October 2021

In this VETgirl online veterinary continuing education blog, Dr. Shelby Reinstein, DVM, DACVO reviews how to do an ophthalmic exam in our veterinary patients! If you’re not good at eye exams in cats and dogs, tune into learn as Dr. Reinstein reviews the important components of every ophthalmic exam.

Also, to get your learning on, make sure to view her amazing VETgirl webinar “Do You See What I see?  Ophthalmic Exam Basics.”

Do You See What I See? Ophthalmic Exam Basics in Veterinary Medicine

By Shelby Reinstein, DVM, DACVO

Ophthalmic History
Obtaining a thorough history is crucial in the management of any case and specific questions should be posed for an ophthalmic case.  The first step is identification of the presenting complaint and predominant symptoms that incited the office visit.  Often owners will report squinting, tearing, swelling, or redness as the main cause(s) for concern.  Many ocular conditions also cause discharge from the eye – if this is present, it’s helpful to classify the discharge to help narrow our list of differentials.   Mucoid and mucopurulent discharge are most common, and generally indicate surface eye disease such as eyelid, conjunctival, or corneal disease.  In addition, owners should be questioned about changes in vision, even if that isn’t part of the presenting complaint.  Oftentimes our pet owners aren’t sure if their pet can see normally. However once they are specifically questioned about their pet’s navigation ability in various light conditions, stairs, or unfamiliar environment, for example, they may identify a vision issue.   The duration of the clinical signs and the progression should be discussed, including any response to medications that have been used.  Lastly, it is very important to discuss the overall health of the pet, including any known concurrent disease, routine medications, and systemic clinical signs like weight loss or vomiting.

The Initial Examination
The first step in the eye exam should be looking at the patient from a distance, with minimal head restraint.  This should be accomplished prior to placing the pet on the exam table to avoid inducing squinting, head shyness, or even excessive tearing.  Specifically, the distance exam should focus on pupil sizes and symmetry, facial symmetry, and the position and size of the globe itself.  In addition, the position of the nictitating membrane (3rd eyelid) should be noted.

Neuro-Ophthalmic Examination
A neuro-ophthalmic exam should be performed on every case.  Vision is assessed most often using the menace response.  Occasionally, a maze test is used to help identify subtle visual changes or exaggerate the effects of various lighting conditions.  For animals younger than 4 months of age who have not developed a full menace response, visual placing or a visual cliff test can be used to assess vision.

The cranial nerves associated with the eye should be tested in a systemic fashion.  The pupillary light reflex assesses the function of the retina to perceive light, followed by the optic nerve (CN II) as the afferent arm and the oculomotor nerve (CN III) supplies the efferent fibers.  The palpebral reflex is performed by tapping both the medial and lateral aspects of the eyelids – sensory fibers travel in the trigeminal nerve (CN V) and elicit a blink via motor fibers in the facial nerve (CN VII). The dazzle reflex is a subcortical reflex, and thus is not truly indicative of vision.  When a bright light is shone into the eye, the stimulus travels via the retina to the optic nerve, and then projects onto the midbrain and a blink reflex is mediated by the facial nerve.  This reflex is most helpful in patients with mature cataracts or elevated intraocular pressure, when the menace response cannot properly determine the visual potential.

Ophthalmic Minimum Data Base
Performing a Schirmer Tear Test (STT), fluorescein staining, and measurement of the intraocular pressure should be considered in almost every eye case, as the clinical signs dictate.   A STT should be performed prior to any significant manipulations of the eye, and before any drops are instilled.  While dogs have a very reliable normal value of >15 mm of wetting per minute, cats may not tear reflexively due to high sympathetic tone.

VETgirl image of a cat with a corneal ulcer

Instillation of fluorescein and examination with a cobalt blue filter is the only way to determine the presence and extent of a corneal ulcer.

Tonometry is an estimation of the intraocular pressure (IOP) usually performed using a TonoPen ® or TonoVet ®.  The Schiotz tonometer is considered outdated, difficult to use, and rarely accurate.  Normal IOP is between 20-25 mmHg in dogs and cats and will lower naturally with age.  It is important to realize that these are estimations of intraocular pressure, and the results should be interpreted in light of the clinical signs and ocular exam findings.

Examination Basics and Equipment
It is very important to be systematic when performing an ocular exam.  It is easy to become distracted by the large corneal ulcer and overlook the eyelid mass, for example.  A checklist or exam form is helpful to ensure that each structure is examined for abnormalities.  Most often, the exam is performed starting with the adnexal structures (eyelids, 3rd eyelid, conjunctiva), progress to the surface structures (sclera, cornea), and then continues from the front of the eye to the back (anterior chamber, iris, lens, vitreous, retina).  Drawing or writing detailed descriptions of all abnormalities is recommended so these may be readily compared to follow-up examinations.

A thorough eye exam can be performed using basic equipment, including a focal light source and magnification (such as head loupes or a direct ophthalmoscope).  An indirect lens for fundic examination has benefits (to be discussed later) but is not considered a necessity.  A transilluminator provides a bright, focused light source that can be used for distance examination and PLR and dazzle testing.  Coupled with a source of magnification such as head loupes, the adnexal structures can be examined with this light source.  The direct ophthalmoscope provides a source of light with magnification and focusing ability.  This instrument is used to examine both the ocular surface and the internal ocular structures.  Most direct ophthalmoscopes have 5 apertures that are adjusted using the rolling dial on the front of the attachment.  The macular (small circle) aperture is used to illuminate a small, focused beam of light through the anterior chamber to detect opacities such as aqueous flare or blood.  The large circle is the most often used to examine the vast majority of the ocular structures.  The dial on the side of the attachment changes the diopter strength of the lens, which focuses the light at different depths within the eye.  Green numbers are positive and red numbers are negative and are displayed on the front or side of the attachment.  In general, the cornea is in focus at +20D, the anterior lens at +12D, the posterior lens at +8D, and the optic nerve at 0D.  Location of lesions within the ocular chambers can be determined by rolling the focusing dial up and down to note the relationship of the lesion to these known focusing depths.  The slit aperture provides an optical section and can be used to detect curvature changes, depth of lesions, or the distance between 2 structures (see Methods of Illumination). The intermediate circle, glaucoma (haptic), and semi-circle apertures are not useful in veterinary medicine.  Finally, the indirect attachment has a colored light filter – either red-free (green in color) or cobalt blue.  The cobalt blue filter is used for visualization of fluorescein staining.  The red-free filter, which differentiates between pigmentary changes and hemorrhage, is commonly used in human ophthalmology but is less useful for veterinary patients.

Methods of Illumination
There are various methods of illumination that can be used to determine the density and location of structures or lesions.  Direct illumination is achieved when the light source is shone directly upon the structure of interest and can be done in a diffuse or focal manner.  Diffuse direct illumination is performed by shining the light from a distance and illuminates a larger area with an unfocused light beam.  This technique is useful for examination of the adnexal structures, ocular surface, lens, and the iris.  A structure that is dense in nature (like a cataract) will appear white with direct illumination, as all of the light beams are reflected back at the examiner.  Focal direct illumination is accomplished by bringing the light source into close proximity to the structure of interest in a dark room. This technique is used to detect the presence of cloudy ocular media (aqueous flare) and, when viewed or shone at a tangential angle, can highlight cystic structures and emphasize texture.  Light will transmit through a cystic structure, which would have otherwise appeared solid when illuminated diffusely.  A slit beam can also be used to perform direct focal illumination and highlight topographical alterations.  This technique is particularly helpful when determining the depth of a corneal ulcer, for example.  In addition, distance estimation can be accomplished by examining the reflection off of 2 superimposed structures. Retroillumination takes advantage of the tapetal reflection in the majority of veterinary patients.  By shining the light onto the tapetum, the structure of interest is “back-lit.”  A dense cataract will block this light, and appear dark, whereas a cystic or thinned lesion (such as iris atrophy) will allow light to pass through.

Fundic Examination
The retina can be examined in both direct and indirect methods.  Direct ophthalmoscopy is achieved by setting the diopters of the ophthalmoscope around OD. The advantages of this technique image are that is provides a highly magnified image that is upright (real), does not require full pupillary dilation, and is accomplished with minimally expensive equipment.  The disadvantages are the required proximity to the patient, and the field of view is quite small thus requiring extensive examination to visualize the entire retina.  Indirect ophthalmoscopy provides a much less magnified view (depending upon the dioptric strength of the lens used) but the field of view is subsequently greater.  The disadvantages are that the image produced is inverted and upside down, and also the need for specialized indirect lenses.

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