Is contrast sensitivity relevant to optometric practice?
Closing Date: 09/04/2021
In keeping with guidance from the College of Optometrists (2015), routine optometric assessments typically include the measurement of visual acuity, refractive and binocular vision status, visual fields and ocular health. However, it is less common for optometrists to assess contrast sensitivity (CS). This raises the question as to whether measuring contrast sensitivity is relevant to optometric practice. Do CS measurements provide additional information? Are there valid reasons for including contrast sensitivity in a routine eye examination? Should this test be part of a standard low vision assessment?
After a brief refresher of what contrast sensitivity is, this article will explore CS from a medical perspective with a focus on assessing visual function to aid diagnosis and treatment of ocular and general medical conditions. It will then consider the social perspective of contrast sensitivity and its influences, with a particular focus on functional vision, task performance and participation.
Brief revision of contrast sensitivity
The ability to resolve targets varies significantly with the contrast of the target. The visual world a patient inhabits is one of varying target size and contrast and, furthermore, diseases affecting vision may affect the ability to resolve these targets in a selective manner.1 Therefore, the use of high contrast targets for acuity testing has been criticised as non-representative of the visual world and less than sensitive at reflecting visual reduction due to disease. If a patient is shown a sine-wave grating of a constant
spatial frequency, as shown in figure 1, their ability to resolve the grating reduces as the contrast is reduced until a point is reached when it can no longer be resolved. This point, the contrast threshold (the reciprocal of which is called the contrast sensitivity), is different for different spatial frequencies and the plot of the threshold values against spatial frequency is described as the contrast sensitivity function (CSF) as shown in figure 2.
Figure 1: A sine wave grating with a low spatial frequency (left) and a higher spatial frequency (right). As contrast is reduced, the point at which the viewer can no longer discern the lines is called the contrast threshold, the inverse of which is the contrast sensitivityFigure 2: The contrast sensitivity function or curve. Note that the CS differs for different spatial frequencies
Snellen acuity relates to the resolution of a high contrast target of maximum spatial frequency and is therefore represented as the cut-off point on the horizontal axis on this curve. This is sensitive to conditions affecting mainly high spatial frequencies, such as refractive error, but less sensitive if lower spatial frequencies are affected, as with cataract, corneal disturbance and contact lens wear.
Increasingly clinicians are using targets of different contrast to assess the influence on acuity. LogMAR charts are available in different contrasts so assessing the patient’s ability to resolve increasing spatial frequencies at a given contrast value. Computerised acuity charts, such as the Thomson Test Chart 2020 shown in figure 3, allow any contrast value to be pre-set.
Figure 3: Electronic test charts (such as the Thomson Test Chart 2020 here) can display a range of different charts at different percentage contrast settings. For accuracy, the screen needs to be calibrated correctly
Other charts, such as the Pelli-Robson (figure 4), use a constant letter size (approximating to one cycle per degree if viewed at 1m) and gradually reducing contrast as the chart is read by the patient. In theory, varying the working distance would allow the whole contrast sensitivity function to be assessed with a Pelli-Robson chart, but in practice this is rarely needed as high and low
contrast acuity scores combined with contrast sensitivity at 1m with a Pelli-Robson is usually sufficient to suggest any visual compromise. The Mars Letter Contrast Sensitivity Test (initially known as the Lighthouse Letter Contrast Sensitivity Test) is similar in design to the Pelli-Robson Test and has been found to have somewhat improved repeatability.
Figure 4: The Pelli-Robson chart is standard for measuring CS in many low vision clinics. The lowest contrast triplet discerned is recorded as a numerical value
Other contrast charts available for clinical practice include the Vistech (figure 5). This is a poster with five rows of circles upon which is printed a grating. Each row represents a constant spatial frequency but, as the patient looks along the row from left to right, the contrast of the grating decreases until it is no longer possible to state in which direction the grating is orientated (to the left or to the right for example). This way, a score representing the limit of contrast resolution for each pre-defined spatial frequency may be noted and plotted as a CSF curve. The test is limited, however, by the necessarily limited number of targets viewed.
Figure 5: The Vistech CS chart
The primary purpose of a medical assessment is to diagnose and treat conditions with a view to preserve and restore health. In the optometric practice this concerns ocular health in the first place, but it also includes general health. For example, a retinal examination may reveal ocular disease but may also reveal signs of general health conditions, such as cardiovascular disease. Some conditions, however, show reduced contrast sensitivity before the onset of ocular manifestations. In this case, contrast sensitivity could act as a biomarker.
Contrast sensitivity and systemic disease
Optometrists play a key role in the early detection of diabetes.2 Early diagnosis and intervention leads to a better health outcome for diabetic patients. Traditionally, early detection of diabetes in the optometric practice is based on retinal signs (figure 6) and changes in visual acuity and refractive error. However, there is evidence that CS is often reduced in early diabetic retinopathy and in pre-diabetes before changes in visual acuity.3-7 This suggests that neural dysfunction can occur before other structural changes develop.
Figure 6: Diabetic retinopathy. Reduction in contrast sensitivity may be a useful early biomarker
Reduced CS and colour vision can also be indicators of neurodegenerative and neurological conditions.8 These biomarkers can aid in early diagnosis in conditions such as Huntington’s disease,9,10 Parkinson’s disease,11 Alzheimer’s disease,12 multiple sclerosis13 and optic chiasm compression due to pituitary adenomas.14
Contrast sensitivity and ocular disease
Although CS loss could be an early sign of the aforementioned conditions, CS loss is also associated with a number of common ocular conditions.15
Tear film instability and ocular surface irregularity
Patients with tear film or ocular surface abnormalities often complain of reduced vision despite normal visual acuity. It is worth assessing CS in these cases to explain their symptoms. Meibomian gland dysfunction, epiphora, dry eye-related superficial punctate keratopathy and reduced tear-film stability are associated with reduced CS.16-19 Also, an irregular corneal shape due to keratoconus is associated with reduced CS rather than reduced acuity (figure 7).20
Figure 7: Irregular cornea due to keratoconus may impact CS before any major acuity change
Cataract is known to have an impact on CS, although in many optometric practices and cataract assessment clinics, this function is not measured routinely. Cataract extraction and intraocular lens implantation can improve visual outcomes in terms of visual acuity, near vision and CS.21-24 In patients with concurring macular degeneration, CS is likely to improve after cataract surgery.25
Diabetic eye disease
As mentioned in an earlier section, diabetic retinopathy is associated with reduced CS. This tends to be more severe as the disease progresses.26,27 This can lead to problems in functional vision. For example, the risk of falls is associated with DRP severity28 and with low CS in Type 2 diabetes.29
Retinal vascular disease
Retinal vascular disease can impact upon CS as well as visual acuity. The impact of treatment on CS is an important outcome measure for patients with conditions such as central or branch retinal vein occlusion (figure 8). It has been demonstrated that intravitreal ranibizumab injections can improve CS, even in the absence of improved visual acuity.30 Systemic cardiovascular conditions can also impact on CS. For example, variability in orthostatic blood pressure is associated with reduced CS.31
Figure 8: Contrast sensitivity is an important indicator of the effectiveness of management of retinal vein occlusion
CS typically reduces as AMD progresses.32 This is important as CS impacts upon health-related and vision-related quality of life in the many people with AMD. Reduced CS, as well as reduced visual acuity and low luminance visual acuity, have an negative effect on face recognition and reading speed.33-38
In terms of monitoring disease progression, visual acuity and visual fields are the mainstream visual function tests for glaucoma. Visual field impairments are associated with an increase of falls and self-reported difficulties with mobility in glaucoma.39,40 However, binocular visual acuity and CS appear to be the best predictors for the ability to perform daily activities in patients with glaucoma.41 Reduced CS is a common feature of glaucoma42-44 and it increases the risk of impaired face recognition45 and Charles Bonnet syndrome.39 Appropriate advice about contrast, lighting and glare management is indicated. This will also be helpful in view of the common issue of reduced dark adaptation and glare symptoms in glaucoma.46
Retinitis pigmentosa (RP)
It is common for patients with inherited retinal degenerations, such as retinitis pigmentosa (RP, figure 9) to experience poor
visual function due to a loss of CS when visual acuity is relatively unaffected.47 Other symptoms that are associated with RP are visual field restriction, impaired dark adaptation and reduced visual acuity. Each of these parameters have an impact on reading, mobility and scanning.48
Figure 9: It is common for patients with retinitis pigmentosa to have a loss of contrast sensitivity even when visual acuity is relatively unaffected
Cerebral visual impairment
Cerebral visual impairment (CVI) is a complex condition which affects basic processing functions as well as higher processing functions. Clinical presentation is variable. Reduced visual acuity is found in the majority of patients. Reduced CS is common among children with CVI.49-51 Other functional impairments include abnormal optokinetic nystagmus, strabismus, ocular motility disorders and field impairments. The majority of patients with CVI have visual-perceptual disorders.49 Children and families benefit from a thorough assessment of visual functions, including CS, to characterise the visual impairments. This informs the practitioner about the visual world as perceived by the child and aids in optimising the child’s functioning and learning. When one understands what is not seen, one can offer learning material in a format that can be perceived and accessed by the child.
Stroke patients often experience visual field impairments, such as hemianopia and lower field impairments. Primary pathway lesions tend to result in field impairment without reduced visual attention. However, field impairments from lesions in the higher processing pathway are often associated with myriad other features. Visual inattention and functional impairment in the ‘seeing field’ are common. CS can be severely reduced in the ‘seeing field’.52-54 This has consequences for daily functioning and rehabilitation strategies.
Other eye conditions
Reduced CS is associated with albinism.55 Prematurely born children, children with developmental delay, cerebral palsy or Down syndrome have a higher rate of reduced CS, and CS is often reduced in children with amblyopia, even in the non-amblyopic eye. Optic neuritis is also associated with impaired CS.56
In summary, CS is associated with a number of ocular and medical health conditions. Although it can be a helpful tool for early detection of these conditions, it can be difficult to discern to which condition the impaired contrast sensitivity is attributable or indeed if it is attributable to more than one condition. In any case, CS is a helpful tool for understanding the patient’s functional vision.
Task performance and quality of life
When a patient has impaired CS, the underlying condition needs to be addressed first. Any interventions to preserve or restore sight need to be considered. In some situations, impaired CS is permanent and one has to consider options to improve functional vision. In some cases, reduced CS can be a stronger predictor for experiencing poor vision compared to visual acuity and visual field impairment.37,47,57,58 Assessment of CS is therefore indicated if one wishes to understand how a condition changes the visual world for patients and how it impacts on daily functioning and quality of life.56
Mobility performance depends on a number of factors, such as age, visual fields, central visual function, visual attention and CS.59 It is suggested that CS is a stronger predictive factor for mobility performance than visual acuity.60 In terms of visual field impairments, it appears that lower field defects tend to cause more problems with mobility,61,62 whereas peripheral field defects tend to cause more collisions with other pedestrians.63
A thorough assessment of all the different parameters of visual functions can form the basis for mobility training. Patients with visual field impairment could benefit from scanning techniques,64 while reduced CS necessitates the use of contrast to prevent falls in the home, school and work environment. Reduced CS renders it difficult to detect floor boundaries and to judge the depth of kerbs and steps. Tactile ground surface indicators in public spaces can aid mobility. The benefit is greater when these indicators have a high level of luminance contrast.66 The use of a cane or hiking pole can be used to feel the ground ahead, which helps with detecting uneven ground, kerbs and steps.
It is well known that visual acuity has an impact on reading speed. As a rule of thumb for fluent reading tasks, an acuity reserve of 2:1 or even 3:1 is desirable.66 When using magnifiers, one also has to bear in mind that a reduced field of view with stronger magnifiers affects reading speed as well.66 Another visual function that influences reading speed is CS.67,68 For fluent reading, the contrast of the print needs to be several times threshold,69,70 from 4x for spot reading up to 10x for fluent reading. Reduced reading speed and CS are associated with poorer vision related quality of life.38 Tables 1 and 2 provide approximate contrasts of everyday objects and reading material. Table 3 explains how contrast sensitivity values as a percentage relate to log contrast and severity of loss on different test charts.
Table 1: Contrast of commonly encountered printed materials
Table 2: Contrast of a range of everyday objects
Table 3: Understanding contrast sensitivity measurements
CS loss is common in elderly people, especially in the presence of eye conditions such as cataract and AMD. The impact of CS on driving safety is well documented.71,72 However, CS is not routinely tested in the context of fitness to drive. The College of Optometrists recommends regular sight tests for drivers, especially in people aged over 60 in order to ensure that peo ple meet driving standards. Currently, the Driver and Vehicle Licencing Agency (DVLA) vision standards state a minimum requirement of 6/12 (best corrected) Snellen vision.73
If patients attend for a sight test, the optometrist is responsible for giving the correct advice about driving standards. It is not safe to assume people with vision below the driving standards will have stopped driving of their own accord. Documentation of having advised patients of any risks of continued driving is particularly important in the case of driving due to the liability issues in the case of a road traffic accident. If a patient is not meeting driving standards, the optometrist needs to advise the patient that they have a legal duty to inform the DVLA about their condition.
The College of Optometrists’ (2020) guidance recommends that the practitioner should put any advice offered about driving in writing and give it to the patient, keep a copy of this letter in the records, and inform the patient’s GP (providing that the patient have given full consent for this). A report by The General Optical Council (in 2017) revealed that optometrists’ views and actions regarding vision and driving are not consistent. Patients who are unfit to drive do not always receive advice about their lack of fitness from their eye care professionals. In some instances, advice is given, but it is often not fully documented in the records.
Some practitioners find it challenging to initiate a conversation about fitness to drive, perhaps because this can be devastating news for the patient. Other practitioners are unsure about the criteria for driving, as a number plate test does not correlate well with the vision test in the consulting room. One has to bear in mind that optometrists can use their clinical judgement and advise the patient to contact the DVLA if in doubt. It is the DVLA’s responsibility to decide if a patient has to give up their licence. If the vision appears borderline for driving, it can be helpful to carry out a CS test to create a more informed clinical picture. This can also be helpful in terms of explaining to the patient why it is no longer safe to drive. Road signs are designed to be visible at a great distance, although poor contrast sensitivity does affect the ability to discriminate traffic signs (figure 10).74 If a driver only uses familiar routes, visual acuity may seem less relevant to their driving ability. However, the inability to see unexpected obstacles or people due to a loss of CS is a realistic concern. A patient is more likely to willingly give up driving when the evidence makes sense.
Figure 10: Road signs are designed to be visible at a great distance
Visual acuity and CS are important factors for recognising faces and facial expressions.75 Difficulties with facial recognition and interpretation are a frequent complaint in a low vision clinic and can lead to social isolation, embarrassment and difficulties in communication. This problem is even more evident if a patient has a concurrent hearing impairment. Reducing back-light
scatter, by avoiding light sources behind the person being addressed, significantly improves the contrast of facial features.
Case scenario 1
An elderly housebound lady with disciform scars attends the low vision clinic. She has given up knitting due to her eyesight problems. Binocular distance acuity is 0.70 LogMAR (6/30 Snellen) and she reads N25 with her glasses, improved to N8 with a 3.5x illuminated hand magnifier. Her CS is significantly reduced (19% on MARS chart) and she cannot identify any plates on the Ishihara colour test.
In this example, magnification on its own is unlikely to solve the problem of not seeing the knitting stitches. However, if the colour and contrast impairment are taken into account, one can find a more suitable solution. This lady was advised to use thicker wool and needles in a contrasting colour (not too dark and contrasting against the colour of the knitting needles). She was also given some ideas for simple projects that do not require studying instructions from a knitting pattern. Furthermore, she was issued with a base for the 3.5x illuminated hand magnifier, which allowed her to see her knitting work without the need to hold the magnifier (figure 11). The next visit, she brought along some beautiful knitted scarfs, and more importantly, she was able to enjoy her hobby once more.
Figure 11: Modern hand magnifiers may be supplied with a stand
Case scenario 2
A diabetic patient with background retinopathy, a binocular distance acuity of 0.32 LogMAR (6/12 Snellen) and no visual field impairment has lost confidence going down the stairs (figure 12). This has affected her independence. Her CS was reduced to 21% (significant loss). This patient was referred to the Eye Clinic Liaison Officer (ECLO) to arrange mobility training and a home assessment with a view to improve lighting around the staircase, to introduce contrasting banisters and floor markings indoors, and to improve visibility of the outdoor entrance to her home.
Figure 12: Poor lighting around stairs represents a significant safety concern for those with reduced CS
In summary, the measurement of contrast sensitivity is an undervalued test in optometric practice. It is a valuable tool for the detection of general health and ocular conditions, monitoring change and appreciating treatment outcomes. Quality of life and daily functioning can be reduced due to impaired contrast sensitivity with a significant number of ocular conditions and health conditions. Understanding the impact of contrast sensitivity loss is a first step in offering appropriate advice and support.
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