UW Special Testing

The following tests, with the exception of color vision testing, do not rely on the subjective response of the patient to a stimulus, but rather provide a purely objective evaluation of the functioning of the visual system.

The “Outer Retinal Function Tests” demonstrate the functional integrity of the mid- and outer-retinal layers and retinal pigment layer to light stimuli under conditions of light and dark adaptation. If those tests are normal, then subsequent tests in the “Higher Visual Pathways by VEP” list can be done to evaluate the responses of the more central visual portion of the visual system.

Know what you’re looking for? Click these red links to download the Order Request Form for Visual Function Studies or Pediatric Sedation Form.

Outer Retinal Function Tests

ERG: Full Field Electroretinogram (click to open and close)

ERG OR FFERG – FULL FIELD ELECTRORETINOGRAM MEASURES PANRETINAL RESPONSE OF PHOTORECEPTORS AND BIPOLAR CELLS.

OVERVIEW: This test measures the onset (latency or implicit time) and amplitude of the electric field response summed from all retinal cells activated by a flash of light under a given adaptation condition. It measures these parameters first under dark-adapted (scotopic) conditions to assess rod-mediated function and then under light-adapted (photopic) conditions to assess cone mediated function. Our testing parameters adhere to the standards set by ISCEV (International Society for Clinical Electrophysiology and Vision) www.iscev.org and are reported according to these standards.

HOW DONE: The patient’s pupils are enlarged by standard dilating drops. The patient, accompanied by a technician, sits in a dark room for about 20 minutes. Topical anesthetic is applied to the ocular surface followed by an electrode, usually a very thin nylon filament placed below the cornea. The electrode is connected to a recording device. The patient is then positioned in front of a hollow hemispheric bowl and asked to fixate a target while flashing lights are presented and the electrical responses of the eyes are measured and recorded. The patient then light adapts for 10 minutes and the flashes are repeated. The entire test takes about one hour to complete.

For adults and older children this test is usually done in the eye clinic. Very young or disabled persons usually require sedation to complete the test and this is done in the UW Hospital outpatient sedation clinic. For patients whose poor health makes sedation too high a risk, the testing can be done in a standard operating room under general anesthesia.

USEFUL FOR:

Any patient with unexplained visual acuity loss to determine if the cause is outer retinal cell dysfunction. Being a pan-retinal response, the full field ERG is not sensitive to small localized retinal lesions.
Patients suspected of a retinal dystrophy such as retinitis pigmentosa.
Patients on retinotoxic drugs such as Vigabatrin.
To determine if any retinal function is present in eyes with poor media.

HOW REPORTED: The test results are evaluated at a weekly meeting of ophthalmologists and technicians along with any ancillary tests that have been provided. The summary of this discussion along with some differential diagnostic possibilities is sent usually by FAX to the person ordering the test. Copies of the test results themselves are also sent by mail.

mfERG: Multifocal Electroretinogram (click to open and close)

mfERG – Multifocal Electroretinogram measures central response of cones and cone bipolar cells.

OVERVIEW: This test measures the amplitude and “latency”(the time it takes for the photoreceptors to respond to a stimulus of light) of small localized areas of retina within the central 20 to 30 degrees of the fundus UNDER PHOTOPIC CONDITIONS. The mfERG is primarily a measure of functional integrity of the outer retinal layers (central cone photoreceptors and bipolar cells).

HOW DONE: The patient’s pupils are enlarged by standard dilating drops. Topical anesthetic is applied followed by placement of an electrode which is usually a thin conductive filament placed below the cornea. The electrode is connected to specialized recording device. The patient is positioned in front of a a specialized computer display and asked to fixate a target. An array of hexagonal patches are flashed on and off according to a semi-random sequence and the electrical responses of the eyes are recorded. The signals are processed and then displayed as the measured electrical response within each of the hexagonal patches. The test can be done only on unsedated patients able to fixate a target. The test takes about 30 minutes.

USEFUL FOR:

Any patient thought to have retinal cone receptor dysfunction such as those using Plaquenil
Any patient suspected of having a central cone dystrophy such as Stargardt’s Disease.
Any patient with visual loss on subjective testing. Note that this test is sensitive to small localized lesions affecting the cone-driven systems of the retina.
This test is not appropriate for lesions involving primarily the rod photoreceptors, or the retinal periphery.

HOW REPORTED: The results are evaluated at a weekly meeting of ophthalmologists and technicians along with any ancillary tests that have been provided. The summary of this discussion along with some differential diagnostic possibilities is sent usually by FAX to the person ordering the test. Copies of the test results themselves are also sent by mail.

EOG – Electro-Oculogram (click to open and close)

EOG – Electro-oculogram measures retinal pigment epithelial cell integrity

OVERVIEW: This test measures the electrical potential difference between the front and the back of the eye which is produced primarily by the healthy function of the RPE (Retinal Pigment Epithelium). Therefore any disease associated with widespread changes the RPE layer will cause a reduction in the potential difference. Normally the potential difference is much higher when the eye is exposed to light than in the dark.

HOW DONE: Electrodes are pasted to the skin nasal and temporal to each eye and connected to a recording device. The patient places their chin and forehead on the rests in front of a hollow sphere with tiny fixation lights on each side in the horizontal meridian. They are instructed to look from side to side for several cycles and measurements are taken of their light adapted state. Ambient lights are turned off and they are asked to continue periodically looking from side to side and measurements are taken as their eyes dark adapt. The test takes about 40 minutes to complete

USEFUL FOR: Primarily used to substantiate a diagnosis of Best’s Vitelliform Dystrophy since in this condition there may be a paucity of fundus abnormalities and the abnormality of the RPE may not produce any visible changes. The test can also be used to substantiate any widespread abnormality of the RPE, as occuring in Retinitis Pigmentosa or Fundus Flavimaculatus.

HOW REPORTED: The result is expressed as the “Arden Ratio” which is the ratio of highest potential in the light/lowest potential in the dark. Normally the ratio is at least 1.85.

Dark Adaptation (click to open and close)

DA – Dark Adaptation measures regeneration of Rhodopsin

OVERVIEW: Traditionally this test is performed using a Goldman-Weekers Adaptometer, but these devices have become impossible to maintain in working order. Our service estimates the dark adaptation curve with its normal “cone/rod break” by measuring the amplitude of the ERG response to a dim light flash over the course of 20 minutes of dark adaptation.

HOW DONE: The patient has electrodes applied as for full-field ERG. The patient is then exposed to a light (300 cd-s m-2) for 30 sec. to achieve a maximum light-adapted state. A flash of dim light is then delivered every 5 min for 30 min. The growth of the B wave with time after bleach is an objective measure of rhodopsin regeneration and recovery of sensitivity after photo-bleach.

USEFUL FOR: Patients with complaints of poor adaptation to changes in luminance, including vitamin A deficiency and night-blindness.

HOW REPORTED: The results are reported as the rate of increase in B-wave amplitude versus time post bleach compared with normal values.

Retinal Ganglion Cell Test

pERG: Pattern ERG

OVERVIEW: This test measures primarily the function of the central retinal ganglion cells and secondarily the function of the macular photoreceptor cells.

HOW DONE: The preparation and set-up are the same as for the multifocal ERG. The difference is in the character of the stimulus which consists of an isoluminant reversing black-and-white checkerboard. The test requires that a fairly accurate refractive correction be in place so that the checkerboard be in good focus on the retina.

USEFUL FOR: This test facilitates the determination of whether an abnormal visually evoked potential (discussed subsequently) is due to macular disease or due to more central CNS disease.

HOW REPORTED: The test is reported as the presence and age-related normal amplitude of a positive wave (P50) followed by a negative wave (N95). The N95 wave is specifically associated with the function of macular retinal ganglion cells. It is reduced in glaucoma, optic neuritis, and other disorders of the optic nerve. It is used primarily as a research tool but on occasion may be useful as an adjunct to clinical testing.

Higher Visual Pathways by Visually Evoked Potential

Single Flash VEP – Basic (click to open and close)

Single Flash VEP – Basic

OVERVIEW: This test is designed to determine the cortical response to simple flashes of light to the eye. It is typically done only after retinal evaluation show that the retina is fully responsive. This VEP is the simplest determination of whether and how much light stimulus is reaching the visual cortex from each eye.

HOW DONE: Electrodes are placed on the scalp over the occipital cortex with easily removed water-soluble gel and are connected to a recording device. For the flash VEP the patient is positioned in front of a flash device. A bright light is repeatedly presented and the electrical responses are summated and recorded. The test takes about 15 minutes.

USEFUL FOR:

Evaluating basic retino-cortical integrity in an infant suspected of having poor vision or with roving eye movements that preclude fixation.
Patients with visual loss, but normal retinal anatomy and function tests, such as persons suspected of optic nerve dysfunction, including optic neuritis, multiple sclerosis, papilledema, optic nerve gliomas.
Evaluating visual acuity potential in patients with ocular media opacities.
Evaluating the possible misrouting of optic nerve fibers in patients suspected of having albinism.

HOW REPORTED: The flash VEP test typically produces two negative (N1, N2) and two positive peaks (P1, P2). The amplitude and time to onset (latency) are reported with a comparison to normal values.

Pattern VEP – Macular Component of Cortical Response (click to open and close)

Pattern VEP – Macular Component of Cortical Response

OVERVIEW: This test is designed to evaluate the electrical response in the brain to a stimulus that elicits macular function. For the pattern-reversal VEP the patient fixates a checkerboard pattern, the white and black checks reverse their positions every ½ sec. Multiple check sizes may be presented.

HOW DONE: The testing situation, other than the character of the stimulus, is the same as for a flash VEP.

HOW REPORTED: The latency and amplitude of the prominent positive voltage that typically peaks at 100 microseconds (P100) after a pattern change.

USEFUL FOR: This test allows assessment of the speed of transmission from ganglion cells along the optic nerve to the visual cortex. The P100 is delayed in optic neuritis, papilledema, tumors effecting the optic nerve, and as a result of cortical insult e.g. anoxia.

Multifocal VEP – Cortical Aspect of Visual Field Abnormality (click to open and close)

Multifocal VEP – Cortical aspect of visual field abnormality

OVERVIEW: This test records the cortical response to retinal stimulation by similar type of temporal stimulus that is used in the multifocal ERG. The difference is that a 4×4 checkerboard pattern that reverses according to the ‘m-sequence’ replaces the hexagonal patch that flashes on and off by the same sequence. The mfVEP provides an objective measurement of a subjectively measured visual field defect involving the central 20 to 30 degrees of the field

HOW DONE: The patient is positioned in front of the computer screen that has the capability of presenting an array of hexagonal patches that are flashed on and off according to a semi-random sequence and electrodes are placed over the occipital cortex. The electrical responses of the visual cortex are recorded. The variable intensities of stimulation of different areas of the retina are reflected in the intensity of response of different areas of the cortex thus validating the presence of a visual field defect.

HOW REPORTED: The result is reported as the symmetry of response recorded over horizontally and vertically positioned electrodes.

USEFUL FOR: This test provides objective validation for the subjective report of a defect in the central region on a visual field test.

Sweep VEP – Visual Acuity Estimator (click to open and close)

Sweep VEP for Objective Visual Acuity Estimation

OVERVIEW: This VEP test is designed to provide an objective approximation of visual acuity in pre-verbal infants and in adults with unexplained subjective visual acuity loss. In this test, cortical responses are generated by a flickering ‘grating’ or stripe pattern then reducing the width of the stripes to determine the minimum stripe width that is ‘seen’ by the visual system.

HOW DONE: The testing arrangement is the same as the single flash VEP described above with the exception that the light stimulus is presented in a series of light and dark stripes that vary in width. The estimation of acuity based on the cortical responses to these stimuli has been shown to correlate well with visual acuity. It is most commonly done on and well tolerated by infants and children sitting in the mother’s or care-giver’s laps.

HOW REPORTED: The test results are reported as the visual acuity in cycles per degree, which can be converted to Snellen scale for qualified eye-care professionals, but should not be used by social service agencies for determining benefits.

USEFUL FOR: The test is useful in any situation where an objective approximation of visual acuity is desired, especially in cases of infantile amblyopia before reliable acuities can be measured. The test may also be useful for determining vision in children with significant neurological impairment that may limit their behavioral response, e.g. cerebral palsy, global developmental delay, ocular motor apraxia. It is also useful in any situation where an objective approximation of visual acuity is desired, such as in adults whose subjective measurements of acuity are in doubt.

Albinism VEP (click to open and close)

Albinism VEP – Assesses Retino-Geniculo-Cortical Misrouting

OVERVIEW: This test is based on the unique anatomical fact that in albinism a larger-than-normal number of the axons from the temporal retina cross over at the optic chiasm to join those from the nasal retina of the opposite eye in projecting to the contralateral cerebral hemisphere. Normally, approximately one-half of the retino-geniculo-cortical neurons cross, resulting in VEPs from the left and right cerebral hemispheres that are identical. However, in albinism due to the asymmetric projections, stimulation of one eye results in a larger amplitude VEP from the contralateral visual cortex compared with the VEP from the ipsilateral cerebral hemisphere. This cross-over asymmetry is a feature of all types of albinism.

HOW DONE: This test uses the standard set-up for a VEP. Each eye is stimulated separately and the relative intensity of the response is measured over each cortex. Additional occipital electrodes may be added to confirm asymmetry. For infants, the stimulus is a strobe flash. For older children and adults, a checkerboard pattern presented in ‘on-off’ mode is used. Recordings are made from each eye to verify that the VEP asymmetry reverses cerebral hemispheres with eye stimulated.

HOW REPORTED: This test is reported as the amplitude of response of each side of the visual cortex to stimulus delivered to one eye at a time. Normally the responses are symmetric but in albinism the amplitude of the responses is higher from the contralateral visual cortex.

USEFUL FOR: This test supports a diagnosis of albinism.

COLOR VISION SPECIAL TESTS

100 Hue and Lantern Test

OVERVIEW: In addition to providing color vision assessment with Ishihara plates, the service provides Farnsworth AO HRR plates, D15, FM 100 hue testing and the “lantern test,” required by some occupations.

The Ishihara plates can be done in a short time and are available in most eye professionals’ offices. They mostly useful for detecting the type of red-green color confusion found in around 6% of males. Ishihara testing can also be abnormal in severe acquired color vision loss.

In addition to the above tests, our service provides the more definitive and sensitive testing with the D 15 panel, the 100 hue test. These tests help determine the character of the color abnormality and whether it is due to photoreceptor or optic nerve disease. AO-HRR plates are also available, which may be useful as a quick screening tool to identify blue-yellow color deficits.

We also provide the “LANTERN TEST,” which is a specific type of test required in some professional job applications typically related to operating aircraft, trains, or boats.

HOW DONE: In the 100 hue test and in its shorter version, the D 15 panel, the patient is asked to put in order of hue change a series of discs with colored inserts. The former takes about an hour and the latter about 15 minutes. Importantly, the test is performed under standard lighting conditions.

USEFUL FOR: The specialized tests help define the character and possible causes of the color abnormality. The sensitive FM-100 hue test is also important in following patients who are taking ethambutol for tuberculosis since acquired color vision loss is an early indicator of ethambutol toxicity.

HOW REPORTED: The D15 panel is reported as the axis along which most color matching errors were made thus distinguishing congenital from acquired defects and is presented in a display of a circle with 15 points along its circumference. The FM-100 hue test is reported in summary form as the score related to the numbers of errors made in matching and in the more detailed form as a page size disc with errors displayed in the axes in which they occurred.

The request for testing is initiated by filling out the appropriate forms:

Pediatric Sedation Form

Fax or mail the completed form(s) to the address on the form along with the following:

A copy of the patient’s most recent ocular exam
Copies of any ancillary tests that have been done of retinal appearance (fundus photos, OCTs) or function (visual field tests)
Our technician calls patient to explain the tests, answer questions and schedule their testing. A description and a map of our location is then mailed to them

HOW REPORTED:

Within a week after the test is done, a panel of retinal specialists reviews the test results along with the ancillary tests provided. Their interpretation and suggested differential diagnoses are reported to the referring professional later that week. Copies of the raw test data along with another copy of the report is sent by surface mail.

Visual Function Form
Please note that no emails will be accepted, due to HIPAA compliance.

UW Special Testing

UW EYE ELECTROPHYSIOLOGY AND COLOR VISION TESTING

Outer Retinal Function Tests

ERG: Full Field Electroretinogram (click to open and close)

mfERG: Multifocal Electroretinogram (click to open and close)

EOG – Electro-Oculogram (click to open and close)

Dark Adaptation (click to open and close)

Retinal Ganglion Cell Test

pERG: Pattern ERG

Higher Visual Pathways by Visually Evoked Potential

Single Flash VEP – Basic (click to open and close)

Pattern VEP – Macular Component of Cortical Response (click to open and close)

Multifocal VEP – Cortical Aspect of Visual Field Abnormality (click to open and close)

Sweep VEP – Visual Acuity Estimator (click to open and close)

Albinism VEP (click to open and close)

COLOR VISION SPECIAL TESTS

100 Hue and Lantern Test

How to Order Special Tests

The request for testing is initiated by filling out the appropriate forms:

HOW REPORTED:

Meet the experts

Kimberly E. Stepien, MD

Mihai Mititelu, MD, MPH

T. Michael Nork, MD, MS, FARVO

Thomas S. Stevens, MD

James N. Ver Hoeve, MS, PhD