From the Guidelines
To test for crude touch versus fine touch, use a 10-g monofilament to assess large-fiber function and protective sensation, which correlates with crude touch, and perform the two-point discrimination test for fine touch discrimination. When evaluating sensory function, it is essential to distinguish between crude touch, which is mediated by large myelinated A-beta fibers, and fine touch, which involves more complex sensory integration and cortical processing 1. For crude touch testing, the 10-g monofilament is a simple and effective tool, as it assesses the function of large fibers and protective sensation, which is critical for preventing diabetic foot ulceration 1. Some key points to consider when testing crude touch include:
- Using the 10-g monofilament to apply a specific amount of pressure to the patient's skin
- Asking the patient to indicate when they feel the sensation with their eyes closed
- Comparing symmetrical body parts and documenting any differences between sides In contrast, fine touch discrimination can be evaluated using the two-point discrimination test, which assesses the ability to distinguish between two separate sensations 1. Some key points to consider when testing fine touch include:
- Using a caliper or paperclip bent into two points to apply gradual pressure to the patient's skin
- Gradually decreasing the distance between points until the patient can no longer distinguish them as separate sensations
- Comparing results to normal two-point discrimination thresholds, which vary by body region By using these tests, clinicians can assess both crude and fine touch sensation, which is essential for diagnosing and managing various neurological conditions, including diabetic peripheral neuropathy 1.
From the Research
Testing for Crude Touch vs Fine Touch
To test for crude touch vs fine touch, the following methods can be employed:
- Light touch (LT) testing: This involves assessing the patient's ability to perceive light touch, which is typically performed using a cotton swab or a soft brush 2.
- Pinprick (PP) testing: This involves assessing the patient's ability to perceive sharp or dull sensations, which is typically performed using a pin or a sharp object 2.
- Von Frey filaments: These are used to assess tactile withdrawal thresholds and can be used to evaluate fine touch 3.
- Nerve conduction studies: These involve eliciting nerve action potentials at sites along a peripheral nerve and recording the response from another site along the nerve or from a muscle innervated by that nerve, and can provide information about the integrity of the large, myelinated axons, neuromuscular junctions, and muscle 4.
Order of Testing
The order of testing for crude touch vs fine touch may be important, as some studies suggest that testing light touch first may be more comfortable for patients, especially first-time examinees 2.
- Testing light touch first may allow for a more comfortable experience, as pinprick testing can be painful for some patients 2.
- The majority of patients reported no preference for testing order, however, first-time examinees and those who did not recall previous sensory testing, were more likely to describe pinprick testing as "very painful" and report more "very painful" experiences when pinprick testing was completed first 2.
Assessment of Tactile Allodynia
Tactile allodynia can be assessed using von Frey filaments, which involve applying a series of filaments of varying thickness to the skin to determine the threshold at which the patient perceives pain 3.