Sclera: Anatomy and Structure

• The sclera forms the posterior opaque 5/6 part of the eyeball's external fibrous tunic.
• The entire outer surface of the sclera is covered by Tenon's capsule and the bulbar conjunctiva in the anterior region.
• The inner surface of the sclera contacts the choroid, with a potential suprachoroidal space in between.
• The sclera is thickest posteriorly (1mm) and gradually thins anteriorly.

Special Regions of Sclera

• Scleral sulcus: An indentation on the inner surface of the anterior sclera near the limbus.
• Scleral spur: A circular flange in the anterior sclera, lying deep to Schlemm's canal; appears wedge-shaped in section.
• Lamina cribrosa: A sieve-like sclera area where optic nerve fibers pass.
• Scleral apertures (emissaria): Three sets of apertures for vessels and nerves.
  • Posterior aperture: situated around the optic nerve.
  • Middle apertures: located 4-7mm posterior to the equator.
  • Anterior aperture: situated 3-4mm away from the limbus.

Microscopic Structure of the Sclera

• Histologically, the sclera consists of three layers: episcleral tissue, sclera proper, and lamina fusca.
• Episcleral tissue: A thin, dense, vascularized connective tissue layer that covers the sclera proper, continuous with Tenon's capsule anteriorly.
• Sclera proper: An avascular structure primarily of dense collagen fiber bundles crossing in all directions, making it opaque.
  • Mucopolysaccharides are present in the interfibrillar space.
  • Few fibroblasts are also present.
• Lamina fusca: The innermost scleral layer, blending with the suprachoroidal and supraciliary lamina of the uveal tract.
  • Brownish in color due to pigment cells.

Apertures of Sclera

• Anterior apertures: Transmit anterior ciliary vessels.
• Middle apertures: Transmit four vortex veins (venae vorticosae).
• Posterior apertures: Transmit the optic nerve and long & short ciliary nerves.
• The optic nerve is situated 3mm medial and 1mm above the posterior pole.

Blood Supply

• The episclera is supplied by anterior ciliary arteries (anterior to rectus muscle insertions) and long/short posterior ciliary arteries.

Nerve Supply

• The sclera is supplied by branches from the long ciliary nerves anteriorly and short ciliary nerves behind the equator.

Episcleritis

• A benign, recurrent inflammation of the episclera, involving the overlying Tenon's capsule but not the underlying sclera.
• Etiology: Often unknown, but associated with gout, psoriasis, rosacea, or hypersensitivity to toxins.
• Symptoms: Redness, mild ocular discomfort described as gritty or burning, and a foreign body sensation.
• Signs: Diffuse or nodular episcleritis.
  • Diffuse: May involve the whole eye to some extent, with maximum inflammation in one or two quadrants.
  • Nodular: A pink or purple flat nodule surrounded by injection, 2-3 mm from the limbus; nodule is firm, tender, and the overlying conjunctiva moves freely.
• Differential Diagnosis: Inflamed pinguecula, scleritis, or foreign body reaction on the bulbar conjunctiva.
• Treatment: Topical corticosteroid eye drops (2-3 hourly), cold compresses, and systemic non-steroidal anti-inflammatory drugs (e.g., flurbiprofen, indomethacin, or oxyphenbutazone).

Scleritis

• Inflammation of the sclera proper; more common in females and the elderly.
• Etiology: Autoimmune collagen disorders (RA, Wegener's, PAN, SLE, ankylosing spondylitis), metabolic disorders (gout, thyrotoxicosis), infections (herpes zoster ophthalmicus, staphylococcal/streptococcal), granulomatous diseases (TB, syphilis, sarcoidosis, leprosy), miscellaneous conditions (irradiation, chemical burns, Vogt-Koyanagi-Harada syndrome, Behcet's), surgically induced, or idiopathic.

Classification of Scleritis

• Anterior scleritis (98%)
  • Non-necrotizing scleritis (85%)
  • Diffuse
  • Nodular
  • Necrotizing scleritis (13%)
  • With inflammation
  • Without inflammation (scleromalacia perforans)
• Posterior scleritis (2%)

Symptoms of Scleritis

• Moderate to severe pain, often deep and boring, waking the patient in the morning, and radiating to the jaw and temple.
• Localized or diffuse redness.
• Mild to severe photophobia.
• Lacrimation.

Signs of Scleritis

Non-necrotizing anterior diffuse scleritis

• Commonest
• Widespread inflammation involving a quadrant or more of the anterior sclera.
• The area is raised and salmon pink to purple in color

Non-necrotizing anterior nodular scleritis

• One or two hard, purplish elevated scleral nodules,
• Usually situated near the limbus
• The nodules are arranged in a ring around the limbus (annular scleritis)

Anterior necrotizing scleritis with inflammation

• Acute severe form of scleritis
• Characterized by intense localized inflammation
• Associated with areas of infarction due to vasculitis
• Necrosed sclera is thinned out (sclera becomes transparent and ectatic) with uveal tissue shining through it
• Anterior uveitis common

Anterior necrotizing scleritis without inflammation (scleromalacia perforans)

• Elderly females with a long-standing history of rheumatoid arthritis (RA)
• Yellowish patch of melting sclera (due to obliteration of arterial supply)
• Overlying episclera and conjunctiva completely separates from the surrounding normal sclera.
• Eventually absorbs, leaving behind a large punched-out area of
• the thin sclera through which the uveal tissue shines
• Spontaneous perforation rare

Posterior scleritis

• The sclera behind the equator.
• Frequently misdiagnosed.
• Associated inflammation of adjacent structures,
• Exudative retinal detachment,
• Macular edema,
• Proptosis
• Limitation of extraocular movements

Complications of Scleritis

• Secondary glaucoma (due to uveitis)
• Complicated cataract
• Sclerosing keratitis
• Keratolysis

Investigations for Scleritis

• TLC, DLC, and ESR
• Serum levels of complement (C3), immune complexes, rheumatoid factor, antinuclear antibodies and L.E. cells for an immunological survey
• FTA-ABS, VDRL test for syphilis
• Serum uric acid test for gout
• Urine analysis
• Mantoux test (TB)
• X-Rays of chest, paranasal sinuses, sacroiliac joint and orbit.

Treatment of Scleritis

• Non-necrotizing scleritis:
  • Topical steroid eye drops
  • Systemic indomethacin (100 mg initially, then 75 mg) until the inflammation resolves.
• Necrotizing scleritis:
  • Topical steroids and heavy doses of oral steroids, tapered slowly.
  • Immunosuppressive agents (e.g., methotrexate or cyclophosphamide) in non-responsive cases.
  • Subconjunctival steroids are contraindicated.

Staphylomas

• Localized bulging of weak and thin outer tunic of the eyeball(cornea or sclera)
• Lined by uveal tissue which shines through the thinned out fibr coat.

Classification for Staphylomas

• Anterior
• Intercalary
• Ciliary
• Equatorial
• Posterior
• Anterior staphyloma
  • Associated with ectasia of the cornea and iris.
  • Due to a perforating corneal ulcer and injury

Intercalary staphyloma

• It is due to the healing of a perforating injury or a peripheral corneal ulcer
• Leadin to ectasia of weak scar tissue formed at te limbus
• There is localised bulgein limbal area lined by root of iris

Ciliary staphyloma

• Bulge of weak sclera lined by the ciliary body.
• About 2-3 mm away from the limbus
• Thinning of slcera following perforating injury
• Also happens in Scleritis and absolute glaucoma

Equatorial staphyloma

• Bulge of sclera lined by the choroid in the equatorial region
• At the regions of sclera which are perforated by vortex veins
• Causes scleritis and degeneration of sclera in pathological myopia

Posterior staphyloma

• Bulge of weak sclera lined by the choroid behind it.
• Common causes are pathological myopia, posterior scleritis and perforating injuries.
• Diagnosis of staphylomas through ophthalmoscopy.
• The area is excavated with retinal vessels dipping in it (just like marked cupping of the optic disc in glaucoma)