Ophthalmology Exam
Equine

Indications for performing this procedure:  The eyes are frequently a window to what is happening to the animal systemically, and so an optical examination is important both for the sake of the eyes themselves, but also understanding better what is happening throughout the animal's body.  When an eye problem or systemic disease with ocular manifestations is suspected at the time of physical examination, a thorough ophthalmology exam is warranted. 


The Procedure:  For the initial examination, the horse should be restrained only physically, with a halter and perhaps even in stocks,  to allow full assessment of behavior and neurological function.  Once this is complete, the horse may be restrained chemically.  Sedation is usually achieved through the use of Xylazine, with the addition of Butorphanol for analgesia.  In addition, the frontal and auricuolopalpebral nerves are anesthetized using a product like 2% lidocaine.

EQUIPMENT NECESSARY FOR A
THOROUGH OPHTHALMOLOGY EXAMINATION

  • Focal light source

  • Direct ophthalmoscope

  • Magnifying loupe

  • Thumb forceps (blunt-tipped with shallow serrations)

  • Sterile fluorescein strips

  • Schirmer tear test strips

  • Culture swabs

  • Sterile eyewash

  • Catheters

  • Sedative/analgesic--Xylazine and Butorphanol

  • Topical anesthetic--proparacaine

  • Mydriatic solution--1% tropicamide

  • Kimura spatula (for corneal scrapings)

A thorough eye exam will likely include the following steps:

   In a well-lit area

  • Assessment of movement and posture of the animal.
  • Observation of symmetry of the face and eyes. This  includes examination of eye movement, opacities, discharge, masses, and the size and position of the globe.
  • Neurological evaluation/Cranial nerve assessment. This includes response to tactile stimuli, eye motility, blink-ability, pupillary size/movement, and vision.
  • Palpation of the boundaries of the orbit for fractures, masses, etc.
  • Retropulsion of the globe to check for resistance and to facilitate examination of the anterior aspect of the nictitans.
  • Examination of the external aspect of the eyelids, including position, movement, and integrity.
  • Collection of specimens for culture, and performance of tear-tests.


   In a darkened room

  • Establishment of the fundic reflex (reflection through the pupil) with a focused light source.
  • Eversion of the eyelids, and examination for any abnormalities such as inflammation (ulcerations, exudates, swellings), foreign bodies, abnormal lashes, meibomian gland openings on eyelid margins, paresis, and malposition (entropion or ectropion).
  • Examination of the nictitans (after administration of proparacaine) for position, pigmentation, follicles, masses, surface integrity, and foreign body entrapment (especially on the bulbar side of the membrane).
  • Conjunctival exam for transparency, moisture, and normal color.
  • Observation of the sclera, noting pigmented areas, color, contour, swellings, masses, and surface irregularities.
  • Corneal examination for vessels, pigmentation, opacities, and irregularities.
  • Nasolacrimal system evaluation for duct presence/function, tear overflow, ocular moisture levels, and discharge into the nares.  Fluorescein dye can be installed to detect corneal ulcers and to assess patency of the nasolacrimal duct (evidenced by appearance of the dye in the nostril within five minutes).
  • Examination of the anterior chamber for transparency and depth.
  • Study of the iris and notation of contour, mobility, neovascularization, pigment, iridal masses (including normal corpora nigrans), and pupil size and shape.
  • Assessment of pupil shape, symmetry, size, movement, and transparency.
  • Observation of the lens position, transparency, shape, and texture.


   With an ophthalmoscope (still in the darkened room)

  • Vitreous examination for congenital remnants or opacities.
  • Fundic exam. Including examination of optic disc size, shape, location, and vascular pattern (the latter two of which are variable among species), retinal vessels, elevations, separations, discolorations, degeneration, scleral defects, hemorrhage, tapetal vascularization and coloration, and the non-tapetal fundus' uniformity of pigment.

   Other procedures used in the diagnosis of equine optic pathology include rose-bengal staining, cytology, bacterial/fungal cultures, tonometry, assessment of aqueous tear production (Schirmer tear test), and biomicroscopy.


References:

  • Reed, S.M., and Bayly, W.M.  Equine Internal Medicine  1998:739-40.
  • Smith, B.P.  Large Animal Internal Medicine  1996:13, 1341-9.