Table of contents

Principles of the Cornea

The cornea is an important structure involved in the focusing of light onto the retina. Disorders of the cornea are serious and can be sight-threatening

Schematic diagram of the eye. National Eye Institute, CC BY 2.0

Anatomy and Physiology

The cornea (45D) has the highest refractive power of the eye, and in addition to focusing light, it also filters out UV rays.

Anatomy

10.6mm vertical length and 11.7mm horizontal length
7.8mm anterior curvature and 6.5mm posterior curvature
Thickness = 555um in the centre, thicker at the periphery
Dermatan sulphate and keratan sulphate are found in the stroma
Avascular immune-privileged structure (lacks MHC II cells)
Innervated by CNV1 via long ciliary nerves

Layers of the cornea (superficial to deep)

The layers of the cornea. By Mai aeg, CC BY-SA 4.0

Epithelium: stratified non-keratinised squamous epithelium
Bowman's layer
Stroma: thickest layer, and contains collagen type 1
Descemet's membrane: contains collagen type 4
Endothelium: maintains corneal transparency by mitochondrial pumping of water from stroma to aqueous

Palisades of Vogt are radial folds of the cornea at the superior and inferior limbus (the boundary between the cornea and sclera)

Endothelial cell density < 800mm² is considered to be corneal endothelial failure.

Corneal endothelial donors must have >1500mm² density

Special Tests

Specular microscopy

Used to study the corneal endothelium at high resolution
Corneal endothelial cells are hexagonal and the normal adult has a cell density of around 3000 cells/mm2
The number of corneal endothelial cells decrease with age
Highest endothelial cell density is at the periphery

The normal range for corneal endothelial cell density is 1500-3500 cells/mm²

Corneal Topography

Oculus Pentacam report of corneal thickness and surface shape.

Produces an image of the shape of the corneal surface
Principally used in the evaluation of corneal ectasias and astigmatism
This is further discussed in the refraction chapter

Pachymetry and OCT are used to measure corneal thickness

Fluorescein Staining

Used in the identification of corneal epithelial lesions
Corneal epithelial defects stain green with fluorescein.

Management Overview

Epithelial defects

May require re-epithelialization to promote healing
Lubrication with artificial tears
Bandage contact lenses
PO doxycycline

Persistent unresponsive epithelial defects:

Amniotic membrane graft
Autologous serum drops

Doxycycline is a matrix metalloproteinase (MMP) inhibitor which promotes wound healing

Exposure/neurotrophic keratopathy

Management involves closing the lids through:

Taping of the lids
Botox to the levator muscle
Gold weight upper lid insertion for facial nerve palsy
Tarsorrhaphy

Smoking hinders epithelial healing and should be stopped

Limbal stem cell deficiency

Limbal stem cell transplantation

The Palisades of Vogt can be lost in cases of limbal stem-cell failure

Dry eye

Artificial tears and lubricants
Punctal plugs

Keratoplasty

Keratoplasty (corneal transplant), can be full-thickness (penetrating) or partial thickness (anterior or posterior lamellar).

Indications

Optical - keratoconus (commonest), scarring, corneal dystrophies, bullous keratopathy
Therapeutic - removal of infected corneal tissue

Types of Keratoplasty

Type	Description	Indication
Penetrating keratoplasty (PKP)	Donor transplantation of the full thickness of the cornea. They are usually stitched with non-dissolvable stitches so PKP eyes look distinctive	Severe disease affecting vision and structural integrity of the cornea
Tectonic Lamellar Patch	This is a full-thickness small patch of cornea used to plug a perforation until eye the becomes quiet. Then do full PKP	Corneal perforation (infection or melt)
Deep anterior lamellar keratoplasty (DALK)	Partial-thickness transplant of the anterior cornea to the level of Descemet's membrane Because the endothelium isn’t transplanted, lower risk of rejection compared to PKP	Keratoconus Scarring Stromal dystrophies
Descemet's stripping automated endothelial keratoplasty (DSAEK)	An endothelial keratoplasty which replaces Descemet's and endothelium. It also adds stroma.	Fuchs’ endothelial dystrophy
Descemet's membrane endothelial keratoplasty (DMEK)	An endothelial keratoplasty which replaces Descemet's and endothelium. It does not add stroma.	Fuchs’ endothelial dystrophy

The key difference between DSAEK and DMEK is that DSAEK adds stroma. Remember ‘S’ for Stroma.