In addition to confirming previous findings on tactile and thermal sensitivity of HNC patients [
20‐
22], our study investigated other sub modalities of somatosensory perception. We included measurements of chemesthetic sensitivity and texture sensitivity using real food samples. We also explored the link between salivary function and sensory perception, in particular food texture sensitivity.
Oral tactile and food texture sensitivity
The tactile sensitivity observed in the patient group is consistent with previous clinical studies employing point-pressure tests. For instance, HNC patients with hemi glossectomy were less sensitive than control but the difference is only significant when comparing the reconstructed tongue region vs. control, and not when comparing the intact tongue region vs. control [
21]. Patients were less sensitive than the controls, yet the magnitude of the difference highly depends on the type of treatment and the moment at which the assessment was done (before or after treatment) [
20]. Cancer patients with tumours located on the mandible and tongue/floor of mouth had a significant decrease in their tactile sensitivity following cancer treatments, but not in patients whose tumour site is on the maxillary region. The authors suggested the difference was due to the treatment site for maxillary tumours which did not involve the tongue [
22]. These studies suggest that the lowered tactile sensitivity of HNC patients is attributed to the side effect of cancer treatments.
Tactile sensitivity measured using the point-pressure test is a contact-detection sensitivity which stimulates distinct parts of the slowly adapting superficial mechanoreceptors [
29]. These are linked to the perception of surface properties such as roughness, particle sizes, and grittiness [
30]. A reduced tactile sensitivity may translate to an altered perception of some aspects of food textures, as observed in the roughness discrimination test. A previous study demonstrated that participants with lower tactile sensitivity were shown to be less sensitive at discriminating the grittiness/roughness of chocolates [
31]. The reduced sensitivity to roughness in cancer patients could also be attributed to the lack of salivation in the patient group, resulting in reduced lubrication and increased friction thereby increasing the perception of roughness [
32].
Food firmness is perceived through the amount of force needed to fracture the foodstuff [
33], therefore physiological factors such as jaw muscle activity and tongue function may explain the underlying difference in the firmness perception of the two groups. Radiation-induced trismus, which is the restricted mouth opening due to fibrosis of muscles, is common among HNC patients [
34]. Although in this study patients who have self-reported trismus are excluded, it is not unlikely that the patients have a certain level of impairment in their jaw muscle activity [
35]. Moreover, patients with cancer in the oral cavity demonstrated reduced tongue mobility and tongue force [
22], altogether influencing their perception of firmness. Additionally, as the samples were semi-solids that can be masticated without chewing, the incorporation of saliva during this stage plays major importance [
33,
36], thus the lack of saliva may influence the firmness perception of cancer patients.
The amount and viscosity of saliva can either dilute or intensify the perception of food thickness [
37]. Thus, it was expected that cancer patients have altered sensitivity to thickness due to their reduced salivary function; however, no significant difference was observed in this study. This may be attributed to the visual bias, as the difference in visual texture was evident between the thickness samples. As sensory perception is a multidimensional process, visual appearance could also influence the judgement of textural properties [
38].
Chemesthetic and thermal sensitivity
The lower chemesthetic sensitivity may be linked to the release of inflammation-associated factors released by cancer cells which can activate and sensitise nociceptors [
39]. The persistent activation may lead to chronic desensitisation of the receptors [
40]. Other possible explanation may include a more acute mechanism in which the difference between patients and controls may not necessarily originate from the perceived intensity per se but from the time-intensity profile. Application or consumption of capsaicin and menthol either leads to sensitisation or desensitisation depending on the temporal delay [
41]. The procedure established to evaluate the chemesthetic solutions, including the 10-s delay before evaluating the samples and the 3–4 min interstimulus interval period, was based on healthy individuals [
42]. It is possible that the 10-s delay was insufficient for patients to fully perceive the sensation, or that the 3–4 min interval was too short that it caused adaptation while evaluating the proceeding samples.
Patients also demonstrated lower thermal sensitivity, consistent with previous findings [
21,
22]. The authors explained that it could be attributed to the late side effects from the surgery and/or the radiotherapy which resulted in an impairment of the sensory function in the oral cavity. Medications such as NSAIDS, corticosteroids, and opioids used to treat cancer pain may also desensitise nociceptive afferents [
39].
Salivary function
The observed reduction in salivary function of cancer patients is consistent with previous findings [
43‐
47]. Radiotherapy causes tissue damage in the radiation field. In the case of HNC, this includes severe, and sometimes permanent, damage to the salivary gland which influenced the amount and composition of saliva production [
45,
46]. A reduction in parotid and submandibular glands volumes was observed 3 months after radiotherapy in the oral cavity [
46], therefore reducing the salivary quantity. In addition, chemotherapeutic agents such as 5-fluorouracil and doxorubicin used by the patients also induced hyposalivation [
48].
Quantity, but not quality (pH and buffering capacity) of saliva, was significantly different between the two groups. In addition to having less saliva production, cancer patients also produced thicker saliva. This may be attributed to the radiosensitivity of the different salivary glands. Parotid glands, responsible for producing most of the watery saliva, were shown to be more affected by radiation compared to submandibular glands which produce more viscous and mucin-rich saliva [
43,
44,
49].
In terms of salivary quality, most of the patients were assessed more than 1 year after radiotherapy (Table
3) and had acidic saliva (pH < 6.8). Patients who were observed more than 1 year after the end of their radiotherapy showed higher salivary functions compared to those observed less than a year after the end of their radiotherapy. This is consistent with previous studies, which demonstrated a significant decrease in salivary pH after radiation but began to increase between 6 months and 2 years post-radiation, although it did not recover to the initial pH of 7.0 [
45,
46]. These two longitudinal studies also showed that buffering capacity decreased upon radiation but recovered to normal at 6 months post-radiotherapy [
45,
46], which also supported our findings.
In terms of food perception, saliva is an essential component influencing the perception of taste, smell, texture, temperature, and astringency [
32,
50]. The lubricating property of saliva is necessary for mastication, bolus formation, and swallowing, so the lack of it may lead to eating problems [
7]. The correlations between salivary function and the perception of texture were observed in a previous study [
37] but in the present study, the correlations were not evident.
This study presents some limitations, for instance, the cross-sectional design does not permit to infer causality. A longitudinal study following patients across different treatments and time points would have allowed observations on the progression of their somatosensory perception. The study involved a rather heterogenous population regarding the treatment type and duration since treatment, therefore unable to discern whether the changes were caused by certain treatments or the disease itself. Further, as the test was conducted at different times of the day and periods of the year, it may influence the measurements of salivary function. Different testing locations for the two groups could potentially introduce contextual influence on perception. In addition, patients treated with radiotherapy have an enlarged periodontal ligament, which is a valuable indicator of proprioception and texture. It would therefore be interesting to study the contribution of the periodontal ligament to texture in HNC patients.