Impact of salivary stimulation on tear film function in dry eye subjects (2024)

Abstract

Keywords: Dry eye, salivary stimulation, tear film

The lacrimal glands are paired exocrine glands, one in each eye. The main lacrimal gland consists of a large orbital part and a small palpebral part. The lacrimal gland is innervated by both parasympathetic and sympathetic nerve fibers.[1] The lacrimal gland is the main contributor to the aqueous layer of the tear film. It secretes proteins, electrolytes, and water, which help to nourish and protect the ocular surface. Lacrimal gland secretion is primarily under neural control, which is achieved through a neural reflex arc.

Any abnormality with the lacrimal gland can cause less production of tear film that spreads on the ocular surface, causing discomfort to the individual. With each blink, a thin coat of tears spreads like a film over the eye. It consists of superficial lipids, and middle aqueous and bottom mucin layers covering the cornea and conjunctiva. The tear film is composed of water, enzymes, solutes, proteins, immunoglobulins, lipids, metabolites, and exfoliated cells.[2] These three layers work together to maintain the health of the eyes by lubricating and warding off infections and also keep the surface of the eyes smooth and clear.[3]

According to the Tear Film and Ocular Surface Society (TFOS) Dry Eye Workshop (DEWS) II-”Dry eye is a multifactorial disease of the ocular surface, characterized by a loss of homeostasis of the tear film, and accompanied by ocular symptoms, in which tear film instability and hyperosmolarity, ocular surface inflammation and damage, and neurosensory abnormalities play etiological roles.”[4]

Dry eye is the most common reason for ophthalmic consultation. In epidemiological studies performed globally, the prevalence of dry eye disease ranges from 5% to 50%. Based on the data from the National Health and Wellness Survey, 6.8% of the United States adult population (approximately 16.4 million people) has been diagnosed with dry eye disease. The prevalence increased with age (2.7% in those 18 to 34 years old versus 18.6% in those ≥75 years old) and was higher in women than men (8.8 versus 4.5%).[5] The prevalence of dry eye disease (DED) is greatly influenced by geographic location, climatic conditions, and lifestyle of people. The prevalence of DED in North India is 32% within the age group of 21 to 40 years.[6] Visual display terminal (VDT) usage, smoking, and contact lens wearing were associated with increased odds of developing DED.[6]

DED is one of the most common reasons for patients to visit an eye care practitioner. Stinging, burning, redness, watering, scratchy sensation, and blurred vision are common symptoms of dry eyes. Patients having asymptomatic dry eyes present with complications, which include sterile stromal ulcers, blepharitis, conjunctivitis, band keratopathy, keratinization, and corneal vascularization.[7]

There are various tests for the diagnosis of dry eyes. They are tear film breakup time (TBUT), rose bengal staining, Lissamine green staining, Schirmer’s test, tear osmolarity, impression cytology, fluorophotometry, tear ferning test,[8] and measurement of tear meniscus height.[9] The symptomatic as well as asymptomatic dry eye disease can be treated with a wide range of modalities from lid hygiene and change in lifestyle to tissue grafting. The conventional management of dry eye is through the use of artificial tears.[10] Various other modalities of treatment of DED are hot compression, tear substitutes, punctal occlusion, anti-inflammatory therapy, tarsorrhaphy, conjunctival transplantation, and treatment of the underlying disease.[7] The 21^st century belongs to the digital generation with digital media being used for both professional and personal work for an extended period of time, leading to discomfort and dryness of the eye. The present study was carried out to explore the effectiveness of salivary stimulation to decrease the uneasiness and discomfort of the eye as using tear supplements frequently is not feasible.

Methods

The study was conducted on the premises of Sankara College of Optometry, Bengaluru. Permission was taken from the Institutional Scientific Research and Institutional Ethics Committees before conducting the study. Written informed consent was obtained from all subjects after explaining to them the procedure in the language understood by them. Inclusion criteria for the study were subjects within the age group of 18–25 years, minimum sleeping period of 56 h/week, and use of electronic gadgets not more than 6 h/day with a room humidity of 40–50%.[10] The subjects were both males and unmarried females (with a normal duration of menstruation within 21–35 days). Exclusion criteria were subjects with any systemic, ocular, and oral diseases, and those using eye cosmetics and tear supplements. A baseline examination was performed for all subjects along with tear film function tests, that is, Schirmer’s test I followed by TBUT, TMH, and Schirmer’s test II. TBUT and TMH were performed thrice and average readings were recorded. The room humidity was controlled throughout the testing.

The study used the following diagnostic criteria to identify dry eye subjects: TMH <0.20 ± 0.05 mm, TBUT ≤10 s, and Schirmer’s test I and II ≤10 mm in 5 min.

Salivation was induced in the subjects by giving tamarind candy orally (a soft slightly sour tamarind pulp mixed with sugar) for 5 min to all participants. Tear film function tests were repeated in the same order (Schirmer’s test I followed by TBUT, TMH, and Schirmer’s test II) within seconds after finishing the candy, 30 min, and 60 min after the salivation. TBUT and TMH were performed thrice and average readings along with the humidity of the day were recorded.

Examiner bias was overcome by adding a co-investigator for baseline measurements so that the principal investigator was unaware of the baseline readings of subjects. Only after the completion of data collection, the proformas were handed over to the principal investigator by the co-investigator.

Statistical analysis was performed using SPSS version 20.0. A descriptive analysis was performed for demographic details. One-way repeated measure analysis of variance (ANOVA) was performed to compare multiple measures (Bonferroni’s test) of the variables (TBUT, TMH, Schirmer’s tests I and II) recorded from the same subjects at four different time periods (i.e., baseline, immediate, 30, and 60 min). The comparison was done from baseline to immediate, baseline to 30 min, and baseline to 60 min.

Results

Discussion

The study performed by Pramanik et al.,[12] showed that there was a significant increment in the readings of Schirmer’s strip length (though not mentioned as I or II), where the subjects were asked to keep a piece of Lopsy candy (a sour fruit pulp mixed with sugar that is sweet and sour in taste) in the mouth for 5 min and tear production was assessed during salivation. There was a quantitative change in tear film (confounding factors were not taken into consideration). However, the qualitative changes were not recorded. The current study, where all confounding factors were controlled, showed that stimulation of salivation with a tamarind candy resulted in a statistically significant increment in tear film production, both quantitively and qualitatively.

The current study also demonstrated that there was a statistically significant change in tear film functions that lasted until 30 min after the stimulation of salivation.

In the study “Optical Quality and Tear Film Analysis Before and After Intranasal Stimulation in Patients with Dry Eye Syndrome,” Passi et al.,[13] showed that the optical quality of patients with DES improved with the use of intranasal neurostimulation for tear production. Also, Kossler et al.[14] showed that chronic lacrimal nerve stimulation with an implantable pulse generator increased aqueous tear production in Dutch belted rabbits and also chronic lacrimal nerve stimulation did not result in any lacrimal gland damage.

The taste sensation from the anterior two-thirds of the tongue is carried by the seventh cranial (facial) nerve; it also carries preganglionic (secretomotor) parasympathetic fibers, which innervate the submandibular and sublingual salivary and lacrimal glands as well as the glands of nasal and palatine mucosa.[6] Because of the involvement of common pathways, when stimulated (chewing of tamarind candy), activation of efferent fibers of the seventh cranial nerve helps in the enhanced secretion of tears from the lacrimal glands in addition to increased salivation from the salivary glands [Fig. 1].[6]

Our study demonstrated positive changes (both statistically and clinically) in the tear film using tamarind candy, only for 5 min. It can be given on a daily basis for a longer duration as a therapy to see its long-lasting effect on tear film function. However, alternatives to tamarind candy need to be explored because the use of tamarind candy for a long period of time may itself cause side effects with regard to the oral health of the subject. Nevertheless, salivary stimulation can be considered one of the alternative modalities of treatment for dry eye subjects because instilling tear supplements is not feasible always.

Conclusion

This study showed a significant improvement in the quality as well as quantity of tear film until 30 min of stimulation of salivation by tamarind candies in dry eye subjects.

Acknowledgement

Dr.T.R.Raju, Director, Research, Sankara Academy of Vision, Bangalore.

References