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Visual Physiology


Physiology Lab Testing

The retinal-testing lab at Orange County Retina offers several ways to analyze and diagnose retinal issues that cannot otherwise been made through traditional observation. These tests are broken down into two distinct categories; Psychophysical and Electrophysiologic. Both types of testing require interpretation. The electrophysiologic tests are completely objective and cannot be falsified. While the psychophysical tests are scored objectively, they are measuring a patient’s subjective response.


Psychophysical Testing

RAM – Retinal Acuity Meter: This test requires that a sharp, formed images (usually Snellen letters) reach the macula, otherwise the test report may be falsely negative. It is useful in assessing macular function in eyes that may have corneal haze, cataracts, a clouded posterior capsule, or vitreous haze. This test is extremely helpful when planning cataract, corneal transplant and posterior vitrectomy surgery or other opacities in front of the macula

PAM – Potential Acuity Meter: This is similar to the RAM with somewhat different letter types, contrast and optics.

BLE – Blue Light Entoptoscopy: Approximately 90% of persons are able to see the flow of white blood cells passing through their macular capillaries. A bright blue light of 430 nm is projected through the patients media toward the macula. The light is differentially absorbed by the white blood cells, which cast shadow and are seen as tiny dots flowing through the macular capillaries. As with the above two tests this test is also scored objectively scored and correlates well with macular acuity.The three tests above are tests of macular potential. The BLE test is best able to penetrate media opacities for objective scoring. All three of the above tests require a clinical understanding of the type and density of the media opacities to reduce or eliminate a false negative interpretation of macular potential. We perform the three above tests as a battery, and also consider other testing (ERG, VER) for an accurate assessment report.

Contrast Sensitivity (CS)– Contrast sensitivity refers to the ability of the visual system to distinguish between an object and its background. It is important to note that blurred vision is not the same as poor contrast sensitivity. A contrast sensitivity test, checks for the ability to differentiate between light and dark (contrast). This ability, is as much an important sense as 20/20, allowing us to be able to carry out everyday tasks safely. The tests provide useful independent information in relation to a patient’s visual function which is often not be observed through normal visual acuity testing. Measuring contrast sensitivity is an important indicator in quality of life functions i.e., driving and mobility. It may also serve as early detection of AMD, diabetic retinopathy and optic neuritis.

MAIA – Macular Integrity Assessment: This is a variant of a visual field test in which very small targets of variable contrast are projected on the macular geography. The test gives a comprehensive topographical picture of the central portion of the island of Traquair, and ,thus, gives a represention of the most important portion of the visual field. This test of macular sensitivity allows for comprehensive macular disease management. This is a non-mydriatic procedure and utilizes a normative database for statistical analysis. It is extremely useful for monitoring pathological changes of the macula over a period of time.

Color sensitivity: There are three test that allow for the determination or extent of a patients “color blindness”. Perception of color requires macular cone function, neuroretinal integration, optic nerve function and occipital lobe function.Each eye is tested separately. The Ishihara test is a good screening test. If abnormal, or if highly defined color measurements are necessary, the Farnsworth D-15 and FM-100 give more specific information.


Electrophysiologic Testing

These analyses are wholly objective in nature, and electrophysiologic data cannot be falsified. However, a report helpful to the clinician integrates the psychophysical testing along with the electrophysiological teting.Also, when clinical information is available the interpretive reports attempt to more deeply answer certain questions that help the clinician reach appropriate conclusions. In general, the results cannot be falsified.

ERG or Electroretinography–ERG, is an eye test used to detect irregular or uncharacteristic function of the retina. During this test, your Orange County Retina specialist examines the light-sensitive cells of the eye, the rods and cones, and their connecting ganglion cells in the retina. An electrode is placed on the cornea to measure the electrical responses to light of the cells (the rods and cones) that sense light in the retina at the back of the eye.

Orange County Retina patients assume a relaxed position, the eyes are dilated and then administered anesthetic drops, causing them to become numb. The eyelids are then propped open, and an electrode is gently placed on each eye with a device comparable to a contact lens. An additional electrode is placed on the skin to provide a ground for the very faint electric indicators produced by the retina.

There are three individual tests –

Mf-ERG Multi-focal ERG – This is a macular test that graphs multiple small segments of macular cone function. This test is often done after morphologic fundus changes have been recorded with fluorescein angiography. It is useful in assessing certain toxicities (ex: hydroxychloroquine), macular edema, macular ischemia, and overall macular function in determining etiology of visual loss.

EOG – Electro-oculogram: This is a test of the electrical “standing potential” of the eye. This “long-wave” potential is recorded after both light and dark adaptation, and the ratio of light-to-dark is a measure of the health of the entire retinal unit and supporting structures including the choroid, retinal pigment epithelium, photoreceptors and entire neurosensory retina. (Disease example: Bests disease)

VER – Visually Evoked Response: Also known as the VECP or visually evoked cortical potential. The retina is stimulated and the microvolt response is recorded (by computer averaging) at the occiput. Both magnitude and time values are important for interpretation, and both tests are heavily influenced by the cone system.