Reviewing ocular specialty testing

November 30, 2016
Savory Turman, COMT, OCS

Specialty testing is performed for diagnostic purposes, observation of disease processes, and treatment plans

Technicians are key in assuring quality outcomes with specialty testing.

Specialty testing is performed for diagnostic purposes, observation of disease processes, and treatment plans. In some cases, specialty testing establishes a baseline prior to starting medications such as hydroxychloroquine or having neurosurgery.

Due to the implications of testing results, accuracy is key. In many practices, testing often falls to the more experienced technical staff.

Let’s review several common types of specialty testing and their purposes.

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Anterior segment testing

Schirmer’s tear testing is utilized to diagnose dry eye conditions. The eye is anesthetized and a paper strip is inserted into the inferior fornix. The patient’s eyes are closed for five minutes and the paper is then checked to determine baseline tear production.

In the “normal” eye, the results will read between 10 mm and 15 mm. Mild dry eye is considered 9 to 14 mm  and moderate dry eye is 5 to 9 mm. Severe dry eye will produce 4 mm or less of baseline tears in five minutes. Treatment can then be directed accordingly.

 

Endothelial cell count or specular microscopy determines the cell density of the corneal endothelial layer. The cell density of an eye will generally vary.

Most people in their 40s will have a cell density between 2,300 and 3,100 cells/mm2; patients in their 60s experience a cell density of 2000 to 2800 cells/mm2.

While cell loss may be a part of aging, patients may need to be monitored for potential development of Fuch’s dystrophy.

Corneal pachymetry measures cornea thickness and can play an important role in glaucoma diagnosis and in determining candidacy in refractive surgery.

The average corneal pachymetry reading is 540 µu. A thinner cornea may prevent a patient from having LASIK or could indicate a higher intraocular pressure (IOP) than Goldmann applanation tonometry would read.

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Corneal topography maps the cornea. The two main methods of testing are Placido disc and Scheimpflug.

The three-dimensional image given by either method can show abnormal curvature, whether steep or flat, and can also track changes in the surface of the cornea. As with most things, red means “danger,” so large amounts of red in the image shows very steep curvature, which could indicate keratoconus.

Wavefront aberrometry can pinpoint potential visual problems in the eye’s refractive system. When light passes through the cornea and crystalline lens, rays can be distorted, resulting in diminished sight.

The pattern of aberrometry is a diagnostic tool to help determine candidacy of refractive procedures. It can also be useful in predicting positive outcomes of intraocular lens (IOL) upgrades.

Axial length measurements (A scans) provide the distance between the anterior pole of the eye and the retinal surface. This measurement is useful in calculating the IOL positioned during cataract surgery.

 

The average adult eye axial length is 24 mm. Variance of that measurement can be seen in myopia, which results in a “longer” eye, and in hyperopia, which results in a “shorter” eye. During pre-operative testing for cataract surgery, an error of 0.33 mm can result in a dramatic post-surgical IOL power deviation of up to 1.00 D.

Posterior segment testing

B scan ultrasonography (brightness scan) is a two-dimensional, cross-sectional view of the eye and orbit. Ultrasound is useful for detecting retinal detachment, tumors, vitreous hemorrhaging, and inflammation and lesions of the eye and orbit.

The eye is anesthetized, and a probe is placed on the eye, and in some cases, the eyelid. The ultrasound echo reveals a black and white picture of the eye and its structures, with the black images being the absence of echo (or structure).

OCT (ocular coherence tomography) use light rays to show a cross-sectional picture of the retina, helping to diagnose a multitude of retinal concerns, such as macular hole, epiretinal membrane, macular edema, and more. Scans of the optic nerve can determine if there is optic nerve thinning, impactful in the diagnosis and treatment of glaucoma.

Visual field or perimetry comes in the form of static or kinetic method.

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The static method of visual field testing utilizes lights that change in size and brightness. Most static testing is computerized, yet confrontational field testing is a form of static visual field testing. Computerized static visual field testing is highly patient dependent-proper instructions to patients and monitoring compliance is key for accuracy.

Kinetic testing utilizes moving lights of differing sizes and brightness. While computerized methods of kinetic testing exist, the most commonly used tester is the Goldmann bowl perimetry. Technician training is crucial for accurate and thorough Goldmann field testing.

While it is one of the most complained about types of testing, visual field perimetry can provide valuable information regarding glaucoma damage, stroke damage, or even the presence of a tumor. Understanding the anatomy behind the visual pathway and the visual manifestations of perimetry will give the technician guidance for testing and will also engage the technical staff during the procedure, which can be less than exciting.

 

Electrooculography and electroretinography (EOG/ERG) are detailed posterior testing methods that can assist in diagnosis and determining progression of retinal disease processes.

Electrooculography measures corneal-retinal potential, the correlation between the front and the back of the eye. Electrodes are attached by the canthi, and small eye movements register the potential and measure the eye’s position.

EOG is used to test the retinal pigment epithelium. The Arden ratio, which is the ratio of the light peak to the dark trough, determines a normal or abnormal test result. An Arden ratio of 1.8 or greater is considered “normal,” subnormal is 1.65 to 1.80, and below 1.65 is abnormal and cause for further investigation. Diseases such as Best’s disease, retinitis pigmentosa, and Stargardt’s disease can cause an abnormal test result in electrooculography.

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Electroretinography measures electrical responses of retinal cells contributing to the diagnosis of retinitis pigmentosa and other retinal degenerations. ERG consists of putting electrodes around the eyes and the skin of the eye, the patient is then exposed to light stimuli, dark adapted for 20 minutes to induce rod cell functioning, and light adapted again.

The activity of the rod/cone system indicates deviations in the retinal function. By observing the A-wave and B-wave of the test, doctors can diagnose a variety of retinal disorders.

Visual evoked potential (VEP) testing is a measurement of the neurological activity of the eye’s visual system. VEP testing utilizes visual stimuli to “evoke” an electrical response from the brain. Various patterns and contrasts coupled with electrodes on the skin can be useful in the monitoring of optic neuritis in conditions such as multiple sclerosis or in cases of traumatic brain injuries (TBI).

Conclusion

Specialty testing is crucial in modern eyecare practice, and the role of the ophthalmic technician is integral in diagnosing, treating, and observing ocular diseases. Whether a technician is involved in specialty testing directly or merely exposed to it through working closely with the physicians, an overall concept of diagnostic testing is needed to assist in comprehensive patient care.