What is the difference between sclera and conjunctiva in terms of function, characteristics, and treatment?

Sclera vs Conjunctiva: Structural and Clinical Distinctions

The conjunctiva is a transparent mucous membrane that overlies the sclera, which is the tough, opaque white outer coat of the eyeball. 1 These are anatomically distinct layers with fundamentally different structures, functions, and clinical implications.

Anatomical Structure and Location

Sclera

• The sclera forms the protective outer shell of the eye, providing structural stability and opacity to shield intraocular tissues 2
• Composed of densely packed collagenous tissue organized in distinct concentric layers: Tenon's capsule (outermost), episclera, scleral stroma proper, and lamina fusca (innermost, melding into choroid) 2
• Contains heterotypic collagen fibrils (primarily types I and III, with small amounts of types V and VI) arranged in interlacing fiber bundles or defined lamellae 2
• Avascular structure that receives nutrients from the choroid and vascular plexi in Tenon's capsule and episclera 2
• The white, tough "ball of the eye" visible externally 3

Conjunctiva

• The conjunctiva is a delicate, transparent epithelial mucous membrane with its own stroma that overlies the sclera 3, 1
• Firmly attached only at the limbus (where sclera meets cornea) and at the eyelids, allowing free eye movement 3
• Forms a blind sac (fornix) deepest under the upper and lower lids 3
• Contains goblet cells that synthesize mucins, rendering the ocular surface hydrophilic and enabling tear film spread 1
• Moderately tight epithelium with various transport processes for ion, solute, and water homeostasis 4

Functional Differences

Sclera Functions

• Provides mechanical stability during variations in internal pressure and eye movements, preventing retinal and lens distortion that would impair vision 2
• Maintains structural integrity through viscoelastic properties of its connective tissue 2
• Houses specialized structures: trabecular meshwork (aqueous filtration) and lamina cribrosa (optic nerve exit) 2

Conjunctiva Functions

• Protects the ocular surface and produces components of the tear film 4
• Serves as a conduit for drug clearance into systemic circulation or transport to deep ocular tissues 4
• Maintains homeostasis of the tear film through active transport of ions, glucose, amino acids, and other solutes 4
• Larger surface area than cornea, making it an attractive route for drug delivery to posterior eye structures 4

Clinical Disease Patterns

Scleral Diseases

• Scleritis represents severe inflammatory destruction of scleral structural and cellular components, often associated with systemic autoimmune disease (particularly rheumatoid arthritis) 2
• Necrotizing scleritis is the most severe form, potentially destroying all scleral components 2
• Scleromalacia increases surgical complexity and postoperative recovery challenges 1
• Autoimmune etiology with infiltration by macrophages and T-lymphocytes 2

Conjunctival Diseases

• Conjunctivitis (inflammation of conjunctiva) presents with hyperemia, discharge, and papillary or follicular reactions 1, 5
• In atopic disease, conjunctiva shows goblet cell proliferation and epithelial infiltration with eosinophils, mast cells, T cells, and macrophages 1
• Severe conjunctival inflammation can progress to cicatrizing conjunctivitis with scarring, symblepharon formation, and risk of corneal complications 1
• Conjunctival chalasis (redundant conjunctiva) causes chronic irritation and dry eye 1, 5

Treatment Implications

Scleral Pathology Management

• Requires aggressive immunosuppression for inflammatory conditions due to autoimmune nature 2
• Surgical interventions complicated by tissue fragility and risk of perforation 1
• Scleral perforation during surgery carries higher risk of retinal detachment in adults due to liquefied vitreous 1

Conjunctival Pathology Management

• Treatment directed at underlying cause (infectious, allergic, immune-mediated, or medication-induced) rather than nonspecific anti-inflammatory therapy 5
• Mild to moderate conjunctivitis often manageable with topical therapy 1, 5
• Severe cicatrizing disease may require ocular surface reconstructive surgery 1
• Conjunctival approaches for drug delivery (including subconjunctival injection of sustained-release formulations) can maintain therapeutic levels in vitreous 4

Critical Distinction for Clinical Practice

The key clinical distinction is that the conjunctiva is the visible, transparent mucous membrane that becomes inflamed (red, swollen) in conjunctivitis, while the underlying sclera is the structural white coat that, when diseased, causes severe pain and vision-threatening complications. 1, 2 Scleral disease is uncommon but rapidly destructive when present, whereas conjunctival inflammation is common and usually manageable 2, 5.