Cornea Flashcards
What structure of the eye has the highest refractive power?
The cornea (45D)
Function of the cornea
Focusing light and also filters out UV rays.
Lengths of the cornea
10.6mm vertical length and 11.7mm horizontal length
8.7.8mm anterior curvature and 6.5mm posterior curvature
Thickness of the cornea
Thickness = 555um in the centre, thicker at the periphery
What is the stroma of the cornea composed of
Dermatan sulphate and keratan sulphate are found in the stroma
The cornea is described as an immune-privileged structure
lacks MHC II cells
Cornea innervation
CNV1 via long ciliary nerves
Layers of the cornea (superficial to deep)
- 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
What are Palisades of Vogt
radial folds of the cornea at the superior and inferior limbus (the boundary between the cornea and sclera)
What endothelial cell density is considered corneal endothelial failure
Endothelial cell density < 800mm²
What cell density is required for endothelial donors
> 1500mm² density
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
normal range for corneal endothelial cell density
1500-3500 cells/mm²
Corneal Topography
Produces an image of the shape of the corneal surface
Principally used in the evaluation of corneal ectasias and astigmatism
Oculus Pentacam report of corneal thickness and surface shape
What 2 investigations can be used to measure corneal thickness
Pachymetry and OCT
Fluorescein Staining
Used in the identification of corneal epithelial lesions
Corneal epithelial defects stain green with fluorescein.
Fluorescein staining on a patient with corneal abrasion. The arrow indicates a stained section of the cornea where epithelial defects are likely to be found.
Management of 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 in management of epithelial defects
Doxycycline is a matrix metalloproteinase (MMP) inhibitor which promotes wound healing
Management of 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 and epithelial healing
Smoking hinders epithelial healing and should be stopped
Management of Limbal stem cell deficiency
Limbal stem cell transplantation
The Palisades of Vogt can be lost in cases of limbal stem-cell failure
Management of Dry eye
Artificial tears and lubricants
Punctal plugs
Keratoplasty
Keratoplasty (corneal transplant), can be full-thickness (penetrating) or partial thickness (anterior or posterior lamellar).
A patient who has undergone penetrating keratoplasty.
Indications for keratoplasty
Optical - keratoconus (commonest), scarring, corneal dystrophies, bullous keratopathy
Therapeutic - removal of infected corneal tissue
Types of Keratoplasty
Difference between DSAEK and DMEK
The key difference between DSAEK and DMEK is that DSAEK adds stroma. Remember ‘S’ for Stroma.
In what two groups of people is Bacterial keratitis most commonly seen in?
contact lens wearers and in post-op patients
Pathogens in bacterial keratitis
Pseudomonas Aeruginosa - commonest cause of bacterial keratitis in contact lens wearers
Other causes include: Staphylococcus aureus and Streptococci
Presentation of bacterial keratitis
Keratitis typically presents with an acutely painful unilateral red eye with discharge
Pseudomonas infection is seen in prolonged contact lens wear
Investigation for bacterial keratitis
Corneal scraping for microbiology
Management of bacterial keratitis
Topical broad-spectrum antibiotics (e.g fluoroquinolones such as ofloxacin)
Subsequent specific antibiotics are guided by sensitivity testing
Compare presentation of fungal vs bacterial keratitis
Presenting symptoms are milder than bacterial infections. Look for immunocompromise or trauma associated with plants/soil in the history.
Candida
Patients with AIDS/ immunocompromise/diabetes
Plaque corneal ulcer with expanding infiltrate
Aspergillus/Fusarium
History of trauma associated with contact of plants or soil
White, feathery corneal lesions
Management of Fungal Keratitis
Corneal scraping for microbiology
Candida → Voriconazole or amphotericin B drops
Filamentous (aspergillus/fusarium) → Natamycin drops
Severe infection → add Chlorhexidine
What may provide a faster diagnosis than culture
(for fungal keratitis)
Confocal microscopy can provide a faster diagnosis than culture
Acanthamoeba keratitis
A protozoan disease that can be clinically severe and is notoriously difficult to treat.
Pathology of Acanthamoeba keratitis
Caused by the protozoan: Acanthamoeba
Lives in soil, fresh water and the upper respiratory tract.
Presentation of Acanthamoeba keratitis
Initial presentation is similar in bacterial keratitis
Neural ring-shaped corneal infiltrates are specific and develop over time
Investigation for Acanthamoeba keratitis
Culture medium is non-nutrient agar with E.coli
Confocal microscopy shows amoebic cysts
Infection is associated with exposure to freshwater or soil, especially in contact lens wearers
Management of Acanthamoeba keratitis
Topical polyhexamethylene biguanide (PHMB) or chlorhexidine
(Acanthamoeba Keratitis is widely considered to be an orphan disease. Drug companies haven’t adapted treatments because of the rarity of the disease)
Marginal Keratitis
An autoimmune reaction against Staphylococcal toxin, which characteristically affects the peripheral cornea.