Elsevier

Toxicology Letters

Volume 199, Issue 3, 15 December 2010, Pages 203-212
Toxicology Letters

Mini review
Ocular discomfort by environmental and personal risk factors altering the precorneal tear film

https://doi.org/10.1016/j.toxlet.2010.09.001Get rights and content

Abstract

Ocular discomfort (e.g. burning, dry and itching eyes) is among top 2 symptoms in office environments. The ophthalmological explanation is aqueous-deficient dry eye and evaporative dry eye and exposure to allergens, while indoor air pollutants causing chemesthesis generally is the rationale of the indoor environmental community. Review of salient environmental, occupational, and personal risk factors, that alter the precorneal tear film (PTF), reveals at least three mechanisms resulting in ocular discomfort. First, the PTF structure is altered by a physical process that increases the emission rate of aqua loss resulting in hyperosmolarity, gland dysfunctions, and associated discomfort. Second, the structural composition of the outermost lipid layer of the PTF is altered by aggressive aerosols and combustion products, both indoors and outdoors, that facilitate loss of aqua, and possibly chemesthesis. Third, strong sensory irritating pollutants cause chemesthesis by trigeminal stimulation. In general, organic and inorganic indoor air pollutant concentrations are too low causing chemesthesis, but the odor may cause reported discomfort. The total risk of ocular discomfort is exacerbated by physical alteration of the PTF by visual tasking and climate conditions (low humidity, high temperature, and draft); further, personal factors like age, gender and use of certain medication also influence the overall stability of the PTF.

Research highlights

▶ Ocular discomfort is among top two in office environments. ▶ At least three mechanisms are responsible for ocular discomfort: physical and chemical alteration of the precorneal tear film (PTF), and chemesthesis. ▶ Physical alteration of the PTF leads to increased rate of aqua loss resulting in hyperosmolarity. ▶ Ambient and aggressive pollutants alter the structural composition of the PTF leading to its instability. ▶ Few pollutants cause chemesthesis at indoor concentrations. ▶The total risk of ocular discomfort is a combination of all mechanisms with possibility of mutual exacerbation.

Introduction

Ocular discomfort (OCDIS) as eye irritation symptoms are abundantly reported in office-like environments; burning, dry, gritty, itching, irritated, sandy, and tired eyes are among top two reported symptoms in public office employees (e.g. Bluyssen et al., 1996, Brightman et al., 2008, Uchino et al., 2008). The symptoms are also among the most commonly reported complaints in the eye clinic (O’Brien and Collum, 2004) and dry eyes is the most common cause of chronic eye irritation in patients over age of 50 (Gilbard, 2009). A large number of women suffer from eye symptoms (e.g. McCarty et al., 1998, Smith et al., 2007, Uchino et al., 2008, Wolkoff et al., 2003).

There are two schools with different etiological approaches of dry eyes. The ophthalmological definition is “Dry eye is a multifactorial disease of tears and ocular surface (OS) that results in symptoms of discomfort, visual disturbance, and precorneal tear film (PTF) instability with potential damage to the OS” (Dry Eye Workshop, 2007a). Thus, dry eyes result in an altered or thinned PTF that elevates the emission rate of aqua (ERA) and tear (aqua) deficiency, and subsequent an increase of the osmotic stress in the PTF. Hypotheses are gland dysfunctions, e.g. meibomian (e.g. Foulks, 2007) that result in compositional changes of the outermost lipid layer of the PTF (OLL) and PTF, and presumed to increase of ERA (Borchman et al., 2010); recently, the hypofunction of the lachrymal glands has been proposed as a model for explanation of symptoms among VDU workers (Nakamura et al., 2010; cf. Chen et al., 2008). The phenomena are partly related to “dry eyes” as an eye disease, while, on the other hand, at least one major causative risk factor of semi-acute eye complaints in office-like environments is considered to be air pollutants causing chemesthesis (sensory irritation) by trigeminal stimulation of the OS.

A major and ever increasing fraction of the work force nowadays uses visual display unit (VDU) as a monitor. Further, the amount of visually highly demanding workplaces increases continuously, and so does the average age of the workforce.

Ocular discomfort has great impact on quality of life (e.g. Gayton, 2009, Smith et al., 2007), but also deterioration of the visual acuity is an important safety factor associated with the PTF quality and health (e.g. Smith et al., 2007, Montés-Micó, 2007). Alteration of the PTF, that affects its structural composition (Wojtowicz et al., 2009), may rapidly result in gland dysfunctions of the eyelids and conjunctiva and ultimately affect the visual acuity (Ohashi et al., 2006). Undoubted, OCDIS results in increased need for breaks, and if not alleviated, deterioration of work performance (e.g. Hedge et al., 1996, Henning et al., 1997), and ultimately in human error (Piccoli, 2003). It is clear that the acute, but episodic, OCDIS in office-like environments and the “dry eye disease” have multifactorial etiologies in common. Thus, efforts to clarify the pathophysiology of the outer eye causing OCDIS are important for better understanding and preventive measures.

This paper deals specifically with effects on the OS by environmental and occupational factors in office-like environments, more specifically the PTF, as opposed to effects in the inner eye (Sheedy et al., 2003). The paper provides the occupational hygienist, physician and ophthalmologist with information that bridges recent important findings in ophthalmology with occupational and indoor air science. This is done by the identification of environmental and occupational risk factors that alter the PTF or cause chemesthesis ultimately resulting in OCDIS; personal factors are also included.

Section snippets

Method

The terms “precorneal tear film” and eye irritation symptoms were searched with: air velocity, discomfort, draft, dry or irritating or itchy eyes, evaporation, exposure, humidity, indoor air quality, inflammation, offices, osmolarity, personal computer, sick building syndrome, temperature, and visual display unit work, volatile organic compounds (VOCs) in databases such as PubMed covering the literature from 2003 up to the present. Exclusion criteria were risk factors as: infection by airborne

Key diagnostic methods for the precorneal film

A large battery of PTF morphology and dry eye diagnostic tests is available (Guillon, 2002, Dry Eye Workshop, 2007b). Some of the more common (Korb, 2002) are listed in Table 1. The preference depends not only on the professional background, but also upon applicability in the clinic, versus in the laboratory and field investigations (cf. Wolkoff et al., 2003).

Environmental and occupational factors affecting the precorneal tear film by aqua loss

Maintenance of the PTF moisture is essential for its stability by avoidance of desiccation. A number of environmental and occupational risk factors significantly affect the ERA in a multifactorial etiology. Increased temperature and low relative humidity increase the ERA, the latter would also be the case at elevated altitudes (cf. Wolkoff, 2008); thus, elevated air velocity on the OS and temperature cause OCDIS (Kaczmarczyk et al., 2010) that probably increases at prolonged exposure.

Essential fatty acids

Omega-3-EFA deficiency results in an altered structure of the PTF (Butovich, 2009; cf. Joffre et al., 2008). Thus, diet appears to be essential to maintain an intact OLL by balancing the EFAs (Butovich, 2009, Roncone et al., 2010, Rosenberg and Asbell, 2010) that are an essential fraction of the meibum (Butovich et al., 2009). Topical application with unsaturated fatty acids is beneficial against inflammation and decreased fluorescein staining (Rashid et al., 2008). Vitamin C is also an

Discussion and conclusion

Ocular discomfort is among top two symptoms in office-like environments with highest prevalence. In order to identify causalities of OCDIS and minimize its prevalence there is a need to integrate mechanisms from different disciplines that alter the PTF resulting in the symptomatology. There appears to be a fluent borderline between the acute OCDIS and the dry eye disease due to their multifactorial etiologies. The objective of our analysis is to see how OCDIS depends on environmental,

Conflict of interest

The author declares having no conflict of interest.

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