Keratoconus is a condition in which the cornea assumes a
conical shape as a result of non-inflammatory thinning of the
corneal stroma.
The corneal thinning induces irregular astigmatism, myopia and
protrusion leading to mild to moderate impairment in the quality
of vision.
It is a progressive disorder affecting both eyes although only
one eye may be affected initially.
EPIDEMIOLOGY:
Keratoconus has its onset at puberty and is progressive until
the third to fourth decade of life when it usually arrests.
It is most commonly an isolated condition, however many
diseases were reported to be associated with it, most importantly
Down’s syndrome, Leber’s congenital amaurosis and connective
tissue disorders.
Other conditions such as atopy, eye rubbing and hard contact
lenses have also been reported to be highly associated with this
disorder.
SIGNS OF KERATOCONUS:
- External signs:
Munson’s sign: It is a V-shaped conformation of the
lower lid produced by the ectatic cornea in downgaze.
Munson's sign
Rizutti’s sign: Is a sharply focused beam of light
near the nasal limbus, produced by lateral illumination of the
cornea in patients with advanced keratoconus.
- Slit-lamp findings:
- Stromal thinning (centrally or paracentrally, most commonly
inferotemporally)
- Posterior stress lines (Vogt’s striae). These
are vertical lines in the deep
stroma and Descemet’s membrane that parallel the axis of the
cornea.
Iron line (Fleischer’s ring)
Epithelial or subepithelial scarring
- Scissoring of the light reflex on retinoscopy.
- Keratometry findings:
- Distortion of the mire lines
- A documented increase in corneal curvature over time is a
sensitive indicator of keratoconus.
- Hydrops of the cornea:
Patients with advanced disease may occasionally present with a
sudden onset of visual loss associated with pain.
On slit-lamp examination, the conjunctiva may be injected and a
diffuse stromal opacity is noted in the cornea. This condition
referred to as hydrops is caused by breaks in
Descemet’s membrane with stromal imbibition of aqueous through
these breaks.
The edema may persist for weeks or months, usually diminishing
gradually with relief of pain and resolution of the redness and
corneal edema ultimately being replaced by scarring.
PATHOLOGY:
The classical histopathologic features of keratoconus consist
of a triad that includes thinning of the corneal stroma, breaks in
Bowman’s layer and deposition of iron in the nasal layers of the
corneal epithelium.
Depending on the stage of the disease, every layer and tissue
of the cornea can however become involved in the pathological
process.
Descemet’s membrane is rarely affected except for breaks seen
in acute hydrops.
The endothelium is usually normal.
MANAGEMENT:
The management of keratoconus varies depending on the state of
progression of the disease. In early cases, spectacles may provide
adequate visual correction but because they do not conform to the
unusual shape of the cornea and the resultant induced irregular
astigmatism, contact lenses provide better correction.
Contact lenses represent the treatment of choice in 90% of
patients.
Corneal transplant:
Penetrating keratoplasty is the most successful surgical option
for keratoconus patients who cannot tolerate contact lenses or who
are not adequately rehabilitated by them.
A patient with keratoconus has an approximately 10-20% chance
over his/her lifetime of needing a corneal transplant.
Patients whose best corrected spectacle visual acuity is 20/40
or worse should be offered corneal transplants instead of contact
lenses.
There is a 50% chance that after a penetrating keratoplasty,
patients may still need contact lenses either because of residual
myopia or post-PK astigmatism.
Keratoconus (either frank or latent) is an absolute
contraindication for keratorefractive surgery.
