BASICS

DESCRIPTION

A disorder of corneal instability, thinning, steepening, and protrusion that develops in the absence of neovascular inflammation. The name derives from the conical shape of the cornea in advanced disease. Clinical disease is characterized by the following:

• Progressive astigmatism ± myopia.

• Loss of best spectacle-corrected visual acuity.

• Decreased tolerance of contact lens wear.

• Central corneal scar formation and opacification.

EPIDEMIOLOGY

Incidence

Highest in puberty and early adulthood.

Prevalence

50–230 per 100,000 (1).

RISK FACTORS

• Eye rubbing

– A significant risk factor in susceptible individuals

• Nocturnal pressure on the cornea

– A suspected risk factor in susceptible individuals

• 1st degree relative with keratoconus

– 3–4% prevalence within families of affected individuals

Genetics

• No clear inheritance pattern (2).

– Some support for incomplete autosomal dominant pattern.

– Less than full concordance in monozygotic twins.

GENERAL PREVENTION

Avoidance of corneal rubbing is recommended.

PATHOPHYSIOLOGY

• Progressive instability and irregularity in anterior and posterior corneal contours lead to irregular astigmatism.

• Breaks in Bowman’s layer result in fibrosis and corneal scarring.

• Breaks in Descemet’s layer lead to acute corneal edema (hydrops).

ETIOLOGY

Not yet fully elucidated.

COMMONLY ASSOCIATED CONDITIONS

• Floppy lid syndrome/lid laxity (odds ratio >19) (3).

• Sleep apnea.

• Atopy.

• Allergic conjunctivitis.

• Congenital disorders of low vision, including the following:

– Leber’s congenital amaurosis, retinopathy of prematurity, and aniridia

• Congenital disorders of developmental delay (and eye rubbing), including the following:

– Down’s syndrome.

• Connective tissue disorders, including the following:

– Ehlers–Danlos Marfan’s and Osteogenesis imperfecta.

• Corneal dystrophies, including the following:

– Fuchs’ endothelial, posterior polymorphous, granular, and lattice

DIAGNOSIS

HISTORY

• Progressive asymmetric vision and contrast sensitivity loss, glare, distortion, or monocular diplopia

• Progressive asymmetric astigmatism (noticed commonly as distorted vision)

• May manifest clinically as unilateral (but >95% show bilateral topographic changes).

• Eye rubbing/pressure on eye during the day or during sleep (4)

– Many patients directly and forcibly rub the globe (in a pattern often distinct from allergic rubbing)

– May describe a discomfort relieved by rub

– Preferred sleeping side often correlates with more affected eye.

• Difficulty or intolerance of contact lenses

– May describe progression from astigmatism correcting soft-to-rigid lenses.

PHYSICAL EXAM

• Fluctuating refraction with increasing astigmatism and, usually, myopia over time

• Irregular retinoscopic reflex (scissoring)

• Irregular shadowing of red reflex

• External examination abnormalities may include the following:

– V-shaped indentation of lower lid with depression (Munson’s sign)

– Conical prominence and/or displacement of apex on lateral view

– Upper lid laxity ± upper lid lash misdirection

• Slit lamp abnormalities may include the following:

– Iron deposition in basal epithelium around base of cone (Fleischer ring)

– Corneal scarring (mild haze to severe opacification typically at apex)

– Irregular corneal thickness and curvature seen with slit beam

– Prominent corneal nerves

– Posterior stromal striae (Vogt striae) which are typically central and vertical—these may fade with external pressure

– Corneal edema, bullae, and posterior scars in cases of Descemet’s membrane rupture (Hydrops)

• Manual keratometry: Irregular rings ± steep readings

DIAGNOSTIC TESTS & INTERPRETATION

Imaging

• Placido-based keratoscopy or topography may demonstrate some or all of the following (5):

– High keratometric values, inferior steepening (elevated I–S values), high astigmatism, skewed radial axes.

• Scanning-slit beam with placido-disc topography (Orbscan IIz, Bausch & Lomb) or rotating Scheimpflug imaging (Pentacam, Oculus) may demonstrate above abnormalities plus

– Anterior and posterior corneal elevation abnormalities.

– Abnormal corneal thickness profiles with decentered thinnest points.

• Optical coherence tomography (Visante, Carl Zeiss) may demonstrate abnormal corneal thickness profiles with decentered thinnest points.

• Wavefront aberrometry may demonstrate increased higher order aberrations (especially vertical coma).

• High-frequency ultrasound (Artemis, ArcScan, Inc.) may demonstrate abnormal corneal epithelial thickness profile.

Pathological Findings

• All corneal layers may be affected in the disease process, including the following:

– Central epithelial thinning with changes in epithelial cell morphology

– Degenerated basal epithelial cells with accumulation of ferritin particles between cells

– Breaks in epithelial layer

– Fragmentation of and breaks in Bowman’s layer

– Alteration of normal stromal collagen architecture

– Endothelial cell pleomorphism and polymegathism

• Note that pathologic findings may vary

DIFFERENTIAL DIAGNOSIS

• Physiologic astigmatism

• Postrefractive surgery ectasia

• Pellucid marginal degeneration

• Keratoglobus

• Traumatic corneal scar

• Contact lens warpage

TREATMENT

MEDICATION

• For acute hydrops:

– Cycloplegia ± patching or bandage contact lens for comfort

– Hypertonic solution/ointment (5% sodium chloride) b.i.d. to q.i.d.

ADDITIONAL TREATMENT

General Measures

• Important to stop rubbing eyes

• Avoid pressure on eyes during sleep

• Spectacle correction if possible

• Contact lens fitting

– Possible if there is not significant scarring or edema.

– Soft lenses for low levels of regular astigmatism

– Rigid gas-permeable (RGP) lenses are most often successful.

– If unable to fit in an RGP, try piggybacking with soft lens or use a hybrid or scleral lens

Issues for Referral

• Contact lens fitting is often difficult and is best performed by an experienced fitter.

• Patients intolerant of contact lens wear should seek consultation with a cornea specialist to consider surgical options.

• Consider polysomnography to test for undiagnosed sleep apnea.

SURGERY/OTHER PROCEDURES

• Intrastromal ring segments (Intacs, Addition Technology)

– May improve vision and contact lens tolerance

• Corneal stromal crosslinking is an investigational therapy that may help decrease progression of vision loss

• Corneal transplantation is the gold standard

– Full-thickness penetrating keratoplasty

– Trephinated versus noncylindrical variations (manual or Femtosecond-assisted)

– Deep anterior lamellar keratoplasty

ONGOING CARE

FOLLOW-UP RECOMMENDATIONS

• At minimum every 12 months, more frequently if progressing rapidly.

• Every 1–4 weeks during a hydrops episode until it resolves (vision may improve due to corneal flattening after hydrops, if edema persists over months, surgery may be indicated).

PROGNOSIS

• Disease severity and rates of progression are widely variable, but even milder forms of disease may lead to significant decreases in vision-related quality of life.

• Significant visual loss over 5–7 year period more likely if better baseline acuity, steeper keratometric values, or fundus abnormalities (6).

COMPLICATIONS

Corneal perforation is rare.

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