Tests for Potential Vision

Specimen Collection

Tests for potential vision gather data starting from the retina up to the visual cortex. While some tests, like the electroretinogram, test the photoreceptors and bipolar cells, others, like the visually evoked potential, test the entire visual pathway up to the visual cortex. They allow for evaluating the integrity and functioning of the visual system.

Procedures

Visual Acuity

Although this is not a test for potential vision, it is a test that needs to be performed at the very beginning of the ocular examination.[4] Visual acuity is defined as the reciprocal of the minimum resolvable angle. It is a subjective method of assessing macular function. Different components of visual acuity include distance vision, near vision, color vision, and contrast sensitivity, which can be measured using different methods.[5]

• Distant vision – Snellen's chart, Landolt C chart, Sheridan Gardiner HOTV test.[6]
• Near vision – Jaeger chart, Roman chart, and Snellen near vision chart.[7]
• Color vision – Ishiharas pseudoisochromatic plates, Farnsworth Munsell 100 Hue test, Green lantern test and Anomaloscopes.[8]
• Contrast sensitivity – Pelli-Robson, Regan, Arden-Grating charts.[9][10]

Brightness Acuity Test 

It is a subjective test that can be performed in clear or slightly opaque media.[11] A simple device is used to test glare disability. The device can simulate three bright light conditions, which are as follows:

• Direct overhead sunlight - the high setting of the device
• Partly cloudy day - medium setting
• Bright overhead commercial lighting - low setting

Visual acuity is measured in all three conditions, and if reduced in comparison to the baseline acuity, the patient is said to have a glare disability. The glare disability is usually seen due to anterior segment pathologies like cataracts, whereas posterior segment diseases will not cause a significant deterioration in vision on exposure to light.

Photostress Test  

It is a subjective test that can be performed in clear media as well as in the presence of media opacities.[12] It tests only the retinal function and is independent of the rest of the visual pathway. It measures the Photostress Recovery Time (PSRT). PSRT is defined as the time it takes for the visual acuity to return to baseline after the photoreceptors in the retina have been bleached by a bright light source. When a bright light falls on the retina, the visual pigments are bleached, producing retinal insensitivity, which is perceived as a scotoma. The PSRT is the time needed to regenerate the visual pigment, which causes the scotoma to disappear.

Procedure

• The distant visual acuity of both eyes is measured.
• The patient is asked to remove their spectacles.
• The bad eye is occluded, and the normal/better eye is tested first
• A direct ophthalmoscope or any other bright source of light is held 2 to 3 cm in front of the eye being tested.
• The patient is asked to look directly at the light, which is turned off after 10 seconds.
• The patient is asked to wear their spectacles and asked to read one line above the original visual acuity on the distant vision chart as soon as the after images disappear and the optotypes are just visible.
• The time taken to read 2 or 3 optotypes is recorded as the PSRT.
• The same procedure is repeated for the other eye.

Amsler Grid

It is a subjective test that can be performed in clear media as well as in the presence of mild media opacities.[13] It is a grid of horizontal and vertical lines used to monitor a person's central 20 degrees of the visual field. It can be an alternative to a central 10-degree visual field test. There are 7 Amsler charts that can be used for home monitoring of macular disease.

• Chart 1/ Standard grid – It tests the general subjective response to distortions in the pattern. It consists of 400 small squares, each measuring 5 mm and subtending 1 degree on the retina when held at 30 cm. It also has a central white dot. If the patient cannot see the white dot, they can be considered to have a central scotoma.
• Chart 2 – It is used if the patient has a central scotoma, as tested by chart 1. It looks like chart 1 and additionally has 2 diagonal white lines running across the grid. These diagonal lines help in determining the limits of the central scotoma.
• Chart 3 – It is primarily used to detect color scotomas and can also be used to check color desaturation in optic nerve diseases. It can also be used along with red–green dissociative glasses in patients with suspected malingering. It consists of a similar grid as found in chart 1, with the only difference being that the lines of the grid are red in color.
• Chart 4 – It is used to differentiate between a scotoma and metamorphopsia, which can be seen in macular edema. 
• Chart 5 – It is used in patients with metamorphopsia to identify the specific meridians that are affected. The chart has parallel lines with a central white dot for fixation. The chart can be rotated to change the meridian.
• Chart 6 – It is used for detailed central evaluation. It is the same as chart 5, with the only difference being that the parallel lines are placed closer to each other, 0.5 degrees above and below the fixation point.
• Chart 7 – It is used to diagnose subtle macular diseases. It has a fine central grid where each square measures 2.5 mm and subtends 0.5 degrees when held at 30cm. This grid can be used to evaluate the central 8 degrees of the visual field. 

Procedure

• The patient is seated, and an appropriate near add is given, enabling visualization at 30 cm. Multifocal/progressive addition lenses should be avoided.
• The test is performed for each eye separately. The non-viewing eye is occluded.
• The appropriate chart is held at 30 cm with the ambient room lights switched on. The patient is asked to fixate on the center of the grid and then asked whether all four corners of the grid are still visible. Then the patient is asked whether any of the squares in the grid look blurry or wavy.
• The test is repeated with any of the other charts deemed appropriate.
• The patient records the defects by drawing any findings directly on the Amsler recording sheet or by the doctor based on the patient's explanation.

2-Point Discrimination

It is a subjective test for the potential vision that requires clear media for testing.[14] It is ideal for kids and uncooperative patients but can be done on anyone. Two illuminated light points of 2 mm diameter are held 2 inches apart, 2 feet away from the patient. If the patient is able to distinguish the two lights separately, there is good retinal function.

Entoptic Phenomenon

It is a subjective test that can be performed even in the presence of opaque media.[15][16] It is the visualization of reproducible visual perceptions that arise within the eye due to intraocular blood vessels, opacities in the media, colored halos, or phosphenes. The opacities that are far from the retina are not perceived as their umbral shadows do not reach the retina.

The opacities closer to the retina are perceived as their umbral shadows are projected on the retina. Normally, blood vessels on the retina are not visualized as the photoreceptors below them are adapted to the reduced level of illumination. However, if the light is thrown at an unusual angle, the shadow of the blood vessels falls on un- adapted photoreceptors and thus becomes visible. These visible blood vessels are called the Purkinje tree, and this technique is called the Purkinje tree entoptic test.[17]

Procedure

1. Transillumination method – A small flashlight is placed over the closed eyelid at the lateral canthus. The light is constantly oscillated to prevent local adaptation. 
2. Pinhole disc method – The patient is made to look at a bright background through a pinhole in a disc that is constantly moving.
3. Blue field entopic phenomenon – The patient is asked to look at a bright and diffusely illuminated surface with accommodation relaxed. The background is illumination by blue light (350 to 450 nm), which is maximally absorbed by hemoglobin.
4. Luminous spots that move in patterns similar to the retinal capillaries can be seen by the patient. These are believed to be red blood cells because:
   1. No spots are seen in the area corresponding to the foveal avascular zone.
   2. The movement occurs in curved, sinusoidal patterns that simulate capillary loops of the vascular tree.
   3. If intraocular pressure is raised above 50 mm of Hg, the movement ceases.
   4. They are best seen at 350 to 450 nm, which is the peak absorption wavelength of hemoglobin found inside red blood cells.

Maddox Rod Test

It is a subjective test that can be performed even in the presence of opaque media.[18] A Maddox rod consists of a set of toric lenses that converts a point source of light to a line. It is usually used to diagnose different types of strabismus. 

Procedure 

• The patient is made to sit 6 meters away from a light source.
• A Maddox rod is placed in front of the eye to be examined monocularly. The Maddox rod converts the point source of light to a line that is seen by the patient.
• The Maddox rod is rotated - vertical, horizontal, and oblique and the patient is asked what direction the line is oriented in and whether the line is continuous or not.

Potential Acuity Meter (PAM)

It is a subjective test that can be performed even in opaque media. It is designed to check the visual acuity due to the retina, and it bypasses the lens and the cornea. It is a device that attaches to the slit lamp and emits a 0.15 mm diameter point light source in the pupillary plane that is aimed through the clear portions of the cataractous lens.[19] 

It is useful in predicting the post-operative visual outcome after cataract surgery in patients with retinal pathologies like dry age-related macular degeneration and those with choroidal neovascularization.[20][21] The PAM can also be used to predict vision after Nd-Yag capsulotomy in patients with posterior capsular opacity.[22]

Procedure

• The pupil is dilated before the test to allow the light to pass through different parts of the lens.
• The patient is seated at a slit lamp, and the PAM projects a miniature Snellen's chart through the less dense parts of the cataract.
• The patient is asked to read the Snellen letters of different sizes. 

 Advantages

• The PAM shows a strong correlation with post-operative Snellen's visual acuity.[20]
• It is easy to use and easy to explain to the patient.

Laser Interferometry

It is a subjective test that can be performed even in the presence of opaque media.[23] It is able to predict acuity up to 20/20. Two coherent light beams are used in laser interferometry to create a three-dimensional interference fringe on the retina.[24][25] By varying the distance between the 2 light beams, different acuities can be predicted.

The different models available are:

• Heine Lambda retinometer[26]
• Lotmar visometer[27]

 Procedure

• The patient's pupil is dilated.
• The interferometer is used to project a fringe pattern onto the retina.
• The patient is asked to recognize the direction of the fringes – vertical, horizontal, or oblique.
• The fringes are made finer until the patient is unable to identify the orientation.
• The endpoint fringe pitch is noted and correlated to Snellen's visual acuity using a chart provided.

Advantages

• Unaffected by the optics of the eye.
• Easier to use
• More accurate than PAM in predicting visual acuity in patients with a macular hole[28]

Critical Flicker Fusion Frequency (CFFF)

It is a subjective test that can be performed in opaque media. Critical flicker fusion frequency (CFF or CFFF) is defined as the frequency at which flickering light can be perceived as continuous. It is used to assess the processing of temporal vision.[29] The critical flicker fusion threshold (or threshold for flicker fusion, TFF), which denotes the fastest rate of flickering light that the visual system can see, is used to characterize visual processing. 

During the CFF test, the patient is asked to fixate on a light-emitting diode that emits light that oscillates at a gradually increasing frequency. The patient is then asked to press a button when they believe the light to be continuous. Critical flicker fusion thresholds are reduced in retinal and neural diseases but resistant to degradation due to media opacities.[30]

Electroretinogram (ERG)

It is an objective test that can be performed even in opaque media.[31] It is a record of the changes in the eye's resting potential caused by a flash of light. It is the summation of the action potentials of the retina. As a test for potential vision, an ERG can be used to record the integrity of the rods and cones, bipolar cells, and retinal pigment epithelial cells.

Procedure

• Three electrodes are placed on the patient –
  • Active electrode – corneal contact lens, gold wick electrodes in the conjunctival sac, or gold foil electrodes on the eyelids.
  • Reference electrode – placed at the lateral canthus.
  • Ground electrode – placed at the nasal canthus.
• A Ganzfeld stimulus is used to stimulate the retina, and the changes in the resting potential are recorded as a graph.
• A scotopic ERG is first performed after pupil dilation and after dark adaptation for 20 mins. Increasing intensities of the stimulus are presented to the retina. The recordings obtained are referred to as the dark-adapted ERG.
• A photopic ERG is then performed after 10 minutes of light adaptation. Increasing intensities are presented to the retina, and the light-adapted ERGs are recorded.
• There are different types of ERGs –
  • Full-field ERG/ Ganzfeld ERG – there is uniform stimulation of the whole retina.
  • Pattern ERG – records the macular response to an alternating checkerboard stimulus.
  • Multifocal ERG – It records the response of the central 40 to 50 degrees of the retina to a 61-103 hexagonal foci stimulus.

Visually Evoked Potentials (VEP)

It is an objective test that can be performed in cloudy media.[32] It is an electroencephalogram taken from the visual cortex. Since the macula has the largest representation within the visual cortex, the VEP produces a macula-dominated response. The test cannot localize the pathology but, instead, is an indicator of the integrity of the visual system.

Procedure

• Scalp electrodes are usually placed in the occipital area as it is closest to the visual cortex. There are two systems of placement:
  • 10-20 international system
  • Queen square system
• The waves produced by the visual stimulus are then recorded as a graph.
• There are different types of VEP:
  • Flash VEP – flash stimuli are presented to the eye at a rate of 1 to 5 times/second. It indicates if the stimulus was perceived by the cortex or not. It is the test of choice in infants as it can be done even during sleep.
  • Pattern VEP – Checkerboard patterns displayed on a screen are used as stimuli. This can be used to roughly estimate the visual acuity.

Slit Lamp Biomicroscopy

It is an objective technique that can determine the expected visual acuity in the presence of a cataract.[33][34] If reduced visual acuity is exclusively caused by the ocular media (corneal opacity, cataract, vitreous debris), then the doctor's view of the ocular fundus should be equal to the patient's visual acuity. For example, if the patient's visual acuity is 20/100, but the doctor's view of the fundus is 20/40, then there should be a suspicion that the ocular media is not the only cause of reduced visual acuity. Additionally, the brightness, shape, direction, and angle of illumination of the slit lamp can be adjusted to view all the different structures of the anterior segment of the eye. It can also be used to visualize the posterior segment. The most accurate testing requires the pupil to be fully dilated and also requires condensing lenses to enable seeing the fundus. The different lenses that can be used are as follows:

• Hruby lens biomicroscopy - It is a planoconcave lens of 58.6 dioptre power. A virtual and erect image of the fundus is produced. The image has a low magnification, and the field of view is small. The fundus cannot be visualized beyond the equator.[35]
• Contact lens biomicroscopy - 
  • Modified Koeppe lens - It is a posterior contact lens that produces a virtual and erect image.[34]
  • Goldmann's three-mirror contact lens - It is made up of a central contact lens and three mirrors with different angles of inclinations.[36]
• Indirect fundus biomicroscopy - It is a method of noncontact fundus biomicroscopy. A +78 Dioptre or a +90 Dioptre lens can be used to produce a real, inverted, and magnified image. 

Fundus Fluorescein Angiography (FFA)

It is an objective technique that requires clear media to perform.[37][38] It can also be performed in the presence of mild media opacities. It is a diagnostic technique that uses systemic fluorescein dye to allow sequential visualization of blood flow simultaneously through the retina, choroid, and iris tissue. 

Procedure

• The fundus camera is correctly aligned, and the eyes are dilated.
• Baseline red-free images are taken.
• A 21 Gauge butterfly needle is inserted into the antecubital vein, and the fluorescein dye is injected at 1ml/sec.
• A timer is started, and photos are taken, starting 15 seconds before the dye is injected up to 10-20 min.

Optical Coherence Tomography (OCT)

Although media opacities deteriorate the image, OCT is still often used to determine the integrity of posterior segment structures such as the macula and the optic nerve. It is an objective method that may be done in the presence of slightly opaque media but not in the presence of dense cataracts.[39] 

By screening for posterior segment pathologies in eyes with cataracts, the OCT is able to detect if the macula, retina, and optic nerve are contributing to the reduced visual acuity. It produces cross-sectional images of biological tissues with less than 10-micron axial resolution. A near-infrared beam of light is projected onto the retina. The light reflected back from the retinal microstructures is scattered. The echo time delay of the scattered light is compared with the echo time delay of the light reflected from the reference mirror. 

Microperimetry 

It is an objective test that requires clear media to perform.[40] It can also be performed in patients with cataracts to evaluate for macular dysfunction. Microperimetry enables precise topographic correlation between that point's retinal threshold and fundus features. By providing real-time input from the color fundus image, the equipment enables precise positioning of the perimetric target on the fundus area of interest. Microperimetry is primarily a research tool, not a tool utilized in everyday clinical practice.

The following are some potential clinical uses:

• When macular scotomas are measured carefully, this allows the clinician to identify a patient's fixation pattern. This makes it easier to implement different treatment modalities, such as eccentric viewing techniques, and monitor the pathology's functional impact.
• Once the focus pattern is recorded, planning and rehabilitation with low vision aids can proceed more effectively.
• In order to investigate the relationship between physical alterations and their functional importance, research methods compare microperimetry and OCT evaluation of macular disorders.

Procedure

• The test can be carried out in low light with an undilated pupil.
• The first stage is to align the fundus image with the target such that when a target is later projected on the photograph of the fundus at a particular position, the stimulus is automatically presented to the patient at that exact actual fundus location.
• The ambient lighting is set to 4 apostilbs. The range of the stimulus's volume is 0 to 20 dB. Goldmann I to V sizes are the range of stimulus sizes.
• Depending on the patient's visual acuity, the fixation target might vary in size and form and is typically at 100 apostilbs.
• Either numerical data or a color scheme is used for reporting.

Indications

• Diagnosis and follow-up of macular diseases[41]
• Potential macular function in media opacities[42][43]
• Visual prognosis after surgery.
• Testing of the patient that malingers is unresponsive, unable to respond, or has non-organic (hysterical) vision loss.

Potential Diagnosis

Potential vision testing allows the clinician to better determine the cause of vision loss when media opacities are present in combination with other possible visual system disorders.  These other disorders include:

• Macular disease
• Optic nerve disease
• A brain disorder affecting the visual tracts and the visual cortex
• Amblyopia
• Malingering
• A patient that is unable to respond or cooperate with a regular vision test
• A patient with non-organic vision loss[14]

Media opacities that can be distinguished include:

• Cataract
• Posterior capsular opacity
• Corneal opacity
• Corneal irregularity
• Vitreous opacities such as hemorrhage

Thorough knowledge of tests of potential vision allows the healthcare team to determine the most appropriate medical intervention, weigh the benefits and risks of surgery, and allow for appropriate counseling of patients.

Normal and Critical Findings

Photostress Test

If the Photostress Recovery Time (PSRT) of the worse eye is equal to that of the better eye, then the macula is not the cause of vision loss. Since the test is independent of media opacities and brain disorders cause bilateral vision loss, such a result would narrow down the vision loss to an optic nerve disorder.

• If the PSRT of the worse eye is longer than the normal/better eye, then the vision loss is due to macular disease.
• PSRT greater than 50 seconds is considered abnormal. 

Amsler's Grid

• The patient has a central scotoma if the central white dot is not seen. The extent of the scotoma is described with the help of the diagonal lines on Amsler chart 2.[44]
• The patient has a large scotoma if all four corners of the grid in any of the charts are not seen.
• If the central white dot is seen, but any boxes are blurry or missing, the patient has a peripheral scotoma or eccentric viewing.
• If any lines appear wavy or distorted, the patient has metamorphopsia due to macular diseases such as edema, drusen, or epiretinal membrane.

2-Point Discrimination

If the patient can perceive two different lights, it indicates that the macular function is normal. 

Maddox Rod Test

• Grade 1 response - The patient can identify the direction correctly in all three positions. The line appears straight, continuous, and correct in color in all three positions. It indicates 6/6 to 6/9 vision.
• Grade 2 response - The patient can identify the direction correctly in all three positions. The line appears straight but broken and correct in color in all three positions. 
• Grade 3 response - The patient can correctly identify the direction and the color but is unsure whether the line is straight or not. 
• Grade 4 response - The patient can correctly identify only color.

Entoptic Phenomenon

If the patient can perceive shadows of the arborizing retinal vasculature, the macular function is probably normal. If they cannot perceive the vasculature, the macular function is probably poor.[45]

In the blue field entoptic phenomenon test, normally, more than 15 corpuscles are seen. If there is a partial loss in movement, a complete loss, or slow movement, the retinal function is abnormal.

Potential Acuity Meter (PAM)

The smallest Snellen line where the patient can read three or more characters is recorded as the potential acuity. 

Laser Interferometry

The gratings (also called spatial frequency) are made finer until the patient is unable to discern the direction of the gratings. The smallest gratings seen by the patient are recorded and then converted to Logmar or Snellen acuity. A spatial frequency of 30 per degree is equal to 20/20 (6/6) visual acuity.[46]

Critical Flicker Fusion Frequency (CFFF)

The critical flicker fusion threshold (or threshold for flicker fusion, TFF), which denotes the fastest rate of flickering light that the visual system can see, is used to characterize the upper degree of one's ability in visual processing. This is what is used to determine visual acuity. While, according to certain sources, the human eye can detect the difference between modulated and steady light at up to 500 Hz, it is generally accepted that the human eye cannot detect flicker above 50 to 90 Hz.[47]

Electroretinogram (ERG)

The normal ERG has three waves: [See the image on - Normal ERG][48]

• a wave – a small negative wave that originates from the photoreceptors
• b wave – a large positive wave that originates from the bipolar cells
• c wave – a slow-rising positive wave originating from the Retinal pigment epithelial cells

These components are measured using the following parameters: 

• Amplitude – a measure of the maximum deviation. [See the image on - Amplitude of ERG waves]
• Implicit time – The time interval between the onset of the stimulus and the maximum response of a or b wave.
• Latency – The time interval between the onset of the stimulus and the start of a wave. [See the image on - Time responses of ERG waves]

These parameters from the patient are compared to normal ERGs to diagnose the status of the retina.

Visually Evoked Potentials (VEP)

• The VEP is a series of many waves, among which the following are used for clinical diagnosis:[32]
  • P100 wave – a positive wave that occurs at approximately 100 ms after photostimulation
  • N75 wave – a negative wave that occurs at approximately 75 ms after photostimulation
  • N135 wave - a negative wave that occurs at approximately 135 ms after photostimulation
• In each wave, the amplitude and the latency are compared with the normal VEP.
• If the largest amplitude fastest waves are produced with a checkerboard stimulus with 5-6mm squares, the visual acuity can be estimated to be around 20/20.
• If the largest amplitude fastest waves are produced with larger squared checkerboard stimuli, the visual acuity is correspondingly lower.

Fundus Fluorescein Angiography (FFA)

The normal FFA has the following phases:[49]

• Prearterial/Choroidal Phase - occurs 9.5 - 10 sec after injection of the dye. The posterior ciliary artery and choriocapillaris fill during this phase. 
• Arterial Phase - occurs 10 - 12 sec after injection of the dye. The central retinal artery fills during this phase.
• Capillary/Early Arteriovenous Phase - occurs 13 - 15 sec after injection of the dye. The precapillary arterioles and capillaries fill during this phase.
• Late Arteriovenous/Lamellar/Early Venous Phase - occurs 14 - 16 sec after injection of the dye. The post-capillary venules fill during this phase. 
• Venous Phase - occurs 17 - 19 sec after injection.
• Late Staining/Recirculation Phase - occurs 10 mins after injection of the dye—the disc margin and Bruch's membrane stain during this phase. 

Hypofluorescent areas visualized are due to the following:

• Blocked fluorescence - Vitreous hemorrhage, subhyaloid hemorrhage, intraretinal hemorrhages, lipofusceine deposits like drusen, etc., block the underlying retinal and choroidal fluorescence. 
• Vascular filling defects - Retinal venous and arterial occlusions keep the dye from flowing through the vessels producing hypofluorescence. 

Hyperfluorescent areas visualized are due to the following:

• Pre-injection fluorescence - due to auto or pseudo-fluorescence. 
• Early hyper fluorescence - due to abnormal retinal or choroidal vessels
• late/ extravascular hyper fluorescence - due to new vessels on the discs, vitreous inflammation, cystoid edema. 

Optical Coherence Tomography (OCT)

The OCT scan allows cross-sectional imaging of the macular and peripapillary regions. The scans are color coded with red-yellow representing areas of maximal optical reflection and backscattering. Blue-black represents areas of minimal signals. The vitreous is seen as a non-reflective dark space. The vitreoretinal interface is well-defined as the difference between the non-reflective vitreous and the backscattering retina. Layers within the retina can be seen separately, with each layer being discernible.

Microperimetry

The green hue symbolizes a low-intensity stimulus that corresponds to high sensitivity, while the red color represents a high-intensity stimulus (low sensitivity). An empty square represents a stimulus that is not seen, whereas a filled square represents a stimulus that is seen.[50]

Interfering Factors

Subjective Tests

Tests that require a patient response, such as potential acuity meter, laser interferometry, and critical flicker fusion frequency, may be interfered with by:

• patient cooperation
• malingering
• hysterical blindness

Clear Media Tests

For those tests that require clear media such as OCT, FFA, and microperimetry, the presence of opacities can interfere with the results.

Photostress Test

There is no standardization for the procedure, and variables influencing the PSRT include the brightness of the light used and the length of exposure.

Amsler's Grid

• It is a personal chart, and there is often a lack of attention to a rote task performed routinely daily.
• A completion phenomenon occurs, which can compromise the perception of the scotoma/metamorphopsia for the patient. This phenomenon is where the cortex perceptually fills small gaps in line stimuli.
• The test relies heavily on maintaining central fixation and, therefore, cannot be used in children or patients with very poor vision.
• Incorrect working distance, near corrections, performing the test binocularly, or wearing glasses - such as bifocals - that distort lines are all sources of error that can affect the outcome of the test.[44]

Potential Acuity Meter

• Being a subjective test, it is heavily dependent on patient compliance.
• It is less accurate in cases with denser cataracts.
• It overestimates vision in cystoid macular edema and retinal detachment.[51][52]
• In patients with severe glaucomatous damage, the results of PAM are not accurate.[53]

Laser Interferometry

• It overestimates visual acuity in macular edema.[54]
• Underestimates visual acuity in some cataracts. 
• Inconsistent in predicting visual acuity in senile maculopathy.[55]

Complications

• In all tests: Erroneous results may lead to the wrong conclusions, resulting in unnecessary surgery.
• Photostress test: Over-exposure of the retina leads to damage to the photoreceptors.
• Electroretinogram: Corneal abrasions due to the contact lens.
• Amsler grid: The completion phenomenon results in the visual system masking important results, leading to delayed diagnosis.
• Fundus fluorescein angiography - local tissue necrosis due to extravasation of the dye, photosensitivity to the die, change in urine color, urticaria, dye-related anaphylaxis, and death.

Patient Safety and Education

The results from most of these tests can, in most cases, be communicated immediately to the patient. One of the main indications for the tests for potential vision is to manage patient expectations regarding the visual outcome of surgery. It is important to ensure that the patients understand the results of these tests as they are indicative of the outcome of the surgery. This makes tests for potential vision a very important tool in patient safety and education. It is also important to explain the procedure as patient cooperation will affect the result of most of them.

Clinical Significance

Tests for potential vision are performed preoperatively to predict whether patients with anterior segment diseases such as cataracts can benefit from surgery. Tests for potential vision also evaluate the integrity of the retina and of the neural visual system. In cases with dense cataracts where the posterior segment cannot be visualized, several of these tests can evaluate the function of the macular.

If a macular or retinal disease is present in such a patient, the postoperative vision after cataract surgery will depend on the remaining visual function of the macula and thus will be less than optimum. This is useful in the counseling and management of patient expectations after surgery. Another application of these tests can be in estimating visual acuity in patients with hysterical blindness, children, and uncooperative, non-verbal, or unconscious adults. 

Some tests, like the ERG, specifically evaluate the integrity of the retinal pigment epithelium, photoreceptors, and bipolar cells. While others, like VEP, test the intactness of the entire visual system, starting from the retina and reaching up to the visual cortex. The Photostress test, Maddox rod, 2-point-discrimination test, and entoptic phenomenon can be used as gross screening methods in the outpatient setting.

The Amsler chart can be administered for home monitoring of macular disease. Thus, tests for potential vision are important tools to examine the patient with less than 20/20 vision; hence, understanding their principles and learning to administer them is crucial.