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The difference in the refractive indices between the air and the tear film results in the tear film having the greatest optical power of any part of the eye. Eyelid blinks are important for maintenance of the tear film. Between blinks there is progressive thinning of the tear film, which becomes non-uniform on the ocular surface and may break up. This produces an irregular air/tear interface, with a reduction in image quality.1 The longer the period between blinks the greater the effect on the tear film and reduction in vision. Restoration of the tear film occurs immediately following a blink so that the ability to maintain a regular tear film is dependent on the blink rate. A reduction in the blink rate such as, for example, a pause between blinks of 15 seconds has been associated with a change in the shape of the profile of the corneal tear film and up to a 6% reduction in visual acuity.2 More importantly, however, a reduction in blink rate leads to a reduction in contrast visual acuity.1
The ability to distinguish between different layers during surgery, such as the posterior capsule of the lens and anterior vitreous face, or peeling a layer off the retinal surface in vitreoretinal surgery, is dependent on the surgeon having and maintaining good contrast acuity. Blink rates and blink amplitude vary according to vision related behaviour and a reduction in the blink rate occurs with tasks of increasing visual difficulty.3 For example, visual tasks requiring concentration, such as video display terminal use, result in a decrease in average blink rate from 18.4/mm to 3.6 blinks/min.4
In order to determine whether the blink rate of ophthalmic surgeons alters during intraocular surgery, the blink rate patterns of nine ophthalmic surgeons were recorded. Two observers recorded the blink rate during casual conversation and when the surgeons were using the operating microscope. None of the surgeons were aware that their blink rates were being recorded, which was done by two medical students during their ophthalmic attachment—that is, their presence in the theatre was accepted as part of their ophthalmic training. The blink rate for each surgeon was recorded in each condition between four and 10 times. The mean blink rate for each surgeon during casual conversation and while operating are presented in Table 1. There was a significant reduction in the average blink rate between both conditions (16.69/min and 4.75/min, p= 0.0002 paired t test), on average a three and a half fold decrease occurred while operating. It was also noted that the onset of conversation such as the request for an instrument or demonstration of an intraocular structure, was associated with the onset of a blink response.
The reduction in blink rates observed in this study—that is, 16.69 to 4.75, are similar to that found by Patel et al for visual tasks such as video display terminal use. A reduction in blink rate to 4.75 translates to a reduction in contrast acuity of approximately 10% with four surgeons having an expected reduction of more than 60%.1 Such a reduction in contrast acuity is likely to have an effect on the ability to differentiate between different intraocular surgical layers. Blinking occurs between visual fixations and may be timed so as not to interfere with significant visual input.5 Blink rate is low when information memory is operating, and cognitive processes utilising display areas accessible to visual input are disrupted during the blackout period of a blink.5 Blinking is thus suspended during certain cognitive activities to avoid disrupting these processes.5
Tear evaporation is dependent on the width of the palpebral fissure and the surrounding humidity. Evaporation increases with widening of the palpebral aperture leading to a lower tear film stability. This has led to a recommendation that video display unit screens be kept below eye level. During ophthalmic surgery the eye pieces are roughly perpendicular to the face, so that it might be reasonable to lower the height of the eye pieces to reduce the width of the palpebral aperture. Frequent instillation of artificial tears during surgery is not practicable and might also create an abnormal tear film and image contrast. Likewise, increasing the humidity of the theatre may not be feasible. It might be reasonable to make surgeons, and anyone who uses the microscope, aware of this potential problem so that they can train themselves to blink during parts of the operation or investigation where good contrast acuity is not essential and where disruption of the cognitive processes is likely to have a minimal effect.