Contributors: David Gould, Claudia Hartley

 Species: Feline   |   Classification: Miscellaneous

Introduction

  • Remember that the eye is not an isolated organ. Many ophthalmic conditions are actually manifestations of systemic diseases. For this reason, prior to performing an ophthalmic examination it is important to take an accurate history and perform a general clinical examination, particularly if the history or clinical signs suggest the possibility of systemic disease.
  • Similarly, both eyes should undergo a thorough eye examination. Whilst this may sound obvious, failure to examine both eyes is a surprisingly common omission, especially in patients presenting with dramatic, apparently uniocular clinical signs.

Key points to remember

  • A step-by-step approach will ensure that the examination proceeds in a logical manner, and will minimize the risk of missing abnormalities. Work from outside to inside, anterior to posterior.
  • The eye is an unforgiving organ; if you are uncertain of the diagnosis or unconfident about how best to treat the condition, seek advice or arrange referral. For many ophthalmic conditions, early diagnosis and prompt, appropriate treatment is vital to reduce the risk of permanent blindness Blindness or irreparable pathology.

Holding the patient for ophthalmic examination

  • Restraint should be minimal. It is important not to distort the eyelids when assessing signs of ocular pain, the ability to blink and eyelid conformation. If a deep corneal ulcer Persistent corneal erosions is present it is vital not to stress the animal or to increase its intra-ocular pressure by compressing the jugular veins. Support the patient below the jaw. Do not scruff or put pressure on the neck.
    Print off the owner factsheet on Eye examination  Eye examination to give to your client.

Equipment

Equipment required for basic eye examination

  • A room capable of being darkened.
  • A penlight or transilluminator.
  • A direct ophthalmoscope Ophthalmoscopy: direct:
    • For distant direct ophthalmoscopy (to visualize opacities in the visual axis such as cataract).
    • For close direct ophthalmoscopy (giving a high magnification, small field of view of the fundus).
  • A 20D or 30D condensing lens:
    • For indirect ophthalmoscopy with penlight or transilluminator (giving a low magnification, wide field of view of the fundus) Ophthalmoscopy: indirect.

Equipment required for more detailed eye examination

  • A tonometer Tonometry is required to measure the intra-ocular pressure (IOP).
  • Indentation tonometers (such as the Schiotz) are inexpensive, although can be relatively difficult to use, require careful cleaning after each use, and are not as accurate as other types.
  • Applanation or rebound tonometers (such as the Tonopen or Tonovet, respectively) are accurate, reasonably simple to use but relatively expensive.
  • A slit-lamp biomicroscope allows a stereoscopic and magnified (10x-16x) view of the adnexa and anterior segment:
    • Useful for detailed examination of eyelid margins, ocular surface, anterior segment and lens.
    • Allows exact localisation of lesion depth, eg for corneal ulcers, foreign bodies, cataracts Cataract.
  • A goniolens is a contact lens that allows examination of iridocorneal drainage angle:
    • Used to assess predisposition to primary closed-angle glaucoma (goniodysgenesis Glaucoma).

Protocol for ophthalmic examination

1. Lights on from a distance

  • Look for asymmetry, ocular discharge Eye: ocular discharge - overview, signs of ocular pain (subtle signs of ocular discomfort such as increased blink rate may disappear when the animal is faced with the stress of a hands-on examination).

2. Lights on, close-up

  • Assess the periorbita, eyelids (in particular, eyelid conformation) and external globe (conjunctiva/episclera, cornea).
  • Schirmer tear test 1 (STT1) Schirmer tear test should be performed at this stage in all cases of ocular discharge, conjunctivitis, or lacklustre cornea, and must be performed before topical drops are applied.
    STT should not be performed if a descemetocoele (very deep corneal ulcer) or a corneal perforation is suspected as this may sufficient to cause rupture and aqueous leakage.
    • A STT1 reading of >15 mm/minute is normal in dogs.
    • In cats the range is more variable; most cats have STT1 readings >15 mm/min, but some normal cats may have STT1 readings as low as 5 mm/minute. 
    • A STT1 reading of <10 mm/minute in dogs may be indicative of keratoconjunctivitis sicca (KCS Eye: keratoconjunctivitis sicca).
    • However, artificially low readings may be recorded if the STT strip fails to contact the corneal surface for the duration of the test, if the animal is nervous (stress can reduce tear secretion), or if local anesthetic drops have been applied. If in doubt as to the validity of the STT1 test, it should be repeated at a subsequent visit.
  • Test the menace response to assess vision and eyelid function:
    • A brisk, threatening hand movement towards the patients eye should elicit a blink response.
    • Alternative tests of vision include visual tracking (ability of the patient to follow the movement of an object of interest, such as a cotton wool ball or a laser pointer light), obstacle course testing (ability of the patient to negotiate a series of obstacles placed on the consulting room floor) or visual placing (the animal is carried towards a table with its forelegs free. As the table approaches a visual animal should extend its leg(s) in anticipation).
  • Test the palpebral blink reflex to assess periocular sensation and eyelid function:
    • Touching the  medial and lateral canthus should elicit a complete blink.
  • Test the corneal blink reflex to assess corneal sensation and eyelid function plus globe retraction:
    • Lightly touching the corneal surface with a sterile cotton bud or wisp of cotton wool should elicit a blink response and globe retraction.
    • The cornea should be touched away from visual axis to avoid confounding reflex with menace response.
  • Test the vestibulo-ocular reflex:
    • Movement of the head should lead to centralisation of the globes within the visual axis (ie moving the head from side to side or up and down should cause the eyes to rotate in the opposite direction so that the eye remains looking ahead).
    • Movement of the head may also make a nystagmus or strabismus more conspicuous in some conditions.
      The remainder of the examination is best performed in a darkened room.

3. Lights off, focal illumination

  • Use a transilluminator or a pen torch to re-examine the external eye, the eyelids, and periorbita.
  • Test the pupillary light reflex (PLR) to assess the afferent visual pathway and iris constrictor muscle function (efferent pathway):
    • Shining a bright light in the eye should elicit a brisk pupillary constriction in the ipsilateral eye (direct PLR) and contralateral eye (consensual or indirect PLR).
    • Although the PLR tests retina, optic nerve and optic tract function, it is not a test of vision, as it does not assess function of the visual cortex.
  • Test the dazzle reflex to assess the afferent visual pathway and eyelid function:
    • Shining a bright light in the eye should elicit a brief blink response.
    • The dazzle reflex can be inconsistent. A very bright light source and a darkened room are required. Even a partial blink response is deemed positive for the dazzle reflex.
    • Although the dazzle reflex tests retina, optic nerve and optic tract function, it is not a test of vision, as it does not assess function of the visual cortex.
  • Use a transilluminator or a pen torch to assess:
    • Depth of the anterior chamber:
    • Presence of iris abnormalities:
      • Swollen, muddy appearance in active anterior uveitis.
      • Increased pigmentation in chronic uveitis.
      • Mass lesions.
    • Pupil size.
    • Presence of inflammatory material (aqueous flare (protein), fibrin, hemorrhage or hypopyon) in the anterior chamber.
    • Lens position and appearance.

4. Lights off, distant direct ophthalmoscopy

  • The ophthalmoscope is held close to the observers eye in the normal way, but at an arms length away from the patient. This technique is used to compare pupil sizes and to visualize opacities in the visual axis, such as cataracts.

5. Examination of the fundus

  • Since this can be difficult through a constricted pupil, it is advisable to dilate the pupil using a topical mydriatic such as 1% tropicamide Tropicamide
    Do not dilate the pupil if glaucoma is suspected.
  • Using a low intensity of illumination will maximize patient comfort and co-operation.
  • The optic nerve head, superficial retinal blood vessels, tapetal and non-tapetal fundus should be examined for abnormalities.
  • Indirect ophthalmoscopy using a lens and a pen torch gives a wide field of view (at the expense of reduced magnification) and is ideal for examining larger areas of the fundus.
  • Close direct ophthalmoscopy using a direct ophthalmoscope can then be used to examine discrete areas of the fundus.

6. Further tests if required

  • Tonometry to measure the intraocular pressure measurement is essential if glaucoma is suspected. If tonometry is not available in your practice, referral to a veterinary ophthalmologist is advised:
    • Use an indentation, applanation or rebound tonometer.
    • Note that inappropriate patient restraint (such as pressure on the neck leading to occlusion of jugular veins) can markedly increase IOP readings in a normal eye. Take a series of IOP readings and ensure minimal patient restraint.
    • Use of sedation or general anesthesia may artificially reduce IOP readings.
    • Incorrect use of a tonometer can also adversely affect IOP readings. Follow the manufacturers instructions or seek advice from a veterinary ophthalmologist if unsure of its correct use.
  • Topical fluorescein should be applied in cases of reddened and painful eyes, even if no corneal ulceration is apparent on clinical examination. Flush the surface of the eye with water or saline after application, since the dye can pool at sites of previously healed deep ulcers.
  • If an infectious conjunctivitis is suspected, a swab from the conjunctival fornix should be taken:
    • Topical anesthesia and fluorescein staining should be avoided prior to swabbing for bacterial culture as it may reduce bacterial yield.
    • However, topical anesthesia is recommended prior to swabbing for PCR PCR (Polymerase chain reaction) (eg testing for feline herpesvirus-1 Feline herpesvirus disease or Chlamydophila felis  Chlamydophila felis ) as it will allow more vigorous sampling to increase DNA yield.
  • Conjunctival or corneal smears are indicated for ocular surface cytology. Topical anesthesia is recommended Anesthesia: non-depolarizing neuromuscular blockade. Use of a cytobrush will increase cell yield and this should be undertaken prior to fluorescein staining Fluorescein test.
  • A conjunctival biopsy may be indicated for conjunctivitis of unknown etiology. In many cases it can be performed in the conscious animal under topical anesthesia. Its anesthetic effects can be increased by soaking a cotton bud in topical anesthetic then pressing it gently against the area to be biopsied. Then gently tent up the conjunctiva with tissue forceps and snip off a small sample with sharp blunt-ended scissors such as Westcott scissors. Avoid crushing the sample. Place it flat on very thin card (this avoids distortion) and place in routine histological fixation solution.
  • Further specialist tests are indicated in some cases. This may include slit-lamp examination, gonioscopy, indirect ophthalmoscopy Ophthalmoscopy: indirect, ocular ultrasonograph Ultrasonography: eye, electroretinography testing, advanced diagnostic imaging.
    Seek advice from a veterinary ophthalmologist if indicated.