Conjunctiva – introduction

9 Conjunctiva – introduction


The conjunctiva is a very important, but often overlooked, part of the eye. Surface disease is very common in general practice and it is essential to understand the relevance of conjunctival involvement with regard to a purely superficial problem, such as a primary bacterial conjunctivitis, or as part of a more serious intraocular condition such as uveitis. We will consider a few common but often frustrating conditions which affect the conjunctiva – by no means a complete overview of all conjunctival diseases but a selection of those which are perhaps overlooked or difficult to manage.



ANATOMY/PHYSIOLOGY REFRESHER


The conjunctiva is the thin mucous membrane which lines the eyelids and the exposed portion of sclera. It can be divided into three anatomic sections which are continuous with each other. These are the palpebral conjunctiva which lines the eyelids, the bulbar conjunctiva which covers the globe, and the nictitans conjunctiva which covers the third eyelid. The palpebral conjunctiva starts at the eyelid margin and lines both upper and lower lids. It meets the bulbar conjunctiva at the fornix where the conjunctiva is reflected over the sclera of the globe. The bulbar conjunctiva continues to the limbus.


The conjunctiva is composed of a non-keratinized, stratified squamous epithelium and substantia propria. It contains goblet cells in its outer epithelium, which produce the mucoid portion of the tear film, lymphoid tissue in its middle layer, and connective tissue, blood vessels and nerves in the deepest layer. The bright red branching blood vessels can be easily seen, especially in the bulbar conjunctiva. Sensory nerves are from various branches of the trigeminal nerve (cranial nerve V). Variable amounts of pigment are also present. The conjunctiva is freely mobile except at the limbus and eyelid margins. Functions of the conjunctiva include preventing corneal desiccation, increasing eyelid mobility and providing a barrier to microorganisms and foreign bodies.


Like most parts of the eye, the conjunctiva can only respond to noxious insults, be these endogenous or exogenous, in a limited number of ways.


Conjunctivitis, inflammation of the conjunctiva, is the most commonly diagnosed ocular condition encountered in general practice and reaching a definitive diagnosis can be challenging in some cases. The three most common reactions of the conjunctiva are hyperaemia, chemosis and ocular discharge. Pain is a variable symptom, while follicle formation is encountered in many cases of chronic conjunctivitis and is not patho-gnomonic for any particular aetiology.


Hyperaemia varies in severity but should be distinguished from episcleral or scleral congestion. Involvement of these deeper vessels normally indicates some form of intraocular disease, although the conjunctival blood vessels will also be hyperaemic in this instance. Thus the anatomical location of the hyperaemia is very important in differentiating a purely superficial conjunctivitis from a potentially more serious intraocular inflammatory process. Remember that the conjunctival vessels are narrow, branch frequently and are often tortuous in nature. They will move as the eyeball position changes. Deeper vessels tend to be thicker, straighter, perpendicular to the limbus and rarely branch. They are also normally a darker red in colour.


Chemosis is most noticeable in acute conjunctivitis, where swelling or oedema of the conjunctiva can be dramatic. The loose arrangement of cells within the conjunctival stroma allows the rapid and extensive oedema to develop. The bulbar conjunctiva is normally the most likely to be involved and may be so swollen as to prevent visualization of the globe. Hyperaemia may or may not be present. The Shar Pei breed often has the appearance of chemosis which is actually due to an accumulation of mucin in the conjunctiva in the same way as mucinosis is seen in the skin. It is not an indication of conjunctivitis in the breed unless accompanied by hyperaemia and since ocular problems are so common in the breed it is useful to remember this!


An ocular discharge is often the presenting complaint with cases of conjunctivitis. This can be serous, mucoid, mucopurulent or purulent in nature. Consideration of sample collection for viral or bacterial culture should be made and such samples should be taken before the discharge is cleaned away or any topical agents (such as topical anaesthetics or fluorescein dye) are applied to the eye.


Normal commensal bacteria are present in the conjunctival sac – indeed they can be isolated from 70–90% of normal dogs (lower percentages in cats) and thus a positive culture does not necessarily correlate to significant infection. The most common isolates from normal canine eyes are Gram-positive organisms such as Staphylococcus, Bacillus and Corynebacterium spp. The normal flora will vary with the climate (both geographic and seasonal) and the breed. Normal cytology of the conjunctiva (via scrapings for example) will reveal sheets of epithelial cells and occasional bacteria but white blood cells are rare (unlike in disease processes). Examination of pathological conjunctival scrapes can help identify the underlying disease process. For example, keratinized cells can be seen in cases of keratoconjunctivitis sicca, chronic irritation from entropion, and vitamin A deficiency, while goblet cell proliferation is commonly encountered in cases of keratoconjunctivitis sicca. However, many chronic changes in the conjunctiva are non-specific and thus both cytology and biopsy samples can be disappointing.

Related posts:

  1. Rabbit conjunctivitis
  2. Blepharitis
  3. Proliferative (eosinophilic) keratoconjunctivitis
  4. Limbal melanoma

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Sep 10, 2016 | Posted by in SMALL ANIMAL | Comments Off on Conjunctiva – introduction

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