by Noelle La Croix, DVM, Dip. ACVO
To the casual observer a pupil appears black, even though there is a direct transparent pathway from a pupil to a colorful retina. Ambient light enters an eye through its pupil at a multitude of different angles. Some of this light is subsequently reflected by the retina, and exits the pupil in assortment of different angles. The reflected light’s intensity (brightness) is usually below an observer’s threshold for perceiving a clear image of the retina, and this absence of light appears as blackness. However, a condensed (bright and compact) light source can easily generate an observable reflection from a retina. In photography this phenomenon is known as ‘red-eye’ and it occurs when a flash (condensed light) is reflected by a subject’s eyes. Fortunately the reflection of condensed light has been put to practical clinical use in visualizing the retina. It is the underlying principle of all ophthalmoscopy.
Direct ophthalmoscopy uses a condensed light source to reflect an erect (non-inverted) image of a subject’s retina to an observer. It is a simple technique that typically produces highly magnified images of retinae. However, a 15 to 17 fold magnification greatly reduces an observer’s field of view. Only a fraction of a retina can be visualized at any particular moment. A thorough retinal exam by direct ophthalmoscopy is therefore time consuming, and made more difficult by moving animal subjects.
Direct ophthalmoscopy is also limited by relatively dim light sources that cannot be reflected by a ‘cloudy’ media. Retinae are poorly visualized in cases of nuclear sclerosis, or in any eyes with hazy vitreal or aqueous fluid. A direct ophthalmoscope’s light source can also be advantageous. Dimmer lights reflect truer color images of optic nerves than brighter lights. This is particularly useful in diagnosing optic nerve atrophy or neuritis. The bright light source of an indirect ophthalmoscope will generally blanch out the color of an optic nerve.
In indirect ophthalmoscopy, a convex lens is placed between the observer and subject generating an inverted image. A binocular indirect ophthalmoscope emits a bright light that is reflected back to both eyes of an observer. This results in a significant increase in the depth perception of an observer.
An indirect ophthalmoscope typically produces images with lower magnification than that of a direct ophthalmoscope. The associated increase in an observer’s field of view significantly reduces the time it takes to thoroughly visualize an entire retina. The difficulty of this technique involves maintaining a direct line of sight. A light source, lens, and reflected image must be maintained in a straight path from an observer to subject at all times. An observer can only visualize reflected retinal light within a limited field of view dictated by this line of sight. Novices will typically move their head during indirect ophthalmoscopy in an attempt to ‘see around’ the retina. This head bobbing will remove an observer from a line of sight and a retinal image will be quickly lost. With indirect ophthalmoscopy the only way to scan an entire retina is by establishing multiple lines of sight.The bright light source of an indirect ophthalmoscope can also be used to visualize a retina through a cloudy media. The high quality lenses of indirect ophthalmoscopes are designed to reduce both spherical and chromatic aberrations. These instrument can even be used to visualize an undistorted retina behind lenticular, aqueous, or vitreal opacities.
A monocular indirect ophthalmoscope produces an erect magnified image. It is therefore simple to use (like a direct ophthalmoscope) and also has an increased field of view (like an indirect ophthalmoscope). However, it lacks the stereopsis and the larger image size of a binocular indirect ophthalmoscope (Figure 1).
All forms of ophthalmoscopy are best performed in a darkened room. Pupillary dilation allows for more thorough retinal scanning during the procedure. Ophthalmoscopy can be used to routinely monitor the visual and systemic health of both dogs and cats. If you have any questions about ophthalmoscopy, please don’t hesitate to contact a veterinary ophthalmologist.
Noelle La Croix, DVM, Dip. ACVO
Veterinary Medical Center of Long Island
75 Sunrise Highway
West Islip, New York 11795
(631) 587-0800; fax (631) 587-2006
Figure 1: The normal canine fundus as it appears through direct (A), monocular indirect (B), or indirect (C) ophthalmoscopy (photos courtesy of David Ramsey).