Testing for retinal disease is directed by the clinical examination of the patient, which starts with obtaining a thorough patient history, including family history, and a careful review of systems. The patient should undergo a detailed ophthalmic examination, including visual acuity measurement and testing for a relative afferent pupillary defect. In addition, in-office Amsler grid testing can provide evidence of macular disease, if present.
This chapter explores some of the imaging modalities used to evaluate retinal disease. Each modality supplies a limited set of information; no particular modality supplies all of the information about every disease. In order to understand the layers of the fundus and their disease manifestations, it is necessary to employ multimodal imaging, mastering many forms of ocular imaging and rapidly integrating the information gleaned into a more complete whole.
The direct ophthalmoscope provides an upright, monocular, high-magnification (15×) image of the retina. The instrument’s lack of stereopsis, small field of view (5°–8°), and poor view of the retinal periphery limit its use. Therefore, indirect ophthalmoscopy has largely supplanted direct ophthalmoscopy. In indirect ophthalmoscopy, illumination is directed into the eye through an aspheric lens; this lens helps form a flat inverted aerial image between the lens and the observer. The magnification is calculated by dividing the dioptric power of the examination lens into –60. For example, a 20 D lens would have a magnification of –3; the negative sign indicates an inverted image. The binocular indirect ophthalmoscope has a unitary magnification. Through the use of prisms, it reduces the distance between the pupils in the instrument, and, using a mirror, it reduces the distance from the light source to the optical axis, so that all 3 fit into the entrance pupil of the eye (see Chapter 8 in BCSC Section 3, Clinical Optics). Binocular indirect ophthalmoscopes allow for stereopsis, have a field of view that depends on the dioptric power of the condensing lens (higher powers deliver wider angles of view, but at a lesser magnification), and, with ocular steering, can visualize the entire fundus as compared with direct ophthalmoscopes. If the patient’s pupil can be dilated widely, the ora serrata may be seen without any additional instrumentation. If the pupil cannot be widely dilated or the clinician needs to see peripheral retinal details in profile, scleral depression must be performed.
The disadvantages of binocular indirect ophthalmoscopy include low magnification and the inverted image. A binocular indirect ophthalmoscope offers no additional magnification of the aerial image. Indirect ophthalmoscopy can also be performed with a slit lamp by using a non–contact lens, for example, a 60 D or 78 D lens. With a 60 D lens, the magnification afforded by the lens is 1; however, the slit lamp typically has a magnification of 10× or 16×. The slit beam can be used to directly or indirectly illuminate the posterior portion of the vitreous as well as retinal structures. The field of view provided by the lenses is good; a little less than 70° with a 60 D lens and more than 80° with the 78 D lens. Ocular steering can be used to evaluate a large area of the fundus. Higher dioptric lenses can help visualize wide areas of the retina even if the pupil does not dilate. These lenses may have a field of view of 100° or more. With noncontact indirect biomicroscopy, the power and capabilities of a slit lamp with a wide field of view may be employed without having to contact the eye; subsequent ocular imaging can proceed without a problem because no contact is made with the cornea. The main disadvantage is that the image created is inverted.
A direct-contact lens provides 1 of the highest resolution methods to view the fundus. Although these lenses provide no significant magnification, they obviate the refractive power, and potentially any astigmatism, of the cornea. The 3-mirror lens is commonly used in this method. The central portion is used to directly visualize the posterior pole and has a field of view of a little more than 20°. The 3 mirrors can be used to evaluate the midperiphery and far periphery of the retina as well as the angle. The advantages of direct-contact biomicroscopy include high resolution and a noninverted image. The disadvantages include having a limited field of view through any 1 component of the lens, needing to rotate the lens on the patient’s eye while performing the examination, and needing to manipulate the cornea or use a viscous coupling fluid, either of which may hinder subsequent ocular imaging. Indirect-contact lenses provide the largest field of view of any lens used for ophthalmoscopy, often more than 150°. They can be used with ocular steering to easily visualize the ora serrata.
Lenses are chosen at the discretion of the examining clinician, according to what is needed for the specific examination. For example, an opacified posterior capsule with a small posterior capsulotomy may inhibit good visualization of the retinal periphery with a 3-mirror lens, but pose no significant problem for a wide-field indirect-contact lens.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.