Dark Adaptation Essay - Grade: A PDF

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Essay from the elderly perspective on dark adaptation and driving...

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Dark Adaptation and Driving Introduction Dark adaptation is an individual’s ability to cope with varying light intensities [ CITATION Bru15 \l 1033 ] . The human eyes are constantly required to adjust to well-lit and dim environments as we move about daily. Although dark adaptation can be described in brief, the process of the retinal photoreceptors adapting may take up to an hour [ CITATION Uni03 \l 1033 ]. For instance, entering a dark movie theatre from a well-lit setting is the beginning of dark adaptation. An individual would initially be unable to perceive anything as the photoreceptors have decreased sensitivity without a stimulus. Rods, the photoreceptor that is responsible for vision at low illumination levels [ CITATION Geond \l 1033 ] contain a protein called rhodopsin which is formed by Vitamin A, which is oxidized to 11-cis retinal and combines with opsin [ CITATION Uni031 \l 1033 ]. Progressively, the individual’s visual acuity, colour vision and contrast sensitivity would be affected. In an article written by[ CITATION Ang17 \l 1033 ], it was stated that there was an increase in the number of road accidents in United Kingdom (UK) that involved drivers over the age of 70. In 2011, a 16-year-old girl was killed by a driver who had poor eyesight. Moreover, a police officer had given a statement saying that a common factor regarding the older drivers were the underlying health issues. This is a cause of concern since eyesight is possibly the most important aspect required when driving at any time of the day. In this essay, we will discuss and analyze the need for visual tests to be regularly executed for drivers when they are 50 years old and every 10 years thereafter under various lighting conditions.

Why is Dark Adaptation important to Humans? There are certain occupations in which having good night vision is required. Some of these occupations include being a soldier, a police officer and even a night-driver [ CITATION Uma19 \l 1033 ] . According to [ CITATION GOVnd \l 1033 ], individuals driving cars must be able to read a vehicle number plate from 20 meters behind. Visual acuity in both eyes have to be 6/12, measured on Snellen chart and the visual field requirement for driving is 50 degrees on either side of the vertical meridian and 20 degrees above and below the horizontal meridian [ CITATION Sha19 \l 1033 ]. However, for drivers operating lorries and buses, visual acuity measured must be 6/7.5 in the better eye and a minimum of 6/60 in the other eye. The field of vision is slightly more crucial in this case with horizontal visual field being 160 degrees. In the UK, the average age of the bus drivers with full licenses is 54 as of August 2019 [ CITATION GOV19 \l 1033 ]. Out of 40,932,294 bus drivers with full license, 5,463,380 of them are aged 70 and above. Given that over 5 million of the bus drivers with full driving license are over 70 years old, changes in their pupillary size should be accounted for. The size of the pupils are prone to decrease as we age [ CITATION Med06 \l 1033 ]. One such condition is Senile Miosis. It occurs due to the weakening of the dilator muscles and ageing. Usually, it onsets when one hits the age of 70 and above [ CITATION Chr18 \l 1033 ]. This makes it more difficult for them to discern the objects that they are viewing and affects their spatial contrast sensitivity (Sloane, Owsley & Alvarez, 1988). Taking into consideration that they might drive at night, it is a matter of concern that their vision might be impaired at night making them more prone to accidents.

Figure 1. Pupil Diameter as Age Increases (Shahani, 2019)

With reference to the graph above, it is evident that the size of the pupil gets smaller with increasing age. How is Dark Adaptation Measured? Dark Adaptometry is one way to assess the degeneration of the retina. It measures the amount of time taken for the retina to recover its maximum sensitivity to low levels of illumination after being exposed to a bright light stimulus and then back to darkness. The test is known as Dark Adaptometer and the instrument predominantly used is the Goldmann-Weekers Adaptometer (Figure 2) (Deb, 2019).

First, the patient is exposed to a very bright light which is intended to bleach the photoreceptors. Instantaneously, they are exposed to darkness and at that moment, their threshold is plotted. At regularly timed intervals, flashes are repeated and the sensitivity to light slowly increases (Deb, 2019).

With reference to Figure 3 below, the upper section before the plateau represents the threshold of cones. The plateau labelled cone recovery represents the point when cones reach their maximum sensitivity and rods start to be sensitive to a certain extent. The rest of the plotted graph simply portrays the threshold of the rods [ CITATION Tri19 \l 1033 ].

Figure 2. Goldmann-Weekers Adaptometer

Figure 3. Dark Adaptation Curve

(Research Gate, 2005)

(Research Gate , 2008)

Clinical Significance of Dark Adaptation The execution of dark adaptometry allows the diagnosis of conditions affecting vision at night such as Retinitis Pigmentosa (RP), Diabetic Retinopathy and Glaucoma [ CITATION Tri19 \l 1033 ]. These conditions can be managed upon diagnosis. However, it is only considered clinically significant when the patient’s daily routine is disrupted. For instance, if their occupation or lifestyle requires them to drive at night, it will affect the routine that they are accustomed to. Moreover, they will be hazardous to people on the road as well.

Factors affecting Dark Adaptation According to (Kalloniatis & Luu, 2007), one of the many factors affecting dark adaptation includes rhodopsin regeneration. As stated by [ CITATION Tri19 \l 1033 ], opacities in the ocular media can also affect dark adaptation. Rhodopsin regeneration Focusing on the elderly, during the ageing process there is a considerable amount of decline in rod-mediated dark adaptation. This may be due to delayed rhodopsin regeneration (Jackson GR, Owsley C & McGwin G 1999). As a result, the elderly take a much longer time to dark adapt and often encounter vision problems at night. Furthermore, for the elderly people who are driving at night, it becomes even tougher to spot any possible obstacles such as someone dashing across the road from the side. Opacities in the Ocular Media A common opacity in the ocular media would be cataract. Cataract is the progressive clouding of the lens due to tissue breakdown and protein clumping. This condition also reduces the overall visual field sensitivity [ CITATION Rah03 \l 1033 ]. In the UK, the minimum visual field requirement for driving is 50 degrees on either side of the vertical meridian and 20 degrees above and below the horizontal meridian [ CITATION Sha19 \l 1033 ]. As such, drivers with cataract having restricted field of vision would be more prone to accidents and thus, are not being cautious about their own well-being as well as other road users.

Visual Acuity & Glare Sensitivity In a study done by (Gruber, Mosimann, Muri & Nef, 2013), it was stated that with increasing age, sensitivity to glare increases, even without the presence of ocular conditions. Furthermore, measuring an individual’s visual acuity only in photopic conditions is not a good indicator of night driving ability. Visual acuity in mesopic conditions and glare sensitivity are also relevant for driving at night. The elderly are more susceptible to visual glares than the young and middleaged, even more so if they are driving at night with glares from street lights and headlights from oncoming vehicles [ CITATION Jea77 \l 1033 ].

Ageing Process & Miotic Pupils The purpose of this study done by [ CITATION Slo88 \l 1033 ] was to analyze how the ageing process would affect spatial contrast sensitivity at dim illumination and assess if senile miosis actually causes sensitivity loss. In a total of 24 participants, 11 of them were older adults and the mean age of older participants was 73. It was stated that retinal illuminance is substantially reduced in the eyes of the older adults. Their loss in contrast sensitivity became more pronounced with an increase in spatial frequency. However, senile miosis was ruled out as a factor for affecting contrast sensitivity and the authors branched out to other possible causes like the density of the crystalline lens or hazy ocular media. This indicates that even though senile miosis was not the cause of impairing contrast sensitivity, their vision is still compromised due to other factors like cataract.

Discussion & Conclusion These aging-related changes in the retina and rod-mediated dark adaptation could possibly contribute to night vision problems commonly experienced by the elderly. As we age, we progressively lose the mechanisms that allows us to view a broad range of light intensities (Deb, 2019). The results of the studies above prove that the elderly are commonly affected by small pupil diameter, delayed rhodopsin regeneration, ocular media opacities like cataract and glare. I opine that the findings of the studies carried out corroborates my stand of the need to do visual tests. Currently, visual tests are only done in well-lit conditions and thus it is hard to ascertain if one has good driving ability at night. There are multiple factors affecting the visual acuity of the elderly at night as stated above, and I believe that for individuals aged 50 and above to drive, it should be made compulsory for them to retake their vision tests under all lighting conditions. There is a lack of research evaluating appropriate visual tests to assess if an individual is capable and has good night driving ability and further research is necessary in this area. Moving forward, researchers can analyze and study if visual tests under mesopic and scotopic conditions would be appropriate.

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