Keratoconus & Refractive Corneal Surgery

Keratoconus is the prototype non-inflammatory progressive corneal ectasia of the young, producing irregular astigmatism and apical thinning. This note links the classic clinical signs to a logical management ladder and ties in the high-yield refractive surgery facts (LASIK contraindications, graft types) that examiners love to combine with ectasia.

Definition & Classification

Keratoconus is a bilateral (often asymmetric), non-inflammatory, progressive thinning and conical ectasia of the central/paracentral cornea, causing progressive myopia and irregular astigmatism. Onset is typically around puberty, progresses through the second to third decade, then tends to stabilise by the fourth decade.

Corneal ectasias as a family:

Ectatic disorder	Location of thinning/protrusion	Distinguishing feature
Keratoconus	Central/paracentral, apex below centre	Munson's sign, Fleischer ring, Vogt's striae
Pellucid marginal degeneration (PMD)	Inferior peripheral band (4–8 o'clock)	"Beer-belly"/"kissing doves" topography, against-the-rule astigmatism
Keratoglobus	Global thinning, limbus to limbus	Whole cornea protrudes, thinnest at periphery, high rupture risk
Post-LASIK ectasia	Variable, at ablation zone	History of refractive surgery
Terrien marginal degeneration	Superior peripheral, with vascularisation	Inflammatory, lipid line at advancing edge

High-yield: Keratoconus apex is below and nasal to the centre (paracentral), distinguishing it from PMD where thinning is a peripheral inferior band with normal corneal thickness over the protrusion.

Severity grading (Amsler–Krumeich) uses keratometry, refraction, corneal thickness and scarring:

• Stage 1: Eccentric steepening; myopia/astigmatism < 5 D; K < 48 D.
• Stage 2: Myopia/astigmatism 5–8 D; K < 53 D; pachymetry > 400 µm; no scarring.
• Stage 3: Myopia/astigmatism 8–10 D; K > 53 D; pachymetry 200–400 µm; no central scarring.
• Stage 4: Refraction not measurable; K > 55 D; central scarring; pachymetry < 200 µm.

Etiology & Pathophysiology

The cause is multifactorial—genetic predisposition plus environmental triggers, chiefly chronic eye rubbing and atopy.

Pathophysiology pathway: eye rubbing/oxidative stress → ↑ matrix metalloproteinases (MMP) and ↓ protease inhibitors → degradation of stromal collagen and proteoglycans → loss of keratocytes (apoptosis) → stromal thinning → biomechanical weakening → ectasia and irregular astigmatism.

Key histopathological hallmarks:

• Fragmentation/breaks in Bowman's layer (earliest layer affected).
• Stromal thinning with reduced keratocytes.
• Iron deposition in the deep epithelium → Fleischer ring.
• Folds/breaks in Descemet membrane → acute hydrops.

Associations to remember:

System / condition	Link
Down syndrome	Strong association; higher hydrops risk
Atopy (eczema, asthma, allergic conjunctivitis)	Eye rubbing → ectasia
Connective tissue	Marfan, Ehlers–Danlos, osteogenesis imperfecta, mitral valve prolapse
Ocular	Leber congenital amaurosis, retinitis pigmentosa, vernal keratoconjunctivitis
Floppy eyelid syndrome / obstructive sleep apnoea	Mechanical + rubbing

High-yield: Vernal keratoconjunctivitis (VKC) and Down syndrome are the two most repeated associations; both share the eye-rubbing mechanism.

Clinical Features

Symptoms: progressive blurring, frequent change of spectacle prescription, monocular diplopia/polyopia, glare and haloes, and—on slit-lamp—the classic signs.

Eponymous signs (must memorise):

Sign	Description	Layer/mechanism
Fleischer ring	Yellow-brown ring of iron at base of cone (best seen with cobalt blue)	Epithelial iron deposition
Vogt's striae	Fine vertical deep stromal stress lines that disappear on external pressure	Deep stroma/Descemet
Munson's sign	V-shaped bulging of the lower lid on downgaze	Cone indenting lid
Rizzuti's sign	Conical reflection on the nasal cornea when light shone from temporal side	Cone refraction
Charleux / oil-droplet reflex	Dark reflex on distant direct ophthalmoscopy	Irregular surface
Scissoring reflex	Scissoring of red reflex on retinoscopy	Irregular astigmatism

High-yield: Vogt's striae disappear on gentle digital pressure on the globe—a classic single-best-answer discriminator.

Acute corneal hydrops: sudden break in Descemet membrane → aqueous floods the stroma → sudden painful drop in vision with marked corneal oedema. Managed conservatively (hypertonic saline 5%, cycloplegic, patching; intracameral gas e.g. C3F8/SF6 may speed resolution). It is not an indication for emergency keratoplasty—wait for resolution and scarring to stabilise.

Diagnosis & Investigation of Choice

Clinical signs confirm advanced disease, but the investigation of choice for diagnosis and detection of early/forme fruste keratoconus is corneal topography/tomography.

Stepwise diagnostic approach:

Retinoscopy (scissoring reflex) → Keratometry (high/irregular astigmatism, inferior steepening) → Placido-disc / Scheimpflug corneal topography & tomography (Pentacam) → Pachymetry (apical thinning) → Biomechanical assessment (Corvis/ORA) for borderline cases.

Topographic/tomographic markers:

• Inferior–superior (I–S) asymmetry value elevated (Rabinowitz criteria: I–S ratio > 1.4 suspicious).
• Central K > 47 D, or skewed radial axes.
• KISA% index for screening.
• Posterior elevation map (Scheimpflug) is most sensitive for early/forme fruste disease—a posterior float elevation is the earliest detectable change.

High-yield: Posterior corneal elevation on Scheimpflug (Pentacam) is the earliest and most sensitive screen for subclinical keratoconus—and the key screening test before refractive surgery to avoid post-LASIK ectasia.

Management / Treatment of Choice

Treatment follows a stepwise ladder based on severity and progression:

Spectacles → Rigid gas-permeable (RGP) contact lenses → Corneal collagen cross-linking (CXL, to halt progression) → Intrastromal corneal ring segments (ICRS/INTACS) → Keratoplasty (DALK/PK).

1. Mild: spectacles correct early myopia/astigmatism.
2. Moderate (irregular astigmatism): RGP / rigid contact lens is the treatment of choice for visual rehabilitation—it creates a smooth refracting surface via the tear meniscus. Options progress to scleral and piggyback lenses with steeper cones.
3. Documented progression (any age, esp. young): Corneal collagen cross-linking (CXL) is the treatment of choice to halt progression. Riboflavin (vitamin B2) drops + UV-A 370 nm light induce new collagen cross-links, stiffening the cornea. Standard (Dresden) protocol needs corneal thickness ≥ 400 µm after epithelial removal (to protect endothelium). CXL halts progression but is not primarily a vision-improving procedure.
4. Contact-lens intolerant, clear central cornea: ICRS (INTACS) flatten the cone and regularise astigmatism.
5. Advanced with scarring or hydrops sequelae / contact-lens failure: keratoplasty.

High-yield: CXL halts progression; it is indicated in documented progressive disease and requires stromal thickness ≥ 400 µm. RGP lenses remain the mainstay of visual rehabilitation.

Keratoplasty: graft types (very high-yield)

When grafting is needed, the choice depends on which corneal layers are diseased:

Graft	Full name	Layers replaced	Best indication
PK	Penetrating keratoplasty	Full thickness	Full-thickness scarring; failed grafts; hydrops scar
DALK	Deep anterior lamellar keratoplasty	Epithelium + stroma, endothelium retained	Keratoconus with healthy endothelium (preferred)
DSAEK/DSEK	Descemet stripping (automated) endothelial keratoplasty	Posterior stroma + Descemet + endothelium	Endothelial disease (Fuchs, bullous keratopathy)
DMEK	Descemet membrane endothelial keratoplasty	Descemet + endothelium only	Endothelial disease—best visual recovery

High-yield: In keratoconus the endothelium is healthy, so DALK is the preferred keratoplasty—it avoids endothelial rejection and the risk of endothelial graft failure. PK is reserved for cases with deep scarring or after hydrops.

Big-bubble technique (Anwar) is the standard method to bare Descemet membrane during DALK.

Refractive Corneal Surgery — and why keratoconus matters

Refractive surgery reshapes the cornea to change its power. The link to keratoconus: an undiagnosed (forme fruste) ectasia is the leading cause of post-LASIK ectasia, so screening topography is mandatory pre-operatively.

Common procedures:

Procedure	Principle	Notes
LASIK	Flap (microkeratome/femtosecond) + excimer stromal ablation	Fast recovery; flap-related risks; needs adequate residual bed
PRK	Surface ablation after epithelial removal	No flap; good for thin corneas/borderline; slower healing, haze
SMILE	Femtosecond lenticule extraction, no flap	Flapless, biomechanically favourable
Phakic IOL (ICL)	Lens implant, cornea untouched	For very high myopia / thin corneas

Residual stromal bed rule: leave ≥ 250–300 µm of residual stromal bed after ablation to prevent iatrogenic ectasia (many surgeons target ≥ 300 µm). Each dioptre of myopic correction ablates roughly 12–15 µm of stroma per 6 mm zone.

High-yield: The single most important contraindication to LASIK is keratoconus / forme fruste keratoconus / any ectasia, because LASIK accelerates ectatic progression.

Contraindications to LASIK (exam list):

• Keratoconus or other corneal ectasia / abnormal topography.
• Corneal thickness too low / inadequate residual stromal bed (< ~250–300 µm).
• Age < 18 years or unstable refraction (changing in last 1 year).
• Pregnancy and lactation (refractive fluctuation).
• Autoimmune/collagen vascular disease, immunosuppression (poor healing).
• Uncontrolled glaucoma, severe dry eye, active herpetic keratitis, cataract.
• Unrealistic patient expectations.

In a borderline cornea (slightly thin, suspicious topography), PRK/surface ablation is preferred over LASIK because it preserves more biomechanical strength (no flap).

Complications

Of keratoconus:

• Acute corneal hydrops (Descemet break → stromal oedema).
• Progressive corneal scarring at apex → permanent visual loss.
• Contact lens intolerance.
• High irregular astigmatism uncorrectable by spectacles.

Of CXL: sterile infiltrates, corneal haze, infectious keratitis, delayed epithelial healing, endothelial damage if cornea < 400 µm.

Of LASIK/refractive surgery: post-LASIK ectasia (most feared), flap complications (buttonhole, free cap, dislocation, striae), diffuse lamellar keratitis (DLK / "sands of Sahara"), epithelial ingrowth, dry eye, under/over-correction, regression, infectious keratitis, night-glare/haloes.

Of keratoplasty: graft rejection (Khodadoust line in endothelial rejection; epithelial/subepithelial rejection lines), graft failure, suture-related infection/astigmatism, raised IOP, recurrence of ectasia in donor (rare, late).

High-yield: Diffuse lamellar keratitis (DLK / "Sands of the Sahara") is a sterile inflammation under the LASIK flap in the first days post-op—treated with intensive topical steroids (not antibiotics for sterile cases).

Key Differentials

• Pellucid marginal degeneration: inferior peripheral thinning, "crab-claw/kissing doves" topography, against-the-rule astigmatism, normal thickness over the cone.
• Keratoglobus: global limbus-to-limbus thinning; high risk of rupture with minor trauma.
• Terrien marginal degeneration: superior, vascularised, with lipid deposition; slowly progressive.
• Contact-lens-induced warpage: topographic distortion that reverses after stopping lens wear—must exclude before diagnosing keratoconus or clearing for surgery.
• Post-refractive surgery ectasia: identical topography but with surgical history.

Mnemonics & Named Facts

• Signs of keratoconus — "FM-VR" / think the radio: Fleischer ring, Munson's sign, Vogt's striae, Rizzuti's sign.
• CXL = "Cross to halt" — collagen cross-linking is to stop progression, not to improve vision.
• DALK for the apex, DMEK for the back — anterior lamellar graft for keratoconus (anterior disease, healthy endothelium); Descemet grafts for endothelial disease.
• Eponyms: Munson (lid V-sign), Vogt (striae), Fleischer (iron ring), Rizzuti (light reflex), Anwar (big-bubble DALK), Dresden (standard CXL protocol), Amsler–Krumeich (staging).

Recently asked / exam angle

• "Earliest layer affected in keratoconus" → Bowman's layer (breaks).
• "Best/earliest investigation to detect forme fruste keratoconus" → Scheimpflug tomography / posterior corneal elevation (Pentacam).
• "Sign that disappears on pressure" → Vogt's striae.
• "V-shaped lid indentation on downgaze" → Munson's sign.
• "Treatment of choice to halt progression of keratoconus" → Collagen cross-linking (CXL), needs cornea ≥ 400 µm.
• "Best visual rehabilitation in moderate keratoconus" → Rigid gas-permeable contact lens.
• "Keratoplasty of choice in keratoconus with healthy endothelium" → DALK.
• "Absolute contraindication to LASIK" → Keratoconus / corneal ectasia.
• "Sands of the Sahara appearance post-LASIK" → Diffuse lamellar keratitis.
• "Sudden painful corneal oedema in keratoconus" → Acute hydrops (manage conservatively, not emergency PK).
• Image-based: Fleischer ring on cobalt blue, oil-droplet reflex, scissoring on retinoscopy.

Rapid revision

1. Keratoconus = bilateral, asymmetric, non-inflammatory progressive corneal ectasia of puberty onward.
2. Pathology starts with breaks in Bowman's layer; iron ring (Fleischer) in epithelium; stress lines (Vogt) in deep stroma.
3. Top associations: eye rubbing, atopy/VKC, Down syndrome, connective tissue disorders.
4. Signs: Munson (lid V), Vogt's striae (vanish on pressure), Fleischer ring, Rizzuti, oil-droplet/scissoring reflexes.
5. Apex is paracentral, below and nasal to centre—differentiates from PMD's inferior peripheral band.
6. Topography/tomography is the investigation of choice; posterior elevation on Pentacam detects early/forme fruste disease.
7. Visual rehabilitation mainstay = rigid gas-permeable contact lens.
8. CXL (riboflavin + UV-A) halts progression; requires stromal thickness ≥ 400 µm.
9. DALK is the preferred keratoplasty in keratoconus (endothelium spared); PK for deep scarring/post-hydrops.
10. Acute hydrops = Descemet break with sudden oedema—treat conservatively (hypertonic saline ± intracameral gas).
11. Keratoconus/ectasia is the key contraindication to LASIK; screen topography to prevent post-LASIK ectasia; keep residual stromal bed ≥ 250–300 µm.
12. Post-LASIK DLK ("Sands of the Sahara") is sterile—treat with topical steroids; PRK/SMILE are flap-sparing options for borderline corneas.