Pterygium is derived from the greek word ‘pterygion’ meaning wing.
It is a non-malignant, slow-growing proliferation of wing shaped fibrovascular tissue arising from the subconjunctival tissue which can encroach the cornea and eventually the visual axis.
The base of the triangle lies in the interpalpebral conjunctiva while the apex encroaches the cornea.
Strong association of Ultraviolet light - more common in warm and tropical regions.
Why is nasal pterygium more common?
Albedo Hypothesis
Light entering the temporal limbus at 90 degrees is concentrated at the medial limbus due to corneal curvature and thus nasal (medial) pterygium is more common.
Light is reflected from skin of nose back onto nasal limbus
Light focusing on the nasal limbus exposes limbal basal stem cells to increased UV radiation
Longer temporal eyelashes of upper lid and greater downward bowing of the outer ⅔ rd of the upper lid filters the light falling on the temporal conjunctiva and cornea
The flow of tears from temporal to nasal conjunctiva carries with it dust particles that enter the conjunctival sac thus further irritating the nasal limbus
There are 2 anterior ciliary arteries in the nasal side vs 1 on temporal side - thus any irritant causes more hyperemia in the nasal side
Pathology
Elastotic degeneration of collagen
Subepithelial stroma consists of degenerated collagen fibrils which are basophilic and can be stained by elastic tissue stain
Types:
Progressive: Thick and fleshy with marked vascularity. It may have an opaque cap called Stocker’s line (iron deposition)
Atrophic/ Stationary: Thin, attenuated, poor vascularity, no cap
Clinical grades: <2mm, 2-4mm, >4mm
More on pterygium excision surgery- indications and technique later
Image from Rajan Eye Care Hospital