Ocular diagnostic testing: what, when, how? (Proceedings)

The basics

Signalment

     • Glaucoma in Cockers, Bassets

     • KCS in Cockers, Bulldogs, Pugs, Lhasas, Shih Tzu

     • Retinal degeneration in Poodles, Labs

     • Indolent corneal ulcerations in Boxers, Retrievers

History

     • When did the problem start?

     • Can the patient see?

     • Do they bump into things? If so, when?

     • Current medications? When they were started?

     • Environment?

     • What did the owner notice first?

     • Past illnesses, injuries, medications, etc.

Examination

     • Set yourself up for success, Have a quiet dark room, a comfortable exam space, a stool, and an assistant. If presenting for other problems as well, do the eye exam first.Have a systematic plan and stick to it.

     • Gather equipment and proceed to a low lit area. Bright light source, Schirmer tear strips, Fluorescein stain, Proparacaine, Tono-Pen/Tono-Vet/Schiotz, Head loops, Direct ophthalmoscope or lens

Basic eye exam

     • This exam should be the same in every eye case that walks in. Assess symmetry, Menace response, Test dazzle and PLR (direct AND consensual), Palpebral reflex, Tear Test, Fluorescein stain, Proparacaine, Examine the eye with a bright light source and head loop.

Basic eye exam

     • Lids: Lacerated? Inflamed? Alopecia?

     • Conjunctiva, Sclera, Third eyelid: Is it red? Is there hemorrhage? Is the TEL elevated?

     • Cornea: Hazy? Taking up stain? Divot? Laceration? Vessels? Melting?

     • Anterior chamber: Can you see the pupil or is it obstructed by hyphema/lens

     • Lens: Is it in position, is it clear?

     • Examine the posterior segment: Use the direct ophthalmoscope OR Use transilluminator and lens

     • Can you see the optic nerve and retina? Is the retina detached? Bleeding in the posterior segment?

Pre corneal tear film

     • Outer – Lipid. Produced by the meibomian glands. Affects tear film breakup times

     • Middle – Aqueous. Produced by the Lacrimal and Nictitating glands. Evaluated by STT

     • Inner – Mucin. Produced by the Conjunctival goblet cells. Affects tear film breakup times.

Schirmer tear test

     • Indications: Red eye, Discharge

Performing

     • 60 seconds in the ventral conjunctival fornix between the lower lid and the third eyelid.

     • Do not touch the notched end with your fingers.

     • If you like to fold them, do it PRIOR to removing from the plastic package.

     • Always perform test on BOTH eyes.

Normal values:

     • Dog: > 15mm of wetting/minute

     • Cat: typically > 10 mm of wetting/minute

     • Rabbit: > 2-3 mm of wetting/minute

Tear breakup time

     • Evaluates the mucin and lipid layers of tear film

     • Apply fluorescein and allow patient to blink, then maintain lids open until the fluorescein begins to disassociate

     • Time between blink and first dry spot is the tear breakup time. Shorter times may indicate qualitative tear film deficiency

External ophthalmic stains

     • Rose Bengal Dye

     • Tear film assessment

     • FHV-1 dendritic lesion detection and Equine mycotic erosions (early)

     • Sodium fluorescein

     • Absorption spectrum: blue light (490 nm). Converts almost 100% of absorbed light to emitted green light (520 nm)

     • Lipophilic epithelium repels stain. Only the stroma absorbs stain

     • Use: Tear breakup time, Corneal or conjunctival ulcers, The Seidel test, Patency of nasolacrimal apparatus or Jones Test

The siedel test

     • Detects leakage of aqueous humor. Liberal application of fluorescein, gently press cornea, look for green dye indicating aqueous leakage

Jones test

     • Evaluates the nasolacrimal passage. Takes ~5 min in the dog and cat and <10 min in the horse. Longer times may indicate partial or complete obstruction. False negatives from licking nares and with brachycephalics.

Cytology and culture

     • Cytology is used either alone or in combination with culture to provide rapid results that may influence treatment

     • Obtain samples from...

          o A deep, melting, progressive, or infected-looking corneal ulcer.

          o Purulent or granulomatous conjunctival lesions

          o Chronic or severe keratitis/conjunctivitis cases not responding to treatment.

          o Corneal masses

     • Equipment: Cytobrush, Beaver blade, Cotton swab, Slides, Microscope

     • Intraocular pressure Use Proparacaine

     • Duration of effect on normal eyes:

          o Dog: 1 drop → 45 min (max effect 15 min). 2 drop → 55 min (max effect 25 min)

          o Cat: 25 min

          o Horse: 25 min. max effect 5 min)

          o Rabbit: 63 min

     • Blocks Na+ channels →impedes depolarization. Dose dependent. Less irritating than tetracaine 0.5%. Destabilizes tear film. Decreases STT.Slows epithelial healing. Antimicrobial activity?

     • Most common IOP measuring tools: Schiotz, TonoPen, TonoVet

     • IOP, Species Reference: Canine16.7 +/- 4.0. Feline 19.7 +/- 5.6. Equine 17-28 mmHg

Tonopen

     • The TonoPen functions in any orientation

     • The device activates with the push of a button and results are displayed clearly on the LCD screen

     • Visually inspect the tonometer's probe tip or cracks, chips or other irregularities

     • Spray the probe tip with compressed gas before the first use of the day - To help prevent build-up of debris around the probe post, cover the Tono-Pen Vet probe tip with a new Ocu-Film Tip Cover

     • Perform daily routine calibration

     • Slide a new Ocu-Film Tip Cover over the Tono-Pen unit's stainless steel probe until the Ocu-Film ridge is seated in the groove. The Ocu-Film Tip Cover should fit over the probe tip without being too tight

     • Proper fit makes contact with the transducer surface (probe post), but still has pleats along the side of the transducer head assembly

     • Taking a Measurement: After Good is displayed, depress the Operator's Button one time and the Tono-Pen will display a double row of dashes [====] followed by a "beep" tone. The Tono-Pen is ready to measure IOP. Prepare the patient: Instill a drop of Proparacaine onto the eye to be examined, Position the patient — seated, standing, lying down, the Tono-Pen unit will function in any stable position

     • Hold the Tono-Pen unit as you would a pencil, position yourself to facilitate viewing of the probe tip and patient's cornea where contact will be made (for normal corneas, central corneal contact is recommended)

     • Brace the heel of your hand on the patient's cheek for stability while holding the Tono-Pen perpendicular to and within ½ inch of the patient's cornea

     • The cornea is gently touched with the tip of the Tono-Pen and four measurements are taken. The unit then displays an average of the readings along with an accompanying coefficient of variance. Battery power operates the pen for about 600 patients while sanitized tip covers are used to eliminate cross contamination. The number displayed represents IOP in millimeters of mercury (mmHg)

     • A number with a single horizontal bar displayed at the bottom of the LCD is an average of the valid measurements. The display of one of four horizontal bars located along the lower border of the LCD block indicates the statistical reliability of the averaged measurement

Tonovet

     • Rebound Tonometer, *new* in 2004, ejection of a small probe, assesses the motion of the probe as it strikes the cornea and rebounds.

     • Electromagnetic probe. 1 mm diameter *plastic* tip. 6 measurements. Calibrate for "dog/cat" and "horse"

     • Step by Step: Press the measurement button to turn the device on, load the probe, press the measurement button again - the probe will vibrate a few times, this activates the mechanism that prevents the probe from sliding out, "00" is on the display and you are ready, have one person holding the head (and animal) still, the distance between the cornea and the tip of the probe should be the length of the silver collar, use 2 fingers of your left (or non dominant) hand to open the lids, use your right (dominant) hand to hold and operate the TonoVet, press the Measurement button 6 times in a row (successful readings have a short beep, after all 6 reading there is a long beep, bad readings have 2 beeps and shows an error message), the display will have a "d" after the last measurement, if there is a line in the middle or top of the "d" you need to repeat the measurements (too much error), the tonometer will turn off automatically in 2 minutes

Fundic exam - preparation

Tropicamide 1%

     • Onset 10-20 minutes

     • Complete mydriasis in 30 min (dog), 15 min (cat)

     • Effects wane 2 hours post-dose (dog), 10 hours (cat)

     • Short duration (6-8 hours in dogs, 8-12 hours in horses)

     • Tropicamide is less likely to exacerbate ocular hypertension since short acting

     • Dry eye patients: 1% tropicamide does not reduce STT values in dogs, transient decrease in cats

Atropine

     • Long acting (3 days in dogs, 14 days in horses, hypersalivation in cats)

     • Decreases STT values

Direct ophthalmoscopy

     • Commonly used in vet medicine. The fundus image is real, upright, and 14 times magnified

     • Advantages: Greater magnification, Options (grid, slit, altering dioptic power)

     • Disadvantages: Small field of view, short working distance, lack of stereopsis, difficult to see through cloudy media (limited light source), difficulty examining the peripheral fundus, greater distortion when the visual axis is not completely transparent

     • How to: Start 1.5-2" away at 0 D setting, best performed in a darkened or semi-darkened room with a dilated pupil, avoid rubbing noses – use right eye to examine patients right eye and left eye to evaluate patients left eye, rotate wheel toward negative (red) until fundus near disk is visible (-2 to-4D), plus lenses moves focal point anteriorly in patients eye, minus lenses moves focal point posteriorly in patients eye

     • Examine fundus in quadrants, re-focus to evaluate optic disk, usually within 1D of fundus focus, more neg. means disk depression, more pos. is elevation.

Indirect ophthalmoscopy

     • Image is virtual, inverted, and 2-4x magnified. Image is upside down and backwards

     • Advantages: Larger field of view for fundic image, larger and safer working distance, stereopsis, ability to use both hands for patient examination

     • Disadvantages: Little more complicated, line up examiner's eye, hand held lens, and the animal's eye

     • Direct Ophthalmoscopy: Key hole peeping, See 2-3 mm of fundus

     • Indirect Ophthalmoscopy: Aerial photography, See 7-18 mm of fundus

     • How to see the fundus: Direct the light beam into the patients eye from a distance of 50-60cm, obtain green/red reflex, place lens in the path of your light (2 inches from eye), silver ring goes toward the patient, catch reflected light through hand-held lens, hand-held lens creates an image in mid-air, move hand-held lens closer or further from the eye until the retinal image fills the lens

What to see to have looked

     • Develop organized approach: Optic Nerve, Retinal Vasculature, Tapetum, Non-tapetum, 4 Quadrants, Record Each Eye, Move the patient's head, Move around the patient, Look across of the bridge of the nose

     • Check List: Optic nerve (Size, Position, Myelination). Retinal Vessels (Size, Shape, Color of retinal vessels). Evaluate tapetum (Reflectivity, Color, Non-tapetal pigmentation)

Ocular ultrasound

     • 10 - 20 mHz probe

     • Permits examination of ocular contents when opacity of a structure prevents visualization of other structures as well as orbital structures. Can guide fine needle aspiration of intra or extraocular contents