Electrogustometry: strengths, weakness, and clinical evidence of stimulus boundaries

Authors: J.A. Stillman, R.P. Morton, K.D. Hay, Z. Ahmad y D. Goldsmith.

Published in Clinical Otolaryngology. 2003,28, 406-410.

ARTICLE COMMENTED BY: NICOLÁS LEBRÓN BARRERA. SPORTS SCIENCE AND PHYSICAL ACTIVITY STUDEN (2018-2019). UNIVERSIDAD MIGUEL HERNÁNDEZ DE ELCHE

Electrogustometry is a tool used in the clinical setting for individuals who have a loss or alteration of taste, such as after certain surgeries in the middle ear area. One of the strengths of this tool is to be able to facilitate a meaningful comparison between patients, or their progress during their post-operative recovery. As weaknesses, it is necessary to point out the scarce sensitivity of the technique regarding the quality and variety of the flavor and the presence of a certain bias towards the acid flavors. Despite this, electrogustometry is used by health researchers to obtain information on changes in certain taste thresholds. However, the technique may trigger the activation of other sensors, such as temperature or pain sensors, which may make it difficult to interpret the results. With respect to the threshold, it is necessary to differentiate whether the objective is the detection itself or the identification of a specific flavour. In the study by Saito et al. (2001), the lowest stimulus in which the metallic or bitter taste was perceived was recorded as a threshold value. However, the authors agreed that this could be a detection threshold, as a stronger stimulus would be needed for a recognition threshold. Due to the variability in patient responses, psychophysical tests could be used to determine whether the responses are reliable. For example, tests with a forced choice between 2 alternatives (50% randomness) minimized the response bias, making the correct detections around 75%.

The establishment of specific physical-chemical parameters is essential for the reproducibility of the results. Thus, Ajdukovic’s study (1990) presents evidence that current intensity and electrode size are key to obtaining a taste experience. Therefore, perception will change if the electrode size increases, as it could affect several areas of the tongue with different thresholds. This author recommends electrodes of 50-100 mm2 with an intensity of 30 µA (1.53 µA /mm2). In this context, current intensity is a key factor. The device must be a constant current stimulator that causes the appearance of hydrogen ions in the saliva in contact with the anode and thereby decreases the pH. This will result in the stimulation of receptors activated by changes in ion concentration, such as those detecting acid and bitter tastes. The latter are achieved with higher current intensities. Stimulations of receptors that require activation of transduction pathways, such as sweet tastes, cannot be measured with these devices.

After certain surgeries in the middle ear area, denervation of certain taste receptors occurs. In these cases, stimulation of the trigeminal nerve is essential to recover part of the taste sensation. Different patients showed activation levels by electrogustometry at different current intensities (range 50-320 µA). In all cases, the patients perceived a burning sensation rather than an acidic or bitter taste. Similar results were obtained using citric acid, which was perceived as a burning sensation in the mouth.

The majority of the studies showed a relationship between electrical current and pH, so that the application of a greater stimulus would correspond to a greater decrease in pH. This acidification in the anode would produce trigeminal activation. Some studies concluded that with a 60 mm2 electrode, patients with trigeminal nerve involvement only stimulated taste sensations with currents less than or equal to 300 µA (5 µA /mm2). The same intensity-activation ratio is not shown in rats (Herness, 1985).

In summary, the authors concluded that above-average thresholds activate the trigeminal nerve. Two cases of individuals with unilateral loss of taste are presented as an example:

– A 39-year-old woman with altered taste due to central nervous system involvement. By means of electrogustometry, different thresholds are obtained for each side of the tongue. On the left side (16.5 µA-0.84 µA/mm2) and on the right side (163.0 µA-8.30 µA/mm2). Since it takes so much intensity to stimulate itself, the right side seems to be registering more a sense of touch than taste.

– 34-year-old male with posterior perforation of eardrum. Presented hearing loss (20-50 dB) and difficulty identifying citrus taste, confusing it with salty or bitter. Its electrogustomometric thresholds were recovered, being 295 (start) and 149 µA (in the second test) on the left side. On the right side were normal 6.5 (start) and 5.3 µA in the second test.

Finally, it is concluded that in order to make comparisons between electrogustometry tests, several factors must be taken into account:

– Type of threshold determined.

– Point of the psychometric function represented by the threshold.

– Psychophysical procedure

– Magnitudes of measure to carry out the stimulus.

– Characteristics of the device.

In addition, the responses associated with electrostimulation after a tympanic injury are possible. This is due to the activation of the trigeminal nerve. Although high levels of stimulation are necessary, above 5 µA/mm2.