Band Shaped Keratopathy (BSK)
It is a corneal degeneration characterised by the deposition of calcium hydroxyapatite in the epithelial basement membrane, Bowman’s membrane and anterior stroma.
It typically begins just inside the limbus in the periphery. There is a sharply demarcated edge separated from the limbus by a lucent zone.
Why does the lucent zone occur?
The absence of the Bowman's layer in the extreme periphery of the cornea.
Due to the buffering capacity of limbal vessels, which prevent deposition of calcium.
It is initially grey but becomes chalky-white as it progresses. It spreads centrally in a "band-like" distribution and affects the visual axis. The deposits may extend anteriorly and break through the epithelium leading to symptoms such as irritation and foreign body sensation.
In advanced cases (second picture), there is a “Swiss cheese" appearance due to the lucent holes in the calcific pattern caused by corneal nerve penetration through Bowman's layer.
Causes:
Uveitis (Especially Juvenile Idiopathic Arthritis)
Herpetic keratouveitis (especially herpes zoster)
Chronic glaucoma
Dry eye and exposure conditions
Interstitial keratitis
Phthisis bulbi
Chemical exposure: mercury fumes, phosphate containing drops, intraocular silicone etc
Hypercalcemia - all causes of it
Hereditary disorders: Norrie's disease and congenital band keratopathy
Systemic diseases: discoid lupus, gout etc
In patients who are symptomatic or have visual disturbances, superficial keratectomy can be done- manually with or without chelating agent such as EDTA, or with excimer laser phototherapeutic keratectomy.
Image from Rajan Eye Care Hospital