July Pulse Article (PART 1 OF 12, ANATOMY AND DIAGNOSTICS)
July 01, 2018
VOC is pleased to post the first of twelve articles in association with the Southern California Veterinary Medical Association’s “PULSE” journal. Each month, this series will provide clinical updates & reviews for the southern California veterinary community. This initial installment describes the normal canine & feline ocular anatomy as well as the fundamentals of the ophthalmic examination.
The Normal Canine & Feline Ophthalmic Anatomy & Examination
This article is first in a twelve-month series of brief clinical reviews, which will cover those ophthalmic presentations most commonly encountered in general practice. Plan to save each one & keep the complete set handy for reference! This introduction will summarize normal ocular structures and their examination.
The Eyelids– comprising skin, orbicularis oculi muscles, deep tarsal & superficial conjunctival tissues (including conjunctival mucous-producing goblet cells, lipid-producing meibomian glands & the openings of the nasolacrimal drainage system)
The Third Eyelid, comprising a supportive T-shaped cartilage, which surrounds the lacrimal gland of the third eyelid and is covered with conjunctival tissue
The externalScleral&Epsiscleral Tissuesand anteriorCornea(composed of an outer epithelium & basement membrane, central stroma and underlying endothelium with its basement or “Decemet’s” membrane).
TheUveal Tract, composed of the anterior iris & ciliary body & the posterior choroid, these tissues being contiguous.
TheLens, which is suspended from the ciliary body by zonular ligaments & is surrounded by capsular tissue.
TheNeuroretinawhich comprises; the retinal pigment epithelium, photorecpetors, supporting neuroglial cells & retinal ganglion cells which ultimately coalesce to form theOptic Nervewhich exits the globe posteriorly, through the porous lamina cribrosa.
The Normal Canine Fundus
The canine fundus exhibits a considerable variation in appearance, comprising the tapetal (reflective) as well as non-tapetal (non-reflective) tissues. The juvenile canine fundus typically appears bluish in color until three to four months of age. The optic nerve appears as a variably myelinated white to pink structure within the fundus, representing the accumulation of ganglion cells. Radiating from the ONH are 3-4 large veins and 15-20 smaller arterioles.
The Normal Feline Fundus
The feline fundus exhibits a similar variation in normal appearance, comprising a relatively large tapetal as well as non-tapetal region. The optic nerve appears as a small, circular, relatively unmyelinated white to grey structure within the fundus. Three major pairs of arterioles as well as larger venules radiate from the ONH.
The Subalbinotic Fundus
Patients displaying blue or heterochromic irides &/or merled coat coloration, typically display “subalbinotic” ocular tissues. In these animals, the tapetal region may be variably reduced to absent in association with a variable to complete lack of pigment within the non-tapetal fundus. As a result, underlying choroidal vasculature is visible against the white scleral background giving these tissues a start “red” appearance, which should not be confused with pathology such as hemorrhage.
TheOphthalmic Examinationshould comprise the following;
A thorough history including patient signalment, presenting complaint, pre-existing medical/surgical history (including travel history) and/or current medications represents the first part of the routine ophthalmic examination.
Initially, the patient should be allowed to move around freely – demonstrating mentation, neurological status and visual ability. Thereafter, a brief generalized physical examination should be performed. Careful palpation of the skull & orbits, will identify deformity, asymmetry, crepitus or pain
Palpebral reflex (closure of eyelids upon tactile stimulus)
Menace response (eyelid closure and/or head withdrawal in response to menacing gesture)
Dazzle reflex (closure of eyelids in response to bright focal light source being shined into eye)
Pupillary light reflex (PLR) (direct and consensual reflex pupillary miosis in response to a bright focal light source)
A focal light source is used to examine the structures of the eye from anterior posterior, following the normal anatomy as outlined above. The posterior segment may be examined using a focal light source and simple hand-held lens.
Ancillary Ophthalmic Diagnostics
Schirmer Tear Test I (STT1) tear testing (normal value = 15-20mm/wetting/min)
Intraocular Pressure (IOP) estimation (normal value = 15-25mmHg)
Where indicated, additional diagnostics may include blood samples for CBC, chemistry, metabolic, endocrine and/or infectious titer testing, the harvesting of microbial samples for culture & sensitivity testing as well as cytological and/or histological examination and/or advanced imaging (including radiography, B-mode ultrasonography, computed tomography and/or magnetic resonance imaging).
Dr Esson is a board-certified veterinary ophthalmologist with more than twenty years of clinical experience and multiple areas of interest & expertise. His clinicVeterinary Ophthalmic Consulting is family owned & operated and he takes great pride & pleasure in working closely with his friends and colleagues in the greater Southern California veterinary community.