Diagnostic tools: Retinal cameras can be used for glaucoma

October 1, 2009

Screening for glaucoma has not been done on a large scale due to cost and a generally low prevalence of the disease.

Patients who have glaucoma often have no symptoms, which makes detection difficult. Indeed, half of those with glaucoma have undiagnosed disease until they present for a comprehensive eye exam, and some disease goes undiagnosed even after an exam. By the time disease is diagnosed in those patients, they already may have severe vision loss, according to Murray Fingeret, OD.

The question is, how can optometrists improve their ability to detect glaucoma before optic nerve damage occurs? One way is for all ODs to refine their diagnostic skills and optic nerve evaluation, he said.

Understand the limits of each test

In the past, glaucoma screening was limited to IOP measurement and visual fields, Dr. Fingeret explained. Both have limits, however, he said. For example, in one-third of patients who have open-angle glaucoma, IOP never goes higher than 21 mm Hg.1 Also, in those who have glaucoma, IOP can vary by as much as 10 mm Hg throughout the day, and a single IOP reading has only 50% sensitivity to detect glaucoma. Further, IOP readings may be artificially high or low depending on central corneal thickness. Use of systemic medications also may affect IOP.

Screening visual field tests often have low sensitivity/specificity in early glaucoma, which precludes their use as a single test, Dr. Fingeret said.

Still, he added, a positive finding on a screening test performed at the initial stage of the examination could be explored during the eye examination.

During the eye exam, the clinician needs to determine why the screening test (for example, a test for cataracts or a retinal lesion) failed to detect glaucoma, Dr. Fingeret said. Sensitivity of the screening test needs to be high enough to find most moderate to severe cases of glaucoma, he added, and specificity should be high to not overcall too many patients with normal eyes as having glaucoma. Frequency doubling technology perimetry in the screening mode has set the model for this type of test, he said.

The introduction of a scanning laser ophthalmoscope (Panoramic 200, Optos) led to increased interest in the use of nonmydriatic, wide-angle retinal photography as part of the comprehensive eye evaluation. Using the scanning laser ophthalmoscope, the examiner captures images, including those of the retina. The resulting high-resolution image (known as the Optomap) provides a 200° view, or 80% of the retina.

Some clinicians use the scanning laser ophthalmoscope either as an alternative to a dilated retinal examination or as a screening tool to decide which patients require dilation, Dr. Fingeret said, adding that others use it as a screening tool at the start of the exam so that the results complement the dilated fundus exam. If the examination reveals an abnormality, then the doctor can refer the patient along with an accompanying copy of the images, he said.

Ideally, this screening test should be performed at the onset of examination to guide the examination and alert the clinician as to whether a problem is present, Dr. Fingeret said. The pretesting battery could include lensometry, autorefraction, visual acuity, screening fields, and nonmydriatic wide-angle retinal photography, he said. This way, the doctor starts the examination with a sense of what eyeglass prescription, refraction, fields, and optic nerve and retina are, Dr. Fingeret added.

But are the images superior to a dilated view through the binocular indirect ophthalmoscope? Advantages of the high-resolution image are that it is better than no dilation and it allows a view over a wide field, he said, adding that limitations of the high-resolution image include low magnification; tilted, distorted image; and wide field.

Benefits of screening

Another screening technique may be to use the digital retinal camera, found in many optometric practices, as a screening device, Dr. Fingeret said. Advances in digital retinal photography provide non-mydriatic, low-illumination images that have high clarity and resolution. Images usually cover a field of approximately 45°. Most cameras can increase magnification, reducing the field to 30°. The concept is to take an undilated retinal image, centered on the optic disc and including the posterior pole of each eye, at the start of the examination. This screening test then can be used to guide the examination if conditions are found in the photographs. Digital photography does not take the place of an eye exam, he said; instead, clinicians can use it for triage and for telemedicine.

Also, without dilation, one can easily miss subtle signs of glaucoma, Dr. Fingeret said. Indeed, 13% to 18% of the images taken through a nondilated pupil are ungradeable, although selectively dilating patients can improve that number to 4%, he added. Also, fewer ungradeable images exist in a younger population, Dr. Fingeret said. The presence of cataracts and small pupils are the most common reasons for poor images, he added.

Although screening for glaucoma is not conducted in the general population, ODs can perform a screening as part of their exam and use the test(s) to pursue any unusual findings, Dr. Fingeret concluded.

Reference

1. Wong EY, Keeffe JE, Rait JL, et al. Detection of undiagnosed glaucoma by eye health professionals. Ophthalmology. 2004;111: 1508–1514.

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