Appropriate examination of the fundus is often the most challenging aspect of the complete ophthalmic exam for many veterinarians. First, for some individuals, proper ophthalmoscopic techniques can be difficult to master. Second, the normal anatomical relationships of fundus structures are often not well-understood, making interpretation difficult. Finally, the large variation in normal fundus appearance, especially in the canine species, adds to the complexity of this important diagnostic tool. This lecture aims to answer the following frequently asked questions and provide practical tips to help ensure good posterior segment ophthalmoscopic skills.

1.  What is ophthalmoscopy?

2.  What technique should I be using to get a good look at the posterior segment of my canine or feline patient?

a.  Provide an overview of techniques with indications, advantages, and disadvantages of each method

3.  What am I looking at?

a.  Building a fundus

4.  Okay, now I can get an image in focus, but what does it mean?

a.  Is it normal or abnormal? If abnormal, is it an addition or a subtraction?

The fundus is a composite picture formed by the 3 basic tunics of the vertebrate eye (neurosensory, vascular, and fibrous) superimposed on one another. As viewed ophthalmoscopically, those components (from the inner to the outer aspects of the globe) are the retina, choroid, and sclera. Normal fundic findings can be highly variable and are influenced by the animal’s species, breed, age, and coat color. Ability to routinely and reliably evaluate the posterior segment of the eye is essential in any patient with a visual disturbance or for which the differential diagnose includes systemic infectious disease, vascular disorder, hypertension, or central nervous system disease. This author is a strong advocate of indirect ophthalmoscopy (versus direct) and she strongly encourages her audience attendees to master this technique for routine examination of the small animal patient. In this author’s opinion, proper technique for direct ophthalmoscopy is more difficult than indirect technique. The high magnification afforded by direct ophthalmoscopy often precludes accurate interpretation of the disease process in question. Once the clinician has mastered the techniques necessary to perform a good ophthalmoscopic exam, it is essential that the practitioner then become familiar with the wide variation in normal fundus findings routinely seen between species, within species, and from breed to breed.

Examination

Follow these steps prior to performing direct or indirect ophthalmoscopy:

• Ophthalmoscopy is part of the complete ophthalmic examination and should be performed after the minimum ophthalmic data base has been acquired.
• In the initial stages of the complete ophthalmic exam, retroilluminate the patient. With the patient at eye level and using a Finoff transilluminator (or a direct ophthalmoscope set at 0 diopters) at arm ‘s distance, establish both tapetal reflexes. Look for anisocoria (unequally sized pupils) and any opacities present in the ocular media (e.g., cataract).
• Pupillary dilation is essential for ophthalmoscopy. First, make certain the animal does not have glaucoma. Then, dilate the pupils with 1–2 drops of tropicamide (Mydriacyl®). Complete pupillary dilation will occur in 10–15 minutes and will last 8–12 hours in the dog.
• Perform your examination in a very dark room.

Comparison of Ophthalmoscopic Techniques

• Indications
• Advantages
• Disadvantages

Direct Ophthalmoscopy

Detailed examination of specific areas of fundus (e.g., optic nerve, chorioretinal scar, coloboma).

Can use handle to attach other heads (e.g., transilluminator or otoscopic)

1.  Portability

2.  Inexpensive

3.  Image greatly magnified

4.  Direct upright image

a.  Decreased penetration through cloudy media

b.  Small field of view (the image is often too magnified)

c.  Short working distance (closer to the animal’s head)

d.  No stereopsis

e.  Inability to examine peripheral retina

Binocular Indirect Ophthalmoscopy

Examination of the ocular fundus

1.  Examiners hands both free to manipulate patient’s head when wearing a headset

2.  Better visualization through translucent ocular media

3.  Large field of view

4.  Stereopsis

5.  Distance from patient’s head

a.  Inverted reversed image

b.  Headset more expensive than direct ophthalmoscope

Monocular Indirect Ophthalmoscopy (Focal Light Source and Lens)

Examination of the ocular fundus (*recommended technique for the small animal practitioner*)

1.  Inexpensive

2.  Portable

3.  Better visualization through translucent ocular media

4.  Large field of view

5.  Distance from patient’s head

a.  No stereopsis

b.  Inverted/reversed image

Direct Ophthalmoscopy

• Use this technique to achieve greater magnification of specific areas of the fundus in small animals.
• Image viewed is upright, highly magnified, with a small field of view, and there is no stereopsis.
• Extremely difficult technique to use for general fundus screening and visualization is poor with opacification of the ocular media as compared with indirect ophthalmoscopy.
• Step 1: Turn the ophthalmoscope on; adjust the rheostat to the desired light intensity.
• Step 2: Darken the room.
• Step 3: With the instrument set to 0 diopters, hold it to your brow, and view the tapetal reflection from a distance of ∼18–24 inches from the animal’s eye.
• Step 4: Continue to view the tapetal reflection and move to within 1–2 inches of the cornea of one of the animal’s eyes. Providing the lens and cornea are normal, the patient’s fundus should be coming into focus. The dioptric settings may be adjusted slightly so that the fundus comes into clear focus for the examiner (remember** - if the examiner normally wears corrective eyewear and removes his/her glasses to perform direct ophthalmoscopy, his/her own refractive error will need to be adjusted for).
• Attempt to systematically view the fundus. Find a blood vessel and follow it to the optic nerve. Using the optic nerve as a “reference point”, scan the fundus for abnormalities of color, clarity, size, and shape. Change the diopter settings to focus in and out of raised or depressed areas.
• It is often helpful to view the animal’s left eye with your left eye and the animal’s right eye with your right eye.
• Other: On most ophthalmoscopes, the red diopter numbers are “negative” (representing the power of diverging lenses). Higher red numbers push the focus point deeper into the eye. The black (or green in some models) numbers (representing the power of converging lenses) are “positive” and bring the focus point more superficial in the eye, enabling visualization of nearer objects.
• More positive dioptric settings may be “dialed-in” so that opacities in more anterior structures become visible. In general, the posterior lens is in focus at +8D the anterior lens at +12D, and the cornea at +20D. The lens or cornea will only be visualized using direct ophthalmoscopy if an opacity is present in those structures and thus, sufficient light is able to be reflected. If the lens or cornea is normal, all that is visualized is a blurred image of the fundus because these structures normally reflect very little light.
• Viewing apertures:
• Use large circular aperture when pupil is large and small circular aperture when pupil is small. It this author’s opinion, it is advisable to always dilate the small animal patient prior to funduscopic examination, regardless of the method being used.
• Slit aperture is useful for eliciting the Purkinje images or determining whether the optic nerve head is raised or depressed.
• G rid aperture can be used to directly measure the size of lesions or to relate the size of a lesion to that of the optic nerve.
• Green light is red-free and permits differentiation of melanin pigment from blood. With red-free light, blood looks black and pigment still looks brown.
• Blue light filter excites fluorescein dye.

Indirect Ophthalmoscopy

• This technique can be performed using a hand-held lens and a bright focal light source (monocular indirect ophthalmoscopy), or for those with a stronger interest in ophthalmology, an indirect headset can be purchased and will provide stereopsis (binocular indirect ophthalmoscopy).
• Indirect ophthalmoscopy is recommended to the small animal practitioner as the preferred method to visualize the fundus of the small animal patient by this author. The image is significantly less magnified than that seen with direct ophthalmoscopy, has a much larger field of view, and is better for routine screening of the eye.
• Indirect ophthalmoscopy provides the examiner with an inverted, reversed image.
• Monocular indirect ophthalmoscopy is inexpensive and required equipment consists only of a bright focal light source and a 28 or 20 diopter lens. It is performed as follows:
• Step 1: Begin at arm’s length from the patient. An assistant is required to restrain the patient and to help hold the eyelids open.
• Step 2: Darken the exam room (the darker the room, the better the exam).
• Step 3: With a focal light source such as a transilluminator or penlight held close to your lateral canthus of your dominant eye or at your cheek and standing at arm’s length from the patient, visualize the tapetal reflection (you will see a red reflex in animals with poorly developed or no tapetum).
• Step 4: Hold a 20- or 28-diopter condensing lens in your other hand between the thumb and index finger, and use your ring finger and pinky to elevate the upper lid. Initially, hold the lens to one side of the eye until the tapetal reflection is established, and then rotate the lens such that the eye can be observed through it. Initially, hold your lens close to the corneal surface (about 1/2 inch from the eye) and then slowly pull the lens towards you until the image of the fundus fills the lens (usually about 1–2 inches away from the animal’s eye, depending on the diopter power of the condensing lens you are using). When the image is lost (usually because you or the animal moved), immediately rotate the lens away from the patient’s eye (the left ring finger and pinky continue to hold up the upper lid), re-establish the tapetal reflex, and again, rotate the lens back into place in front of the eye so the fundus comes into view. The fundus should appear as a virtual image in front of the lens. (Hint: some people find it helpful to look at the image which is in front of the lens and not at the lens or the eye; this author “sees” the fundic image within the lens).
• Step 5: Maintain alignment with your light source, lens, and patient’s eye and then move yourself up, down, left, or right to examine all quadrants of the patient’s fundus. (Hint: remember that the image is inverted, so you must move in the opposite direction to the image). If the image is lost, move the lens out of the light beam and start again.
• Indirect ophthalmoscopy is relatively easy to master providing you practice often!
• Various lenses are commercially available for indirect ophthalmology and will provide varying magnification and field of view. The less the magnification, the greater the field of view. Although a 20 or 2.2Diopter lens is considered “standard” for most board-certified veterinary ophthalmologists in practice, I routinely begin all of my exams using a 28 diopter lens, and then work my way up to higher magnifications by making use of the 2.2D, 14D, and if necessary the 5.5 MAC lens.

Interpretation of Findings

When viewing the fundus, be systematic.

Assess

1.  Reflectivity (Uniformly or focally increased? Uniformly or focally decreased?) - Note it is very helpful to learn how to “shimmy” the lens to help determine is a lesion is hyper or hypo-reflective. This will be demonstrated in the lecture.

2.  Vascularity (Attenuated? Engorged? Tortuous? Out of focus due to elevation or depression?)

3.  Optic nerve (Color? Degree of myelination? Raised? Depressed?)

4.  Pigmentation of tapetal versus nontapetal areas.

Ask yourself: Normal versus abnormal? If abnormal a is it due to an addition (something is there that shouldn’t normally be there such as fluid, blood, or a granuloma) or subtraction (the retina and possibly underlying choroid has undergone a selective or uniform degeneration such as what we see in inactive chorioretinal lesions from distemper or end stage retinal degeneration diseases)?

References

References are available upon request.