Ocular Functional and Structural Investigations

Color Fundus Photography

Standard fundus cameras, which are typically found in ophthalmology clinics, require pupillary dilation to capture images of the macula (central retina), optic disc, and retinal periphery. Dilation is needed to overcome the technical limitations of these cameras, which require the incident light beam projected on the fundus to follow a different optical path through the cornea and lens than the reflected imaging beam in order to avoid reflections from the cornea and lens. Newer nonmydriatic cameras have become available that capture high-quality images of the central retina or optic nerve in patients with pupils as small as 3–4 mm ( Fig. 43.1 , A–C ). Two-dimensional retinal images may also be obtained with scanning laser ophthalmoscopy (SLO), which generates an image based on the amount of light reflected by a single wavelength of infrared laser. Newer cameras utilizing multiple scanning lasers and pseudo-coloration (e.g., Optos plc, Dunfermline, Scotland, UK) are capable of capturing ultra-widefield retinal views of up to 200 degrees; they have particular utility for assessing disorders that affect the retinal periphery (see Fig. 43.1 , D ).

Fundus photography is primarily used to document and allow scrutiny of the ocular fundus, relying on the discernment of disease pathology by an experienced specialist who interprets the images, similar to the interpretation of a fundus examination performed during ophthalmoscopy. Although not yet routinely used, automated image analysis methods to detect abnormalities in retinal images have also been developed ( ).

In the context of waning proficiency with direct ophthalmoscopy, due to limited training, interest has recently been spurred in the development of nonmydriatic fundus cameras in settings such as emergency departments and neurology offices ( ). Nonmydriatic fundus photography offers significant advantages over direct ophthalmoscopy: it offers a much wider field of view (45 vs. 5 degrees), circumvents the technical barriers of direct ophthalmoscopy, and yields images that can be used to monitor a patient’s visual or neurological disorder—images that can be shared with consulting specialists or the patients themselves to educate them regarding their condition. Disadvantages include limited availability, lack of portability of tabletop devices, and cost.

Recent technological advances have facilitated the development of handheld nonmydriatic fundus cameras that yield a relatively wide field of view and may be used in critically ill patients, but cost and availability limit their use. Fundus photography devices that attach to the built-in cameras of smartphones have also generated considerable interest, offering many of the advantages of fundus photography and great portability. However, they can be difficult to use, especially without pupillary dilation, and image quality tends to be considerably inferior to that from tabletop cameras.

There has been growing interest in the use of fundus photography to research neurological disorders. Changes in retinal vascular caliber associated with cerebrovascular disease and cognitive impairment can be analyzed with fundus photography. These findings raise the possibility that routine fundus photography may eventually play a role in the early identification of medical and neurological conditions ( ).

Fundus Autofluorescence Imaging

Autofluorescence imaging records the light emitted by native fluorophores in the retina after excitation by light of a blue or green wavelength. The principal naturally occurring fluorophore is lipofuscin, contained within cytoplasmic granules of retinal pigment epithelial cells. Images may be captured by a properly equipped fundus camera, an SLO-OCT machine, or a wide-field SLO. A normal fundus autofluorescence (FAF) image shows a low-intensity background autofluorescence with reduced intensity in the foveal region related to the absorption of blue light by macular luteal pigment ( Fig. 43.2 , A ). The optic nerves and blood vessels, which contain no fluorophores, appear very dark. Changes in the normal pattern of FAF most often occur in retinal diseases that disrupt retinal pigment epithelium (RPE) cells (see Fig. 43.2 , B ). Autofluorescence imaging is also useful for identifying optic disc drusen at or slightly below the surface of the optic nerve head (see Fig. 43.2 , B and C ).

Optical Coherence Tomography for Assessing Retinal Disorders

OCT imaging is an indispensable component of the diagnosis and monitoring of retinal disorders in which pathology is evident by morphological changes in the retina. It is also extremely valuable to distinguish cases of subtle macular pathology from other causes of visual loss, such as retrobulbar optic neuropathy.

Retinal artery occlusion

The retinal arterioles originating from the central retinal artery give rise to capillary networks supplying the inner retinal layers, including the retinal nerve fiber layer, ganglion cell layer (GCL), inner plexiform layer (IPL), and most of the inner nuclear layer ( ). The outer retinal layers, which include the photoreceptors, receive oxygen through diffusion from the retinal choriocapillaris, which is supplied by posterior ciliary arteries originating from the ophthalmic artery. Thus an acute central or branch retinal artery occlusion results in cytotoxic edema in the inner retinal layers that is evident as hyperreflective signal change and thickening ( Fig. 43.4, A and B ). In areas with permanent injury, atrophy ensues over weeks, with loss of thickness of the retinal layers extending beyond the GCL into the inner nuclear layer, which includes the bipolar cell soma (see Fig. 43.4 , C ). The outer retinal layers that receive oxygen through diffusion from the retinal choroid are preserved in an isolated occlusion of a central or branch retinal artery. In contrast to the optic atrophy that ensues following occlusion of a central retinal artery, optic atrophy from primary optic neuropathies is characterized by selective thinning of the retinal nerve’s fiber layer and GCL without involvement of the deeper inner retinal layers.