Computer Vision Syndrome
Presbyopia and Dry Eye

Dry Eyes

Background

Computer users commonly experience symptoms related to dry eyes. These symptoms can include:

There are several reasons why computer workers are at greater risk for experiencing dry eye. An important finding has been that the blink rate is significantly reduced during computer work. Patel et al (1991) measured blink rate by direct observation on a group of 16 subjects. The mean blink rate during conversation was 18.4 blinks/min and during computer use it was 3.6 - more than a five-fold decrease. Tsubota and Nakamori (1993) measured blink rates on 104 office workers. The mean blink rates were 22 blinks/min under relaxed conditions, 10 while reading a book on the table, and 7 while viewing text on a computer. Their data support the fact that blink rate decreases during computer use, but also show that another task such as reading a book can decrease the blink rate.

Although both book reading and computer work result in significantly decreased blink rates, a difference between them is that computer work usually requires a higher gaze angle resulting in an increased rate of tear evaporation. Tsubota and Nakamori measured a mean exposed ocular surface of 2.2 cm2 while subjects were relaxed, 1.2 cm2 while reading a book on the table, and 2.3 cm2 while working at a computer. The size of the ocular aperture is related to the gaze elevation - as we gaze higher the eyes are wider open. Since the primary route of tear elimination is through evaporation, and the amount of evaporation is roughly linear with ocular aperture area, the higher gaze angle when viewing a computer screen results in faster tear loss. Even though reading a book and computer work both reduce the blink rate, the computer worker is more at risk because of the higher gaze angle.

Probably the most common clinical assessment of the tear stability is evaluation of the tear breakup time (TBUT). This test is performed by instilling fluorescein in the tears (so that they can be seen with a slit lamp), holding the lids open, and then recording the length of time until the tears breakup and expose the dry cornea. Normal individuals have a TBUT of 25-35 seconds with a range of 15-45 seconds.

Breakup times of less than 10 seconds are considered to be clinically significant. Individuals with marginal TBUT are at greater risk for having dry eye symptoms.

There are many individuals with marginal TBUTs who will not have ocular drying with �normal� blink rates, but who will have regular ocular drying while working at a computer.

There are other factors that may also contribute to the dry eye. For example, the office air environment is often low in humidity and can contain contaminants - there is an extensive literature on sick building syndrome. It is also likely that the higher gaze angle results in a greater percentage of blinks that are incomplete. It has been suggested that incomplete blinks �don�t count� (Korb, 1993) because the tear lipids pool along the lower lid margin and during a full blink the upper eyelid grabs the lipids and spreads them over the aqueous tear layer to form the lipid or oily layer of the tears. An incomplete blink does not allow this to occur - therefore the evaporation-retarding properties of the lipid layer are lost.