After reading IMMPACT, DFNS, NeuPSIG,EFNS and a few other sources it still is not clear if von-Frey monofilaments can be used to study high-threshold mechanonociceptors? (different sources say yes others say only LTM).
Quantitative Sensory Testing and Mapping: A Review of Nonautomated Quantitative Methods for Examination of the Patient With Neuropathic Pain:
" Probes with a diameter less than 0.4mm and an angle less than 120 degree allow for testing of sharp sensitivity. von-Frey monofilaments are larger than this. meaning that they would not be adequate for HTM?"
“ In pain research VFM can be used to administer painful stimuli on hypersensitve skin in human experimental pain conditions … in the presence of stimulus-evoked pain of the skin, apart from the perception threshold, a pain threshold can be determined as well.” Keizer D Clinc J Pain 2007 22;85-90
There is a clear answer on your question: Yes, they can. However, niow there are practical issues. First, Forces have to be really very high to reach pain threshold in all subjects you want to test. Second, commercially available sets of von Frey hairs are very moderates qualities throughout, when they are based on nylon filaments. For that reason I used to prepare them myselves with forces ranging from 0.5 mN . 4100 mN. The reason for that was that you need a uniform contact area to make vFh forces meaningful and tresholds depend on tip shape (!). With vFh like this you can prepare psychometric functions (I attached a paper showing results and experimental effects which demonstrate it). Still at highest forces there are problems remaining (like forces depending on ambient temperature and humiditiy, not least repetitive use makes especially the stronger hairs progressively softer due to warming up by inner friction of nylon; the people authoring the review you cited above may not be fully aware of these problems - 0.4 mm nylon filaments have to be cut short like a nipple to be able to produce sufficient force and then are not controllable any more) and we proceeded to develop a pinprick-like test device, which simply uses calibrated stiff stainless steel rods of various forces. This is now a standard instrument in the DFNS QST test battery (by the way I am the DFNS chief instructor on QST). If you need more information do not hesiate to come back with further questions, kind regards
That is excellent news. I've seen the calibrated pin prick/stainless steel devices (amazing work by the way) . Thank you Walter and thanks for the readings.
Dear Walter,
You mentioned that
" 0.4 mm nylon filaments have to be cut short like a nipple to be able to produce sufficient force and then are not controllable any more "
Are there any guidelines or standardized/validated published methods for doing this type of modification to the nylon filaments that you are aware of or which you could recommend for review?
Respectfully,
Dear Anthony, there no published guidelines, but this is the method used since von Frey cut wild bore hair to modify force (original papers hard to access and all in German, anyway). You can easily produce different force vFh yourself using this method. In the 1998 paper I had sent previously that is exactly how we did it. Regards, Walter
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