Conjunctiva and subconjunctival tissue

Chapter 31 Conjunctiva and subconjunctival tissue





Anatomy


The conjunctiva is a thin, translucent, vascular mucous membrane that lines the inner surface of the eyelids and the anterior surface of the eyeball as far as the limbus. This tissue is arranged in a sac-like fashion and is composed of a palpebral region (covering the inner aspects of the lids), a bulbar region (covering the surface of the sclera), a forniceal region, and a medial semilunar fold.


While the palpebral conjunctiva shares its blood supply with the eyelids, the bulbar component is supplied by the anterior ciliary arteries. The nerve supply to the conjunctiva is through the lacrimal, supraorbital, supratrochlear, and infraorbital branches of the ophthalmic division of the trigeminal nerve.


The conjunctival epithelium varies from 2 to 5 cells in thickness and is continuous with the corneal epithelium at the limbus and with the skin at the margin of the lids. While the bulbar conjunctiva is lined by a stratified non-keratinized squamous epithelium, the forniceal and tarsal regions are lined by columnar and cuboidal types, respectively. A key constituent of the cellular architecture is the presence of goblet cells, which accounts for 10% of the basal cells of the conjunctival epithelium. These cells are more prevalent in the medial forniceal and palpebral regions, and play a vital part in secreting the mucin component of the tear film. There is an increase in the number of these cells during chronic inflammation of conjunctiva, while conditions like pemphigoid and vitamin A deficiency causes a decrease. Other cell types which nestle within the epithelial layers include melanocytes, Langerhans’ cells, and intraepithelial lymphocytes.


Beneath the epithelium lies a loose structure called the substantia propria. This structure contains different cell types which mediate immune responses (mast cells, plasma cells, eosinophils, and lymphocytes) interspersed in a vascular network. This arrangement of immune cells, commonly referred to as conjunctiva-associated lymphoid tissue, existing in a vascular environment, is continuously exposed to potential external infective agents and allergens thus serving as a perfect setting for inflammation to set in.


Beneath the conjunctiva lies a fibroelastic tissue, Tenon’s capsule, which surrounds the eye ball from the corneoscleral junction to the optic nerve. Tenon’s capsule is thicker in children and contains more fibroblasts. Hence surgeries like trabeculectomy performed in children, especially without adjuvant procedures like intraoperative use of antimetabolites, may fail due to the aggressive healing response induced by these fibroblasts.1



Conjunctiva in systemic disease


A careful flashlight examination of the conjunctiva in a brightly lit environment often provides comprehensive information about a potential underlying systemic disorder. Information about color, lustre, abnormalities in vascularization, and pigmentation help to suspect an underlying ocular or a systemic cause. A slit-lamp evaluation can then be carried out to focus on the specific area of the pathology.



Vitamin A deficiency


This systemic condition affects organs throughout the body. The ocular manifestation is termed xerophthalmia and affected individuals present with night blindness, conjunctival xerosis, Bitot’s spots, corneal xerosis, keratomalacia, and the “xerophthalmic” fundus.


In this condition, the conjunctival epithelium is transformed from the normal columnar to the stratified squamous type. There is an associated loss of goblet cells, formation of a granular cell layer, and keratinization of the surface. The conjunctiva loses its normal lustre and is altered into a dry or unwettable one (Fig. 31.1). It is almost always bilateral. A classic ocular sign is Bitot’s spots, which is a superficial, scaly, gray area on the interpalpebral region of the bulbar conjunctiva (Fig. 31.2). Corynebacterium xerosis can colonize these spots, and produce a foamy appearance because of the gas-forming nature of these organisms. If untreated, the condition involves the cornea, causing corneal xerosis and finally corneal melting, or keratomalacia.




The diagnosis of xerophthalmia is often clinical and does not require any additional investigations. In doubtful cases, impression cytology of the superficial layers of the conjunctival epithelium may be helpful to show the loss of goblet cells and keratinization of epithelial cells. Oral administration of vitamin A is preferred because it is safe, cost-effective, and highly effective. In affected children above 12 months of age, retinol palmitate (110 mg) or retinol acetate (200 000 IU) are given orally immediately and the dose is repeated the following day. An additional dose should be given 2 weeks later to boost liver resources. Children between 6 and 11 months should receive only half the above-mentioned dose, and children less than 6 months one-quarter of the dose.


Parenteral administration is indicated in those children with conditions such as persistent vomiting, severe stomatitis, and attendant difficulty in deglutition, severe diarrhea with malabsorption, and septic shock. Such children can be treated with intramuscular injection of 55 mg of water-miscible retinol palmitate (100 000 IU), which replaces the first oral dose. This is repeated the next day. Children of less than 1 year are treated with vitamin A in half the prescribed dosage. After the acute phase is over, dietary supplements with provitamin A-rich foods, should be provided.2









Alkaptonuria


This is a rare autosomal recessive disorder in which an affected individual’s urine turns a dark brown-blackish color when exposed to air. It is linked to chromosome 3q21-q24, caused by a deficiency of homogentisic 1,2-dioxygenase.5 This deficiency results in accumulation of homogentisic acid, which gets deposited in various tissues and organs. Systemic features include pigmentation over the face and nails, calcific and atherosclerotic heart disease, and arthritis. Ocular manifestations include a brown to black pigmentation of the nasal and temporal sclera especially in the area of the horizontal rectus muscle insertions. Pigmentation of the cornea has been reported.


Jun 4, 2016 | Posted by in OPHTHALMOLOGY | Comments Off on Conjunctiva and subconjunctival tissue
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