Why can't spectacles fully correct keratoconus?

Keratoconus causes irregular astigmatism where the corneal surface is non-uniformly curved. Spectacle lenses can only correct regular astigmatism (two perpendicular meridians with consistent curvatures). Rigid contact lenses work by creating a smooth tear film layer over the irregular cornea, providing a regular refracting surface.

Why is vision worse in the morning with Fuchs' dystrophy?

During sleep, the closed lids prevent tear evaporation, and the cornea absorbs more water. In Fuchs' dystrophy, the failing endothelial pump cannot adequately remove this excess water, causing significant stromal edema overnight. Vision improves during the day as the open eye allows evaporation to help dehydrate the cornea.

When does a pterygium need to be removed?

A pterygium requires surgical removal when it threatens to grow over the visual axis (central cornea), induces significant astigmatism affecting vision, causes chronic irritation unresponsive to lubrication, or interferes with contact lens wear. Small, stable pterygia that are not causing symptoms are typically monitored.

Common Corneal Conditions for ABO

Why Corneal Conditions Matter for Opticians

The cornea provides about two-thirds of the eye's refractive power, so any condition that changes its shape, clarity, or regularity directly affects vision and lens correction. Opticians encounter patients with corneal conditions when dispensing specialty lenses, managing post-surgical prescriptions, and recognizing when to refer for evaluation.

Keratoconus

Keratoconus is a progressive condition where the cornea thins and bulges outward into a cone-like shape. It is the most common corneal ectasia, typically presenting in the teens to twenties.

Key Features

Progressive irregular astigmatism that cannot be fully corrected with spectacles
Corneal thinning, usually inferior-temporal
Scissors reflex on retinoscopy
Munson's sign: V-shaped indentation of the lower lid when looking down
Fleischer ring (iron deposit at base of cone) and Vogt's striae (fine stress lines)

Correction

Early: Spectacles may provide adequate vision
Moderate: Rigid gas permeable (RGP) contact lenses mask the irregular corneal surface with a smooth tear lens
Advanced: Scleral lenses, piggyback systems (RGP over soft lens), or hybrid lenses
Surgical: Corneal cross-linking (to slow progression), intrastromal corneal ring segments (INTACS), or corneal transplant for severe cases

Keratoconus causes IRREGULAR astigmatism that spectacles cannot fully correct. Rigid contact lenses work because the tear film between the rigid lens and the irregular cornea creates a smooth refracting surface, effectively neutralizing the corneal irregularity.

Fuchs' Endothelial Dystrophy

Fuchs' dystrophy is a progressive disease where corneal endothelial cells deteriorate, losing their ability to pump water out of the stroma. This leads to corneal edema and clouding.

Vision is typically worse in the morning (overnight lid closure reduces tear evaporation, allowing more stromal swelling) and improves during the day
Guttae (excrescences on Descemet's membrane) are the hallmark finding
Advanced cases develop epithelial bullae (painful blisters on the corneal surface)
Treatment: Hypertonic saline drops (draws water from cornea), hair dryer technique, and eventually corneal transplant (DSEK/DMEK)

Corneal Abrasion

A corneal abrasion is a scratch or defect in the corneal epithelium, the most common corneal injury. Symptoms include pain, tearing, foreign body sensation, and photophobia. The epithelium regenerates quickly (24-72 hours for most abrasions). Contact lens wear should be discontinued until the abrasion heals completely.

Pterygium and Pinguecula

Pinguecula: A yellowish, raised growth on the conjunctiva near the limbus. Benign and usually does not require treatment.
Pterygium: A wing-shaped growth of conjunctival tissue that extends onto the cornea. Can induce astigmatism if large enough and may require surgical removal if it threatens the visual axis.

Corneal Edema

Corneal edema (swelling) causes haziness and reduced vision. Common causes:

Contact lens overwear (oxygen deprivation)
Fuchs' dystrophy (endothelial pump failure)
Post-surgical (cataract surgery can damage endothelial cells)
Elevated IOP (acute angle-closure glaucoma)

If a contact lens wearer complains of hazy vision first thing in the morning that clears during the day, consider whether they are sleeping in their lenses. Overnight wear reduces corneal oxygen, causing epithelial and stromal edema. Higher-Dk materials or switching to daily disposables can help.

Attempting to correct keratoconus with spectacles alone when the condition has progressed. Once irregular astigmatism becomes significant, spectacles cannot provide adequate vision because they correct regular astigmatism (two perpendicular meridians) but not irregular astigmatism (non-uniform surface). Rigid contact lenses are needed.

Key Takeaways

Keratoconus causes progressive irregular astigmatism best corrected with rigid contact lenses
Fuchs' dystrophy damages the endothelial pump, causing morning corneal edema
Corneal abrasions heal quickly (24-72 hours) as the epithelium regenerates
Pterygium can grow onto the cornea and induce astigmatism
Corneal edema from any cause reduces vision and indicates compromised corneal health