EGM-thresholds
A statistically significance difference was observed between the values prior to starting the therapy and one week after (p=0.02, Cohen’s d-Test=0.35). At that time, patients had complaints of hypogeusia (n=6), allogeusia (n=1) and dysgeusia (n=1). When the third measurements were made, hypogeusia was reported by every patient, while 3 patients also complained of dysgeusia. A further deterioration in EGM-threshold values was observed in the subsequent measurements, which included the follow-up measurements following RCT. Table 1 shows the mean EGM-thresholds of the evaluated patients.
Structural alterations associated with the fungiform papillae
The fPap shape and vascularisation of the tip of the tongue exhibited a significant deterioration when therapy was being delivered based on the classification criteria of Negoro. It appeared that the fPap parameters (form and vascularisation) had worsening tension once the treatment had been completed.
Prior to RCT, there were significant correlations among the EGM-thresholds and morphology (r=-0.719, p<0.05) as well as vascularisation of fPap (r=-0.719, p<0.05). Once RCT had been initiated, a strong positive correlation was only detected between EGM and vascularisation (r=0.763, p<0.05) one week subsequent to the initiation of therapy. In the follow-up measurements, no additional correlations were observed.
Discussion
In the current study, we demonstrated that an improvement in EGM thresholds does not occur concurrent to the morphology and vascularisation of fPap. Vascularisation is the first parameter that appears to show signs of improvement. A further important result is that additional deterioration of EGM-thresholds appears to occur even after the RCT treatment has been completed. Our findings support the theory that when nerve fibres responsible for innervating taste buds are damaged by irradiation, taste cells could be indirectly destroyed due to the fact that mature taste cells need functional nerve contact in order to be maintained [8].
After receiving RT treatment for HNC, patients commonly complain of taste dysfunction [1]. When receiving a 6-8-week daily radiotherapy course, a loss of taste generally manifests by weeks 3 to 4 and it is common for all taste modalities to be impacted. Previous studies have reported the worsening of EGM-thresholds during RCT for HNC [3]. The time at which taste cells are lost predominantly occurs when patients begin to experience functional taste loss, which is initially seen one week into the course of radiotherapy [9].
Low-level radiation delivered during the early stages could cause the receptors to be damaged. In a study, it was observed that when curative radiation therapy was given for HNC, salivary function had decreased by the end of week 1, and measurements clearly indicated that after the second week of treatment, taste function had been impaired [9]. Initial evaluations of taste changes reported by patients receiving radiation therapy have indicated that significant changes occur when the treatment is ongoing, and that taste sensations do not return to pre-treatment levels [1]. Such taste problems could be caused by direct impacts on the taste receptors, the reduction in the production of saliva leading to secondary infection, and limited delivery of tastants to receptor areas.
With regard to chemotherapy, taste disorder aetiology is not only because of damage caused to taste receptors and neurons by RCT, but it is probable that it is caused by multiple factors. Apart from chemotherapeutic agents, other drugs can also alter taste and must be taken into account when assessing taste in such patients [10].
In summary, there is no improvement in EGM-thresholds concurrent with the morphology and vascularisation of fPap. There has been minimal focus in the literature on taste dysfunction in patients with HNC, and it is probable that they are underestimated. Additional long-term studies of nerve fibres at the tongue tip subsequent to chemotherapy or radiotherapy in patients diagnosed with tonsillar squamous cell carcinomas are therefore necessary.