No abstract

Recent studies demonstrate neuropathic changes with respect to vibration sensitivity for different measurement frequencies. This study investigates the relationship between vibration perception thresholds (VPTs) at low and high frequencies at two plantar locations and diabetic peripheral neuropathy (DPN) severity in diabetes mellitus (DM) subjects with DPN. We examine differences of VPTs between participants with DM, with DPN, as well as healthy controls. The influence of anthropometric, demographic parameters, and DM duration on VPTs is studied. Thirty-three healthy control group subjects (CG: 56.3 ± 9.9 years) and 33 with DM are studied. DM participants are subdivided into DM group (DM without DPN, n = 20, 53.3 ± 15.1 years), and DPN group (DM with DPN, n = 13, 61.0 ± 14.5 years). VPTs are measured at the first metatarsal head (MTH1) and heel (30 Hz, 200 Hz), using a customized vibration exciter. Spearman and Pearson correlations are used to identify relationships between VPTs and clinical parameters. ANOVAs are calculated to compare VPTs among groups. Significant correlations are observed between DPN severity (by fuzzy scores) and VPTs at both locations and frequencies (MTH1_30 Hz vs. fuzzy: r = 0.68, p = 0.011; Heel_30 Hz vs. fuzzy: r = 0.66, p = 0.014; MTH1_200 Hz vs. fuzzy: r = 0.73, p = 0.005; Heel_200 Hz vs. fuzzy: r = 0.60, p = 0.032). VPTs in CG and DM groups are significantly smaller than the DPN group, showing higher contrasts for the 30 Hz compared to the 200 Hz measurement. The correlations between fuzzy scores and VPTs confirm the relevance of using low and high frequencies to assess a comprehensive foot sensitivity status in people with DM.

In humans, plantar cutaneous mechanoreceptors provide critical input signals for postural control during walking and running. Because these receptors are located within the dermis, the mechanical properties of the overlying epidermis likely affect the transmission of external stimuli. Epidermal layers are highly adaptable and can form hard and thick protective calluses, but their effects on plantar sensitivity are currently disputed. Some research has shown no effect of epidermal properties on sensitivity to vibrations, whereas other research suggests that vibration and touch sensitivity diminishes with a thicker and harder epidermis. To address this conflict, we conducted an intervention study where 26 participants underwent a callus abrasion while an age‐matched control group ( n = 16) received no treatment. Skin hardness and thickness as well as vibration perception thresholds and touch sensitivity thresholds were collected before and after the intervention. The Callus abrasion significantly decreased skin properties. The intervention group exhibited no change in vibration sensitivity but had significantly better touch sensitivity. We argue that touch sensitivity was impeded by calluses because hard skin disperses the monofilament's standardized pressure used to stimulate the mechanoreceptors over a larger area, decreasing indentation depth and therefore stimulus intensity. However, vibration sensitivity was unaffected because the vibrating probe was adjusted to reach specific indentation depths, and thus stimulus intensity was not affected by skin properties. Since objects underfoot necessarily indent plantar skin during weight‐bearing, calluses should not affect mechanosensation during standing, walking, or running.

Determining vibration perception thresholds (VPT) is a central concern of clinical research and science to assess the somatosensory capacity of humans. The response of different mechanoreceptors to an increasing contact force has rarely been studied. We hypothesize that increasing contact force leads to a decrease in VPTs of fast-adapting mechanoreceptors in the sole of the human foot. VPTs of 10 healthy subjects were measured at 30 Hz and 200 Hz at the heel of the right foot using a vibration exciter. Contact forces were adjusted precisely between 0.3 N–9.6 N through an integrated force sensor. Significant main effects were found for frequency and contact force. Furthermore, there was a significant interaction for frequency and contact force, meaning that the influence of an increasing contact force was more obvious for the 30 Hz condition. We presume that the principles of contrast enhancement and spatial summation are valid in Meissner and Pacinian corpuscles, respectively. In addition to spatial summation, we presume an effect on Pacinian corpuscles due to their presence in the periosteum or interosseous membrane.

Mechanical skin properties (MSPs) and vibration perception thresholds (VPTs) show no relationship in healthy subjects. Similar results were expected when comparing MSP and VPT in individuals with diabetes mellitus (DM) and with diabetic (peripheral-)neuropathy (DPN). A healthy control group (33 CG), 20 DM and 13 DPN participated in this cross-sectional study. DM and DPN were classified by using a fuzzy decision support system. VPTs (in µm) were measured with a modified vibration exciter at two different frequencies (30 and 200 Hz) and locations (heel, first metatarsal head). Skin hardness (durometer readings) and thickness (ultrasound) were measured at the same locations. DPN showed the highest VPTs compared to DM and CG at both frequencies and locations. Skin was harder in DPN compared to CG (heel). No differences were observed in skin thickness. VPTs at 30 and 200 Hz correlated negatively with skin hardness for DPN and with skin thickness for DM, respectively. This means, the harder or thicker the skin, the better the perception of 30 or 200 Hz vibrations. Changes in MSP may compensate the loss of sensitivity up to a certain progression of the disease. However, the influence seems rather small when considering other parameters, such as age.