cornea

1. Pterygium
• Disease Entity
• Pterygium, from the Greek pterygos meaning “wing”, is a common ocular
surface lesion originating in the limbal conjunctiva within the palpebral
fissure with progressive involvement of the cornea.
• The lesion occurs more frequently at the nasal limbus than the temporal with
a characteristic wing-like appearance.

2. • Etiology
– The pathogenesis of pterygia is highly correlated with UV exposure.
– An increased incidence is noted in latitudes nearer the equator and
in individuals with a history of increased UV exposure (outdoor
work).
– Some studies have shown a slightly higher incidence in males than
females, which may only reflect a higher rate of UV radiation.
• Risk Factors
– UV radiation, proximity to the equator, dry climates, outdoor lifestyle

3. • General Pathology
– Histologically, pterygia are an accumulation of degenerated subepithelial
tissue which is basophilic with a characteristic slate gray appearance on
H&E staining.
– Vermiform or elastotic degeneration refers to the wavy worm-like
appearance of the fibers.
– Destruction of Bowman layer by fibrovascular ingrowth is typical.
– The overlying epithelium is usually normal, but may be acanthotic,
hyperkeratotic, or even dysplastic and often exhibits areas of goblet cell
hyperplasia.

4. • Pathophysiology
– The large number of theories that exist to explain the pathogenesis of pterygium
growth underscores the uncertainty of the etiology.
– The increased prevalence in hot dry climates and regions nearer to the equator
suggest a role of environmental factors such as UV radiation and dryness.
– Actinic changes seen on histopathology similar to actinic keratoses on the skin also
supports the role of UV radiation.
– It has been suggested that radiation activated fibroblasts may result in excessive
production of material resulting in pterygia.
– Other proposed theories include choline deficiency, inflammation, disregulation of
angiogenesis, immune system abnormalities, tear film abnormalities, as well as the
possible role of a viral stimulus.
– Coronea MT proposed that pterygium occur due to albedo concentration in the
anterior eye (albedo's hypothesis).
– Light entering the temporal limbus at 90 degree is concentrated at medial limbus
and this is responsible for predominance of medial pterygia

5. • Degenerative Versus Proliferative Disorder
– Pterygium has long been considered to be a chronic
degenerative condition based on histologic examination.
– Classically described as an “elastotic degeneration”,
pterygium tissue is characterized by abnormal subepithelial
tissue containing altered collagen fibers demonstrable with
elastic stains.
– There are, however, behavioral and etiologic features of
pterygium that suggest a proliferative growth disorder, not
unlike properties seen in benign tumors.
– As mentioned, pterygium has been linked to dysfunctions in
DNA repair mechanisms, cellular proliferation, migration, and
angiogenesis

6. • Pterygium surgery can be divided into four main
groups, in order of increasing complexity:
• 1. Bare sclera excision
• 2. Excision with conjunctival primary closure or transposition
• 3. Bare sclera excision with antimitotic adjunctive therapies
• 4. Ocular surface transplantation techniques, including
conjunctival or conjunctival-limbal autografts and amniotic
membrane transplantation (AMT).

7. • Mitomycin C
• Mitomycin C (MMC) is an alkylating agent that inhibits
DNA synthesis in all cells. This leads to cell death due to
the inability to repair the genotoxic injury caused by
alkylation. MMC causes a reduction in the number of
cellular division, especially when it comes in contact
with cells in the late G1 and early S phase of the cell
cycle. It therefore has an antiproliferative effect and
reduces fibrovascular tissue formation, which has a
direct effect in reducing recurrence after pterygium
excision

8. • MMC is used in pterygium management in three ways:
• 1. Preoperative subconjunctival injection of MMC directly into the
• pterygium tissue at the limbus, 1 month before bare sclera excision
• of the pterygium (0.1 mL of 0.15 mg/mL MMC). The recurrence
• rate reported was 6% during a 2-year follow-up period.66
• 2. Intraoperative application of surgical sponges soaked in MMC
• solution directly to the scleral bed at the time of pterygium excision.
• Typically with 0.02% MMC for 30 seconds to 5 minutes.51–53,62,67
• Recurrence rates in bare sclera excision reported with this application ranged from 3% to 43%.
• 3. Postoperative use of topical MMC eye drops. Typically, 0.02% MMC
• eye drops are used; however, concentrations vary between 0.005%
• and 0.04%, administered four times daily for 1–2 weeks. Recurrence rates with postoperative MMC drops
are reported between
• 0% and 38%.53,58,59 Application of MMC eye drops are associated
• with complications including ocular surface toxicity, iritis, limbal
• avascularity, scleral melts or calcific plaque, corneal decompensation, scleral or corneal perforations,
secondary glaucoma, and cataract formation.

9. • Several organisms have the ability to penetrate intact corneal epithelium and this group of
• organisms can be remembered using the following mnemonic ‘CHANeLS’.
• C— Corynebacterium
• H— Haemophilus influenza
• A— Acanthamoeba
• Ne— Neisseria gonorrhoea/ meningitidis
• L— Listeria
• S— Shigella
• Microbial keratitis caused by this group of organisms need to be treated aggressively to prevent
• permanent visual loss.
• ASCRS. Special Report: Acanthamoeba Keratitis. July 2007. Available at: http:// www.ascrs.org/
sites/
• default/ files/ resources/ Acanthamoeba%20Keratitis.pdf
• Tjia KF, et al. The interaction between Neisseria gonorrhoeae and the human cornea in organ
culture.
• An electron microscopic study. Graefes Arch Clin Exp Ophthalmol 988;226:34 – 5.