Background: The trigeminal nerve is a mixed cranial nerve responsible for the motor innervation of the masticatory muscles and the sensory innervation of the face, including the nasal cavities. Through its nasal innervation, we perceive sensations, such as cooling, tingling, and burning, while the trigeminal system mediates the perception of airflow. However, the intranasal trigeminal system has received little attention in the clinical evaluation of patients with nasal pathology. Summary: Testing methods that enable the clinical assessment of intranasal trigeminal function have recently been developed. This study aims to present the current clinical methods that can be utilised in everyday practice, as described in the literature. These methods include four assessment techniques: (1) the quick screening test of trigeminal sensitivity involves patients rating the intensity of ammonium vapour presented in a lipstick-like container. (2) The lateralisation test requires subjects to identify which nasal cavity is being stimulated by a trigeminal stimulus, such as eucalyptol or menthol, while the other side receives an odourless stimulus. (3) The trigeminal sticks test evaluates the trigeminal function similarly to the olfactory function using sticks filled with trigeminal stimulant liquids. (4) The automated CO2 stimulation device is used for measuring trigeminal pain thresholds, utilising intranasal CO2 stimuli to define the pain threshold. Key Messages: Assessing intranasal trigeminal function clinically may prove useful in evaluating rhinology patients, particularly those who encounter nasal obstruction without anatomical blockage and those experiencing olfactory disorders with suspected trigeminal dysfunction. Despite their limitations, the presented methods may provide useful information about nasal patency, chemosensitivity, and pain sensation in the daily clinical practice of such patients, leading to better therapeutic decisions.

The trigeminal nerve, the largest cranial nerve, is divided into three branches: ophthalmic, maxillary, and mandibular nerves. It innervates both nasal cavities, with the anterior part of the nasal cavity innervated by branches of the ophthalmic nerve (anterior ethmoidal nerve and infraorbital nerve) and the posterior part innervated by fibres of the maxillary branch (posterior superior medial nasal nerve and nasopalatine nerve) [1].

The intranasal trigeminal system mediates sensations, such as burning, warming, irritation, tickling, cooling, and stinging, through special receptors known as transient receptor potential (TRP) channels. These receptors are sensitive to specific temperature ranges and chemicals [1, 2]. TRP channel receptors, including subtypes such as TRPV1, TRPV3, TRPM8, and TRPA1, play a fundamental role in trigeminal perception [1, 3]. For example, TRPV1 is activated by temperatures above 42°C and chemical stimuli, such as capsaicin, eugenol, camphor, acids, and CO2. These activations are associated with sensations of tingling, sharpness, burning, and pain. TRPV3 responds to temperatures starting at 39°C and chemical substances, such as thymol, causing a warm sensation. TRPM8 is activated at temperatures of 8°C–23°C and by compounds such as menthol and eucalyptol. The stimulation of TRPM8 evokes a cooling and fresh sensation [1, 3]. Temperatures below 18°C and compounds such as allyl isothiocyanate activate the TRPA1 receptor, resulting in a dull, burning, and painful sensation. Furthermore, we perceive nasal airflow during breathing through specific trigeminal fibres (TRPM8 receptors) activated by nasal mucosal cooling [1, 3‒6]. Recently, solitary chemosensory cells have been identified as trigeminally innervated nasal epithelial cells with bitter receptors located at their apical ends, contributing to nasal defence mechanisms [7, 8].

The main challenge when assessing the trigeminal system is that most trigeminal stimuli also activate the sense of smell. However, trigeminal sensations can be separated from olfactory sensations using various methods [9]. These methods include (a) using stimuli that exclusively activate the trigeminal system, (b) instructing subjects to focus on the trigeminal sensation and disregard the olfactory sensation, (c) using tasks that rely solely on trigeminal activation, and (d) evaluating patients with anosmia.

The trigeminal nerve plays multiple roles with contribution in olfaction, nasal patency, and nasal defence mechanisms, holding a dominant position in nasal health status. Therefore, its clinical assessment may provide information for a more accurate and evidence-based clinical management. However, it seems more practical to assess intranasal trigeminal function only in specific rhinological conditions at clinical practice. For example, it can be initially used in patients for whom there is evidence of potential low trigeminal sensitivity such as patients scheduled for septoplasty, patients with empty nose syndrome, patients with nasal obstruction without specific clinical findings, patients suspected of having olfactory dysfunction [6, 10‒16].

Various methods have been described for assessing intranasal trigeminal function, including psychophysical, electrophysiological (event-related potentials, negative mucosal potentials, and magnetoencephalography), imaging techniques (functional magnetic resonance imaging to measure the ratio of oxygenated haemoglobin/deoxygenated haemoglobin in the brain and rarely PET), reflexes (pupillary responses to intranasal trigeminal stimulation), and changes in respiration (trigeminally induced breathing changes during deep and moderate propofol-induced sedation and apnoea threshold elevation in the elderly) [1, 3, 17‒24]. However, in this review, our focus is on measurement techniques that can be easily applied by the majority of rhinology clinics providing relatively quick information about intranasal trigeminal function. We are not referring to electrophysiological and imaging techniques that require specialised equipment only available in a limited number of specialised centres.

The use of intensity ratings in response to ammonium vapour, a known stimulant of trigeminal receptors such as TRPV1 and TRPA1, has been proposed as an easy and reliable screening tool to measure intranasal trigeminal function [25]. As most trigeminal irritants typically activate multiple types of receptors [1, 3], the ratings are expected to provide information about olfactory dysfunction and subjective nasal obstruction.

Method

The trigeminal stimulus is presented using a commercially available lipstick-like container (AmmoLa®, Devesa Dr. Reingraber GmbH, Muggensturm, Germany) (shown in Fig. 1). Patients rate its intensity on a visual analogue scale (VAS), ranging from 0 to 100, where 0 represents no feeling of stimulus and 100 a very strong stimulus.

Fig. 1.

Intensity ratings of trigeminal stimulation. The arrow shows the lipstick-like container of ammonium vapour.

Fig. 1.

Intensity ratings of trigeminal stimulation. The arrow shows the lipstick-like container of ammonium vapour.

Close modal

The authors correlated the results with detailed measures of olfactory sensitivity in patients with olfactory dysfunction. They suggested using the 10th percentile as the cut-off score to differentiate normal from abnormal intranasal trigeminal function. Ratings below 15% indicated the possibility of intranasal trigeminal dysfunction and olfactory dysfunction. Intranasal trigeminal ratings exhibited a positive correlation with olfactory testing (TDI score). Furthermore, a significant positive correlation between the trigeminal ratings and the odour discrimination scores was particularly observed in post-infectious cases [25].

Weaknesses and Strengths

This test is simple, inexpensive, easy to use, and not time consuming, as it takes only approximately 2 min to perform. Firstly, it can support the clinical assessment of patients complaining of nasal obstruction. Specifically, patients without anatomical findings of obstruction but experiencing a blocked nose should undergo screening. In the case of low-intensity ratings, further investigation is necessary before making any decisions regarding surgery. Secondly, it can be helpful in detecting potential disparities between trigeminal and olfactory functions. If a patient presents with olfactory dysfunction and high trigeminal ratings (>94%), physicians should conduct further investigations to explore medical conditions associated with olfactory loss but without an intranasal trigeminal dysfunction (e.g., congenital anosmia). However, its reliability is lower than that of other trigeminal tests, and the availability of normative data is limited [25] (Table 1).

Table 1.

Characteristics of each trigeminal test

Intensity ratings of trigeminal stimulationLateralization testTrigeminal sticks testCO2 pain threshold test
Price $$ $$$ $$$$ 
Availability of normative data Limited Less limited Limited Limited 
Commercial availability Yes No No No 
Reliability Yes (low) Yes Yes Yes 
Validity NR Yes NR Yes 
Changes with age No Yes Yes Yes 
Changes with gender NR Yes Yes Yes 
Changes with CRS NR Yes Yes NR 
Duration of test 2 min 40 min 20 min 7 min 
Software available No No No Yes 
Correlation with airflow NR Yes (low) NR NR 
Correlation with rated airflow NR Yes NR NR 
Correlation with trigeminal Yes Yes Yes Yes 
Correlation with olfactory Yes Yes Yes Yes 
Intensity ratings of trigeminal stimulationLateralization testTrigeminal sticks testCO2 pain threshold test
Price $$ $$$ $$$$ 
Availability of normative data Limited Less limited Limited Limited 
Commercial availability Yes No No No 
Reliability Yes (low) Yes Yes Yes 
Validity NR Yes NR Yes 
Changes with age No Yes Yes Yes 
Changes with gender NR Yes Yes Yes 
Changes with CRS NR Yes Yes NR 
Duration of test 2 min 40 min 20 min 7 min 
Software available No No No Yes 
Correlation with airflow NR Yes (low) NR NR 
Correlation with rated airflow NR Yes NR NR 
Correlation with trigeminal Yes Yes Yes Yes 
Correlation with olfactory Yes Yes Yes Yes 

NR, non-reported.

References used: [3, 14, 20, 21, 25‒29].

In Summary

Strengths: simple application, short duration, commercial availability (for ammonium).

Weaknesses: high probability of confusion of olfactory and trigeminal stimulation, lack of normative data.

This is the most widely used trigeminal test in rhinology. It is based on the observation that when a pure odorant is administered to only one nostril, humans can barely identify which nostril was stimulated [1, 26‒28, 30‒32]. We are not good at localising pure odorants that exclusively activate the olfactory system, but improvement can be achieved through training or by means of mixed olfactory/trigeminal odours [1, 27, 30, 31, 33‒35]. As most odorants also stimulate the trigeminal system, especially at greater concentrations, pure olfactory, odorants are uncommon [1]. In the lateralisation test, stimuli are simultaneously presented to both nasal cavities, and the subject is asked to determine their lateralisation, which can only be done through the trigeminal nerve [26, 27, 29‒32, 36].

Method

A trigeminal stimulus is applied to one side, while the other side receives an odourless solution. Menthol or eucalyptol is typically used as the trigeminal stimulus in this test, with eucalyptol being the most commonly used stimulus in most studies. Lateralisation scores for eucalyptol and menthol show high congruency and correlation [34]. Both substances activate the intranasal trigeminal nerve through TRPM8 receptors, providing a cooling and fresh sensation along with a sense of improved nasal patency [1, 3, 37]. Two polyethylene bottles with a volume of 250 mL are presented to a blindfolded subject (shown in Fig. 2). Each bottle is equipped with a pop-up spout inserted into each nostril. One bottle contains 15 mL of 50% menthol in propylene glycol. The second bottle is filled with 15 mL of either odourless propylene glycol solvent or white mineral oil. By pressing the handheld squeezing device, both bottles release a 15 mL puff of air into both nasal cavities. It should be noted that there have been reports indicating that the test can be performed without the need for a handheld device [21, 27].

Fig. 2.

Lateralisation test. The yellow asterisk indicates the handheld device of lateralisation test, while the blue one the polyethylene bottle containing 15 mL of 50% menthol in propylene glycol. The arrow indicates the pop-up spout placed into nostril.

Fig. 2.

Lateralisation test. The yellow asterisk indicates the handheld device of lateralisation test, while the blue one the polyethylene bottle containing 15 mL of 50% menthol in propylene glycol. The arrow indicates the pop-up spout placed into nostril.

Close modal

Immediately after each attempt, the subject indicates whether the trigeminal stimulation (i.e., cold, fresh sensation) was perceived on the right or left side. The lateralisation test consists of 40 intranasal stimuli presented in a pseudorandomised sequence, with each nasal cavity stimulated 20 times. A 30–40 s interval is required between each stimulus. Thus, lateralisation test is scored out of 40.

Previous studies have shown that a score below 26 can be considered as a threshold for low intranasal trigeminal sensitivity [13, 28]. The lateralisation test demonstrated that individuals older than 35 years had lower lateralisation scores (mean 29.8) than those younger than 35 years (mean 34.2). In addition, there was no significant gender effect on the lateralisation scores of healthy subjects [26].

Weaknesses and Strengths

The lateralisation test is a relatively time-consuming procedure, lasting approximately 40 min for 40 stimuli, but it is a simple test for the examiner. Although there is a lack of extensive normative data, the existing literature provides sufficient information compared with other tests. The handheld device is not commercially available, but it can be easily constructed (e.g., using 3D printing). Its relatively small size makes it portable. Moreover, the lateralisation test can be conducted without the handheld device, using only two bottles. This test is a good choice for assessing trigeminal function especially when subjective nasal obstruction cannot be explained with clinical findings. The shortened version of the lateralisation test has been also found to have good test-retest reliability [14] (Table 1).

In Summary

Strengths: simple application, little probability of confusion of olfactory and trigeminal stimulation, test-retest-reliability.

Weaknesses: need of apparatus, no commercial availability, long duration, moderate lack of normative data.

The trigeminal sticks test has been recently developed, following a similar approach to the Sniffin’ Sticks test for olfactory function (Burghart Medical Technology, Wedel, Germany) [14, 38]. The difference between the two tests lies in the fact that the trigeminal sticks are filled with substances containing potent trigeminal compounds, such as menthol, diallylsulphide, ethanol, propanol, eucalyptol, or camphor [9, 39‒41]. These six substances activate different chemosensory receptor channels of the trigeminal nerve, namely, TRPM8, which is activated by eucalyptol, camphor and menthol; TRPA1, which is activated by diallylsulphide; and TRPV1, which is activated by propanol and ethanol [1, 4, 18]. The investigator uncaps each pen for approximately 3 s and presents it to the patient, holding it about 2 cm in front of both nostrils [14] (shown in Fig. 3).

Fig. 3.

Trigeminal sticks test. Pens with red cap, containing menthol, are used for threshold score. Pens with green cap, containing trigeminal substances, are used for identification score and along with pens with blue cap, containing odorous substances are used for discrimination score.

Fig. 3.

Trigeminal sticks test. Pens with red cap, containing menthol, are used for threshold score. Pens with green cap, containing trigeminal substances, are used for identification score and along with pens with blue cap, containing odorous substances are used for discrimination score.

Close modal

The test comprises three distinct subsets: (1) trigeminal threshold, (2) trigeminal discrimination, and (3) an identification task. This is combined with a shortened version of the lateralisation test (20 stimuli given as described above).

Method

Trigeminal Threshold

The trigeminal threshold is assessed using various concentrations of menthol, diluted in propylene glycol, in a geometric series (1:2 dilutions). During this test, subjects wear a sleeping mask to prevent visual identification. They are presented with 10 different triplets of pens in a randomised order: one pen with menthol and two pens with a “blank” or odourless solvent without menthol [36]. The subjects are then asked to identify the pen that elicits a different sensation compared with the other two pens, described as a cool, burning, stinging, or irritating sensation. There should be a 30 s interval between the presentations of the three pens, each lasting approximately 10 s. An ascending staircase procedure is performed with the triplets of pens presented to the subjects, with seven reversals. The average of the last four reversals of the staircase determines the threshold [14]. Threshold testing is scored out of 10.

Trigeminal Discrimination

The discrimination between trigeminal and odorous sensations is evaluated using six triplets of sticks. In each triplet, two pens contain an odorous substance (from the olfactory identification test of the Sniffin’ Sticks test), and one pen contains a substance with a trigeminal valence (ethanol, menthol, diallylsulphide, propanol, camphor, or eucalyptol). Blindfolded subjects are asked to identify, within each triplet, the pen that elicits a strong trigeminal sensation. The interval between the presentations of the triplets of sticks lasts for 30 s, with a 3 s interval between the individual pens. Discrimination testing is scored out of six [14].

Trigeminal Identification

The identification of the trigeminal sensation is evaluated using four trigeminal pens used in the discrimination test. These four pens (ethanol, menthol, diallylsulphide, and eucalyptol) are presented to the subjects in a randomised order with intervals of 30 s. In addition, five cards with descriptors of the substances, namely (1) astringent and pungent, (2) warm and burning, (3) sneezing, tickling, and scratching, (4) prickling, and (5) cold and fresh, are provided to the subjects [32]. The correct answers are “cold and fresh” for menthol, ethanol and eucalyptol, and “astringent and pungent” for diallylsulphide. Identification testing is scored out of four. Based on 86 healthy subjects, the range for the threshold subset was 7.20 ± 2.47–8.71 ± 1.29, for discrimination 4.12 ± 1.24–4.63 ± 1.27, and for identification 1.46 ± 0.90–2.70 ± 0.79 [14].

Threshold, identification, and lateralisation performance decreases with age. Patients with chronic rhinosinusitis (CRS) and olfactory disorders had higher detection thresholds than controls. The discrimination and identification scores were found better in the controls than in patients with olfactory disorders. Finally, lateralisation scores were lower in patients with CRSwNP and CRSsNP than in controls [14].

Weaknesses and Strengths

The test-retest reliability of the trigeminal sticks test has been found to be satisfactory for the threshold and identification subsets, similar to that of the Sniffin’ Sticks test [14]. The trigeminal sticks test is a simple and practical tool that takes approximately 20 min to assess trigeminal chemosensory function. It is portable, but the sticks are not yet commercially available. The main limitation of this test is the simultaneous activation of the olfactory along with the trigeminal system. Therefore, participants are instructed to focus on the trigeminal sensation and disregard the olfactory sensation. However, data about this test are limited, as only one study has used it [14] (Table 1).

In Summary

Strengths: relatively simple application, test-retest-reliability.

Weaknesses: high probability of confusion of olfactory and trigeminal stimulation, no commercial availability, relatively long duration, lack of normative data.

This examination requires a device that presents intranasal CO2 stimuli with a steady flow at varying intervals [20].

Method

The components of the device include a high-pressure bottle of CO2, a solenoid valve that allows the flow of CO2, a high-precision control valve for configuring the desired flow, a sensor for accurately measuring the CO2 mass flow, a response button for collecting the subject’s reply to the stimulus, a filament tube and a microcontroller that facilitates the automation of the procedure (shown in Fig. 4). Moreover, the device provides a user-friendly interface with an interactive menu that enables the selection of different testing modes and the presentation of the results to the user/doctor.

Fig. 4.

CO2 pain threshold test. The white asterisk shows the high-pressure bottle of CO2. The green asterisk shows the device, while the blue one the pc with the software. The yellow arrow shows the nasal cannula, and the blue one the response button.

Fig. 4.

CO2 pain threshold test. The white asterisk shows the high-pressure bottle of CO2. The green asterisk shows the device, while the blue one the pc with the software. The yellow arrow shows the nasal cannula, and the blue one the response button.

Close modal

The test can be performed separately in each nasal cavity or simultaneously in both using a double nasal cannula. The ascending staircase method is used to detect the pain threshold. The subject is instructed to press the response button when he or she feels a slight burn or irritation after a stimulus, and then a stimulus of a shorter duration is presented. If the subject does not respond, a stimulus of a longer duration is presented. The goal is to collect at least three turning points. A turning point is defined as a stimulus that elicits a response from the subject when the previous one does not, similar to how an audiogram is performed. The intranasal pain threshold of the trigeminal nerve is calculated as the average of the three stimuli that were classified as turning points.

Literature with this device shows that CO2 pain responsiveness is lower in patients with olfactory loss than in individuals with normosmia. The pain threshold not only correlates with olfactory function but also with ageing, with females being more sensitive to CO2 stimulations [20].

Weaknesses and Strengths

The CO2 pain threshold test is a simple and automated procedure with good test-retest reliability that can be used for the rapid assessment of trigeminal function in relation to pain [20]. A strong advantage of this test is that it does not activate the olfactory system because CO2 has little or no smell. It is not a lengthy procedure, with a mean test time of about 7 min, according to our experience. Examiners should keep in mind that short inter-stimulus intervals (e.g., less than 20 s) may lead to habituation and yield unreliable results [13, 35, 42]. A weakness of the method is the lack of commercially available devices. It is also not easily portable, mainly due to the CO2 bottle. The lack of normative data requires further studies to establish its reliability (Table 1).

In Summary

Strengths: simple and automated procedure, relatively short duration, no confusion of olfactory and trigeminal stimulation, test-retest-reliability.

Weaknesses: need of apparatus, no commercial availability, lack of normative data.

The intranasal trigeminal function is involved in nasal airflow perception, olfaction, and pain sensation after the activation of specific trigeminal receptors [1, 4]. In clinical practise, we can assess the various aspects of the intranasal trigeminal nerve by means of the described tests above, although there is some overlap between them. Specifically, (1) the intensity ratings in response to ammonium vapour and the trigeminal sticks test mainly reflect the interaction of trigeminal with the olfactory system, (2) the CO2 pain threshold test has a direct relationship with pain sensation, and (3) the lateralisation test predominantly reflects nasal patency sensation.

The intranasal trigeminal system has not been studied as extensively as the olfactory system. Most odorants stimulate both the olfactory and trigeminal nerves at high concentrations [9, 30, 32, 34]. Both systems exhibit close interaction at the central and peripheral neural processing levels [43, 44]. However, the trigeminal system is more resistant to environmental, toxic, and irritant factors. The ammonium intensity rating is a simple screening test that may contribute to the evaluation of trigeminal function and possibly to some extent of olfactory function [25]. In addition, the trigeminal sticks test is a new psychophysical technique easily used in everyday clinical practice as many Rhinologists have the experience how to use similar olfactory tests (e.g., Sniffin’ Sticks battery test). A limitation of this technique, in a similar way to the lateralisation test, is that the trigeminal stimuli can activate the olfactory system simultaneously, although the subjects are asked to focus on the trigeminal sensations during the test. Wysocki et al. previously described a method through which we can assess true trigeminal thresholds; however, it is a time-consuming procedure [45]. The fact that these tests can discriminate between patients with olfactory dysfunction and normal subjects reflects the interaction of the two systems and the trigeminal contribution to what subjects perceive as smell. However, this should not be interpreted by clinicians as options for alternative olfactory tests. Olfactory function should always be tested by ordinary available tests, such as the Sniffin’ Sticks test and the UPSIT. Additionally, the trigeminal sticks test appears to be able to distinguish patients with CRS from normal subjects [37].

TRPV1 receptors are associated with intranasal pain sensation [1, 3]. CO2 is a component of inhaled and exhaled air, almost odourless; thus, it is a good option for intranasal stimulation of the trigeminal nerve [20]. The automated CO2 stimulation diagnostic device described above can detect the intranasal pain threshold of the trigeminal nerve with speed and accuracy. CO2 pain responsiveness is lower in patients with olfactory loss than subjects with normosmia, and it is correlated with olfactory function. Moreover, it has been shown that trigeminal system is more sensitive in females and young subjects [20]. Another study using CO2 has suggested that nasal anatomy relates to trigeminal sensitivity, although its impact does not appear to be significant [46]. A disadvantage of this device, similar to the majority of the described methods, is that it is not commercially available and its construction requires special personnel.

Several studies on nasal obstruction have assessed intranasal trigeminal function using the lateralisation test. For example, patients with empty nose syndrome seem to have impaired intranasal trigeminal function [13]. Using the same test, patients with CRS were shown to have reduced intranasal trigeminal function [15]. Interestingly the lateralisation scores of symptomatic patients with nasal septum deviation were lower on the deviated side of the nasal cavity than those of asymptomatic controls with a same nasal septum deviation [47]. In terms of olfactory dysfunction, patients with olfactory loss exhibited lower trigeminal sensitivity than healthy subjects [26]. As the lateralisation test lasts approximately 40 min, it is difficult to be part of every rhinological assessment. Thus, our suggestion is to be included in the diagnostic procedure of specific conditions, such as in patients with a sense of blocked nose without clinical findings of nasal obstruction.

The anterior part of the nose is considered the most important for nasal patency, being the location of the nasal valve a structure which regulates nasal resistance and it houses the majority of chemosensory receptors [48]. In addition, it seems that the anatomy of the anterior nasal cavity is related with the intranasal trigeminal sensitivity [49]. Perceived nasal patency is significantly affected by air humidity, and it has been suggested that we perceive nasal airflow through mucosal cooling rather than air temperature alone. Furthermore, the peak heat loss posterior to the nasal vestibule is significantly correlated with the subjective perception of nasal patency in normal healthy subjects [5, 50]. The subjective sensation of a patent nose can also be induced by menthol after activation of TRPM8 receptors, which mediates the cooling sensation. Patients with olfactory loss may experience a subjective change in nasal patency after chewing gum containing menthol, depending on their olfactory test score [51]. However, it should be noted that menthol does not affect nasal mucosal temperature and nasal airflow, making clear that subjective feeling of patent nose is resulted only by trigeminal receptors activation [29]. The sensation of nasal patency can also be evoked and modulated by a specific olfactory stimulant that selectively induces olfactory perception. Therefore, patients may benefit from being exposed to odours, as they do with menthol inhalation in the common cold [52].

A low score in the lateralisation test, intensity ratings of ammonium vapour, and trigeminal sticks test signifies reduced intranasal trigeminal function. Conversely, a low threshold score in the CO2 pain test indicates normal or increased trigeminal function. A variety of nasal conditions referred to in this review, such as olfactory dysfunction, nasal obstruction without clinical evidence, CRS, empty nose syndrome, etc., are implicated with trigeminal dysfunction. Thus, an impaired score related to these conditions may be useful in the diagnostic procedure helping for better therapeutic decisions. For example, surgical treatment for nasal obstruction does not always result in symptom relief [6]. In such cases, a preoperative dysfunction of the intranasal trigeminal system may predispose patients to suboptimal outcomes from surgery. In the literature, a poor correlation between nasal airway measurements (e.g., rhinomanometry) and subjective ratings of nasal patency is often found. The missing gap may be the lack of information about trigeminal function. Ultimately, an improvement of more than 3 VAS points after septoplasty can be predicted by a lateralisation test score of >31.5 (88% sensitivity) [16].

Summarizing the clinical value of the described tests, clinicians should keep in mind that when testing intranasal trigeminal function using the quick screening test of intensity ratings or a more detailed approach such as the trigeminal stick test, the results will be contaminated by olfactory stimulation. Measurements of olfactory function cannot be replaced by the above-mentioned trigeminal tests and should always be performed by ordinary olfactory tests. If we want to quickly assess the trigeminal function related to pain, we can use the CO2 pain threshold. If the goal is to assess the trigeminal function related to nasal patency sensation, then the lateralisation test is the appropriate choice.

The measurement of intranasal trigeminal function contributes to a better understanding of the complex functions of the nose. Easily applicable clinical tools, such as the tests described in this review, can be used in everyday clinical practice despite their limitations. However, an improvement in the availability and reliability of normative data is required. Additional data from both normal subjects and patients can serve as a basis to possibly modify guidelines for the management of unexplained nasal obstruction and olfactory dysfunction.

The authors have received written informed consent for the publication of details and any accompanying images of participants.

The authors have no conflicts of interest to declare.

The authors did not receive any funding.

Konstantinos Garefis: literature search and drafting of the manuscript. Dimitrios Markou: drafting a part of the manuscript. Angelos Chatziavramidis: design and manuscript review. Vasilios Nikolaidis and Konstantinos Markou: manuscript review. Iordanis Konstantinidis: conception of idea, design, and expert manuscript review.

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