), the stimulus needs to be applied with enough pressure to allow for the differences between stimuli to be perceived. For example, a common error can occur if the clinician or student is reluctant to use the “sharp” stimulus for fear of hurting the patient.
Varying the pattern of stimulus application
The patient can often pick up on a pattern of application of the stimulus, such as sharp-dull-sharp-dull-sharp-dull, and may adapt to the pattern whether or not he/she can actually perceive the difference. For this reason, it is important that the health care practitioner vary the pattern of application in an unpredictable way to improve the reliability of patient responses.