Cornea: Structure, Conditions and Types

If you are preparing for Ophthalmology, you must know about one of the most important parts of the eye, the Cornea. In this detailed blog post, get ready to accelerate your understanding of the Cornea and your Ophthalmology preparation. 

Read thoroughly because this is one of the most critical topics for your NEET PG Ophthalmology preparation. 

Let’s begin.

The shape of the cornea 

• Aspherical 
• Diameter - 11.5 mm to 12 mm
• Refractive index - 1.37 
• Thickness - 500-600 microns or 0.5 to 0.6 mm

Structure of Cornea 

Upper layer - epithelial layer

• Multilayer consisting of a single layer of columnar cells.
• Columnar cells are called basal cells.
• Next are two-three layers of wing cells. 
• The uppermost two layers are stratified squamous epithelium, non-keratinised. 
• This layer has a microvilli layer.
  • Microvilli helps to increase the surface area, helping in attachment of tear film to cornea.
• In corneal dystrophy the problem lies in desmosomes

Second layer - bowman’s membrane

• Part of stroma, acellular membrane. 
• Doesn't regenerate.

 The third layer, the thickest layer, the majority part includes - stroma. 

• Consists of collagen lamellae.
• Type-1 collagen.
• A ground substance composed of - glucose amino glycans (GAGs).
• GAGs - keratin sulphate and chondroitin sulphate. 
• Other compositions include fibroblasts/ keratocytes. 

Fourth layer- Dua’s layer

• Acellular. 
• Strongest and toughest layer.

Descemet membrane 

• Two parts - Banded and non-banded parts. 
• Banded part - In uterus, regenerative. 
• Non-banded part - secreted by endothelial cells. 
• No elastic tissues.
  • Therefore, the break leads to haabs striae, Vogt’s striae 
• On gonioscopy: Schwalbe’s line is seen.

Endothelium 

• Single layer of polygonal cells.
• Does not regenerate. 

Physiology of Cornea 

• Avascular 
• Dehydrated 
• Primary metabolism - Aerobic metabolism 
• Nutrition from - Aqueous humour
• Pump function by Sodium Potassium ATPase pump  and barrier function helps to keep the cornea in dehydrated state.
• Whenever endothelium is not functioning well, hydration occurs leading to edema in the cornea.

Number of endothelial cell (normal range)

• Adults - 2500 to 3000 cells/mm square
• Children - 3500 to 4000 cells/mm square 
• When endothelium is damaged leads to: 
  • Pleomorphism: changing of shape. 
  • Polymegathism: Enlarging 
• If <500 cells/mm square leads to 
  • Stromal edema 
  • Epithelial edema 
  • Bullous keratopathy – Sign of corneal decompensation

Factors responsible for transparency of cornea

• Regular arrangement of corneal epithelium. 
• Regular arrangement of stromal villi.
• Distance between two lamellae is half of wavelength of light.
• Being assisted by - GAGs. 
• Endothelium takes care of the pumping function and barrier function. 
• Normal IOP, less than stromal pressure. 
• Avascularity 

Investigation related to Cornea 

Keratometry
Measuring the central optical zone of 3 metres.

Keratoscopy or placido's disc

• Regular spacing seen in normal cornea.
• Rings are closer in steeper cornea.
• Rings are far apart in flatter cornea.

Corneal topography 

• Steep cornea, rings are closer and viseversa.
• Name of instrument is Orbscan. 

Corneal tomography 

• 3-D image of both anterior and posterior cornea.

Penta Cam

• 3-D image of both anterior and posterior cornea.
• Based on scheimpflug imaging (rotating camera)
• It is a non-invasive imaging device used in ophthalmology. 
• Used to measure the anterior and posterior curvature of the cornea, the thickness of the cornea, and the anterior chamber depth. 
• It generates a three-dimensional image of the eye and is used in the diagnosis and treatment of various eye conditions such as glaucoma, cataracts, and keratoconus.
• Axial map- colour coded, steeper ones will be warmer (red colour), flat ones will be cooler (blue).
• Corneal thickness measured. Thinner cornea- warm colours, thicker cornea- cool colours 
• Measures pupillary diameters.
• Uses of Penta cam:
  • Assessment and screening of keratoconus 
  • Preop assessment before corneal refractive surgeries.

Conditions related to cornea.

Corneal Opacities

3 types:

• Nebular: <1/3^rd of cornea gets involved – faint white in colour 
• Macular: 1/3^rd to 2/3^rd of cornea gets involved – little darker than nebular
• Leucoma: >2/3^rd to full thickness of cornea gets involved – dense white
• Diminution of vision is more in nebular because of scattering of light. Whereas leucoma obstructs the light.

Management: 

• Optical iridectomy: when it covers the pupil.
• Penetrating / lamellar keratoplasty
• Corneal tattooing by brown or black 
  • Brown colour by applying gold and black colour by applying platinum. 
  • And wash with hydrazine hydrate which helps to fix it 
  • Apply bandage

Optical iridectomy 

• Used to treat various eye conditions such as glaucoma and iris cysts. 
• During the procedure, a small hole is made in the iris to improve fluid drainage and reduce pressure in the eye.
• This can be performed using a laser or with a traditional surgical technique. 
• It is a relatively safe and effective treatment option for certain eye conditions.

Corneal Dystrophies 

• Corneal opacifying disorders
• Genetic 
• Bilateral and symmetrical and generally progressive. Exception - Posterior polymorphous dystrophy is unilateral.

Classification according to layers involved:

• Epithelial/ sub epithelial corneal dystrophy (basement membrane)
• Epistromal corneal dystrophy (bowman’s membrane and anterior stroma)
• Stromal corneal dystrophy
• Endothelial corneal dystrophy 

Epithelial and subepithelial dystrophy/ epithelium basement membrane dystrophy

• Presents with recurrent erosions, due to defective hemi desmosomes attachment.
• Sometime asymptomatic 
• They are of 2 types- 
  • Microcystic/ map dot/ fingerprint dystrophy
  • Meesmans dystrophy: It contains intra epithelial cysts.
• Treatment of overall dystrophy 
  • For meesmans- Lubricants, bandage contact lens. 
  • For microcystic - anterior stromal puncture (stimulates scarring of bowman’s membrane), PTK (Photo therapeutic keratectomy) – Removing epithelium by excimer laser.

Bowman’s membrane dystrophy

• Clinical feature - Recurrent erosion 
• 2types: Rees buckler dystrophy and Thiel Behnke.
• Rees buckler dystrophy seen as Reticular pattern subepithelial opacities. 
• Histology: Bowman’s layer is replaced by connective tissue 
• Thiel Behnke dystrophy: Bowman’s membrane is replaced by fibrofatty tissue. 
• Histology: Saw tooth appearance/ honeycomb appearance
• Treatment - lubricating eye drops, bandage and contact lenses. 

Stromal Dystrophy

• Treatment: Replace the cornea by penetrating keratoplasty

Endothelial dystrophy

• Clinical features: Present with corneal oedema, due to dysfunction of endothelial. 
• Treatment - Hypertonic saline eye drops (5% NaCl), hairdryer. 
  • In case of bully rupture- bandage contact lens, antibiotics and cycloplegics
  • Keratoplasty: penetrating keratoplasty / posterior lamellar keratoplasty
• Names - Fuchs endothelial dystrophy, posterior polymorphous dystrophy

Fuchs endothelial dystrophy 

• Common in females, 
• Associated with open angle glaucoma. 
• Accelerated and bilateral endothelial cell loss
• On specular microscopy beaten bronze endothelium is seen ? leads to bullous keratopathy
• Corneal guttate is a feature of Fuchs endothelial dystrophy.
• Posterior lamellar keratoplasty
  • It is unilateral 
  • Metaplasia of endothelial cell 
  • Seen in early childhood. 
  • Associated with glaucoma and Alport syndrome. 

Test for the Conditions Related to Cornea

Pentacam

• Pentacam is a device that evaluates cataracts, glaucoma, and other eye problems. The pentacam measures- axial map, pachymetry, and anterior and posterior elevation map.

• Pentacam, where all four measurement types are present. This is a normal finding where it gets thick at the green part and thin with the blue color. 

Measurements of Pentacam

• Axial Map: This map assists the cornea's curvature, and can be depicted with a color coding system. 
  • The steeper part of the map has a warm color, like - red.
  • The flat part of the map is depicted in a cool color- blue. 
• Pachymetry
  • It shows the thickness of the cornea.
  • Wherever there is a steeper cornea, the thickness will be less. So, the thinner cornea shows Red color on the pentacam.
  • Whereas the thicker cornea shows Blue color on the pentacam.
• Anterior Elevation MAP
  • This map considers the anterior surface of the cornea's curvature.
  • The curvature is compared with a best-fit sphere. 
  • When it is above, this best-fit sphere is represented by - warmer colors.
  • When the best-fit sphere is below, it is presented with cooler colors. 
• Posterior Elevation MAP
  • This map considers the posterior surface of the cornea's curvature.
  • The baseline is above the best-fit sphere.
  • Above the best-fit sphere, pentacam is represented by - warmer colors. 
  • Below the best-fit sphere, the pentacam is presented by- cooler colors. 
  • Pentacam tells us the anterior chamber depth. 

Uses of Pentacam

• It includes keratoconus diagnosis, screening, and management.
• If the patient is suitable for pre-assessment of corneal refractive surgery.
• Pentacam HR - Helps in the pre-op assessment of a patient's intraocular phakic IOLs.
• Pentacam AKL - Measures axial length, helpful for IOL power calculations.

• Microbiological investigations 
  • Staining 
  • Culture

Layers 

Expected Topography

• Expected topography: Progressive flattening from center to the periphery by 2-4D, with the nasal area flattening more than the temporal area.

Anterior and Posterior elevation map

• Progressive flattening from the center to the periphery by 2-4D, with the nasal area flattening more than the temporal area.
• The accuracy of the finding is depicted by QS value (quality sensitive).
• The value should be - More than 95 for us to rely on the map.

Keratoconus

• Regular astigmatism: uniform steepening along a single corneal meridian that can be fully corrected with a cylindrical lens (BCVA of 20/20 or better)
• Expected topography: symmetric “bow-tie” along a single meridian

Pachymeter

• It is a device used to measure corneal thickness. 

Image description - Pachymeter - Shows the thickness of the cornea.

Specular Microscopy 

Ulcer

Sores that heal slowly or keep coming back are known as ulcers. 

• They can take on a variety of shapes and show both within and externally in your body.
• Also, any interruption in the epithelial layer leads to an ulcer. 

Organisms That Can Penetrate Intact Epithelium

This is a mnemonic to remember the names of organisms.

• N - Neisseria gonorrhoeae 
• N - N. meningitis 
• L - Licheria
• D- Diphtheria
• H - Homophilous 
• S - Shigella 

Pannus 

• Any superficial vascularisation and some degenerative change 

Clinical Features

• Pain 
• Redness 
• Photophobia 
• Discharge 
• Blurring of vision

Keratitis 

• Inflammation of the cornea leads to ulcer formation 

Q. What is the most common etiology?

Ans. Staph aureus 

Q. Most common bacteria causing keratitis in India? 

Ans. Staphylococcus epidermidis

Q. Most common infection after refractive surgery?

Ans. Mycobacterium chelonae  

Q. Which bacterial infection resembles fungal keratitis?

Ans. Nocardia

Other causes 

• Streptococcus Pneumoniae
• Pyogen

Pneumococcus causes 

• Ulcus serpens also called hypopyon corneal ulcer 

Types

• Perforating 
  • Pseudomonas 
  • Exotoxins Collagenic effects ? Perforation 
• Localized 
  • TE ? Healing ? Corneal opacity 
• Sloughing Ulcer
  • Infection by virulent organisms ? replaced by inflammatory exuadtes ? Pseudocornea

Investigations

• Corneal Scraping ? Kinura Spacula
• Conjunctival swap 
• Staining 
  • Grain stain
  • Gimsa stain 
  • AFB – Acid fast stain 
• Culture
  • Blood agar
  • Chocolate agar
  • Lowenstein Jesun 
  • Cooked Meal broth 

Clinical Features

The following are the clinical features of Nocardia. It resembles features fungal keratitis. 

• It has fluorescein stain-positive ulcer. 
• 3 types of corneal ulcer
  • Perforative - Infection by pseudomonas releases toxins having a collagenolytic effect.
  • Localized - If the ulcer is healing, it leads to corneal opacity.
  • Sloughing corneal ulcer - It is a virulent organism infecting the cornea. It is replaced by inflammatory exudates, leading to the pseudo cornea. 
• Leucoma adherence - Iris is adhered to it to block it.
• Acid-fast stain - it is mainly for mycobacterium and Nocardia.

Treatment of Bacterial Keratitis

The treatments for bacterial keratitis are mentioned below.

• Empirical monotherapy - This is a very effective treatment. Moxifloxacin achieves the best penetration.
• Duo therapy - This therapy uses fortified antibiotics. It takes care of both gram-positive and negative stains. The combination of two drugs is given - cephalexin with gentamicin. 
• Antibiotic ointment is another treatment. 
• Oral anti-inflammatory drugs.
• Lubricating eye drops 
• Vitamin A and C contain supplements.
• If corneal thinning – Tetracycline, Doxycycline (Oral)

Management of Non-Healing Ulcer

Management of non-healing ulcers is done in the following ways. 

• Debridement - This process means removing the necrotic tissue. These tissues are the dead  cells in the body organ.
• Another way to manage it is by using lubricating eye drops. 
• Chemical cauterization like these two is used to manage non-healing ulcers. 
  • Trichloroacetic acid (TCA), carbolic acid
• Bandage contact lens - This process helps corneal thinning. 
• Surgical means - This is amniotic membrane grafting.

Keratomycosis 

• Yeasts → Candida albicans
• Filamentus fungi
  • Aspergillus

Q. Which is the most common fungal infection cornea? 

Ans. Aspergillus fumigatus is the most common fungal infection in the cornea.

Q. Why will you get fungal keratitis? 

Ans. Trauma by vegetative matter causes fungal keratitis.

Q. What is the most common fungal infection for Endophthalmitis 

Ans. Candida

Q. What is the most common cause of orbital cellulitis 

Ans. Mucomycosis

Clinical Features

• Satellite nodule
• Dry/ rough ulcer
• Hyphae

Investigation

Investigations can be carried out in the following ways.

• Microbiological - It is corneal scraping. The ulcer is scraped with care with a platinum loop or sterile needle. Because microorganisms may lie deep or near the ulcer edge, this is crucial to reaching the infective material. 
• Firstly, there is staining of the culture.
  • SDA - It stands for Sabouraud dextrose agar.
    • With the use of this agar media, the culture reporting should be given in one week.
  • PDA - Potatoes dextrose agar, used for cultivating and isolating yeast and molds from foodstuffs.  
• 10% KOH is used.
• Can be seen through light or a simple microscope 
• Confocal biomicroscopy - The hyphae of fungi can be observed through this.
• Calcofluor white- A widely used stain for detecting fungi and bacteria.
• Gram stain - Checks for bacteria at the site of suspected infection.
• PCR - Polymerase chain reaction method. This is a highly accurate way to diagnose infectious diseases. It helps in achieving an early diagnosis of the cornea. 
• Corneal biopsy - Helps treat complex corneal problems.

Treatment

• Drug of choice for treatment of fungal keratitis is - Natamycin 5%.
  • If it is responding, taper it in 7 days.
  • If not responding, add 1% voriconazole with natamycin.
• Another option is Econazole - 1%.
• For severe cases fluconazole tablets (Monitor LFT) voriconazole can be used.
• O.15% Amphotericin can be used 
• Silver Sulfadiazine cream can also be used in fungal keratitis.
• For recurrent fungal keratitis Caspofungin can be used
• For severe cases
  • Systemic 
    • Fluconazole
    • Voriconazole 
  • Subconjuctival injection of Fluconazole
  • Intra cameral (inside the chamber)

Viral Keratitis

• Viral Keratitis- When the lens is in your eye, the cornea may become contaminated, leading to infectious keratitis. Both infectious and noninfectious keratitis can be brought on by improper contact lens care or excessive contact lens usage. 
• Herpes infection is the most common, found in the cornea. 
• The infectious cause of corneal blindness is Herpetic Eye DS.
• Herpes Simplex (HSV 1) viruses are mostly transmitted through sores, saliva, or surfaces near or in the mouth. 
• This infection occurs above the waist.
  • Primary infection- It causes minimal corneal involvement.
  • Secondary infection - It is recurrent and has severe corneal involvement.
• HSV 2 is caused below the waist.
• Pathognomic features of viral keratitis show decreased corneal sensation.
  • On examination observe- superficial punctate keratitis.
  • Formation of linear ulcers can be seen again. 
  • There are knob ends filled with viruses. 
  • A dendritic ulcer is a feature of Herpes simplex. 

Investigation of Viral Infection

• Most of the time, treatment is clinical. 
• PCR - Polymerase Chain Reaction is one of the investigation methods.
• Multinucleated giant cells may also be used.

Treatment

• Topical therapy is given with - 0.15% ganciclovir gel, to be given five times.
• Topical acyclovir ointment with- 3% is used.
• Tetracycline atrophy - 1% is used.
• A topical steroid is given under an antiviral cover if the epithelium is healed and not involved (Under anti-viral cover)
• This is mainly for the hypersensitive reaction of stroma or endothelium, topical ointment.
• Iodxivurine
• IOP control; avoid PG analogs in case of managing IOP. It promotes hepatic activity.
• Systemic antiviral drugs are those which inhibit viral replication. 
  • The viral infection depends on whether it requires a therapeutic dose of acyclowave.
  • These drugs are given in both stromal keratitis and endothelial involvement. 
  • If epithelial is involved topical treatment is used. 

Causes of Decreased Corneal Sensation

The causes of decreased corneal sensation. 

• Leprosy - The illness impacts the upper respiratory tract mucosa, peripheral nerves, skin, and eyes.
• Viral keratitis - It is the Infection of the cornea
• Diabetes - It reduces the corneal nerve fibers. 
• Chronic degenerative condition of the cornea, like - band-shaped keratopathy 
• Absolute glaucoma - A condition when the entire eye loses all visibility. 
• Varicella zoster 
  • Two types- one causes chickenpox, and the other causes herpes zoster.
• Herpes zoster ophthalmic 
  • Either there is a skin lesion, eye lesion, and trigeminal neuralgia. 
• Hutchison’s rule - The eye will be involved if the nose tip is involved. As it indicates nasociliary nerve involvement. 
• Vesicles are present on the nose. Whenever there is the involvement of the nose and skin lesions, this is called Hutchinson’s sign.
• Zoster infection is mainly found in immunocompromised patients. 
• Nerve involvement - Causes cranial nerve palsies. Three motor nerves are involved.
• The most common nerve involved is the frontal nerve. 

Ocular Manifestations

• Acute 
• Chronic – Eye Disease
• Recurrent – Infection 

Acute Infection

Clinical Features 

• Pain
• Redness
• Photophobia 
• Blood Vision 
• Discharge 
• Decreased Corneal sensations.

Investigation

• Diagnosis is mostly clinical 
• PCR
• Gimsa staining ? Multi nucleated giant cells

Chronic Eye Disease

• It can lead to neurodermatitis, mucus plaque, scleritis, and lipid keratopathy.
• Mucus plaque keratitis - managed through acetylcysteine eye drops, mucus stains to rose Bengal. 
• Lipid keratopathy 
  • Irregular deposition of lipids around the cornea 
  • The most common cause is herpes 
• Relapsing eye disease - Reactivation of the issue happens after several years.
• Investigation - not generally required, but PCR can be used.

Treatment

• Topical treatment is the form of a cream that can be applied in any place.
• Drugs used are - Oral acyclovir- 800mg, 5 times a day
• Another drug used is Valacyclovir - 100mg TDS, 3 times a day
• Therapy should not exceed more than 14 days. 
• If it exceeds that, it is because of the toxicity of antivirals. Also called Metaherpetic keratitis.
• Go for lubricating eye drops. 

Acanthamoeba Keratitis

Clinical Features 

• Rare infection 
• Happens in soft contact lens users.
  • Tap water 
  • Swimming with contact lens 
• Pain is more due to perineural invasion, i.e., radial perineuritis. 
• On examination, typical ring lesions and pseudo dendrites can be observed.

Q. Which is the most common infection after contact lens use? 

Ans. Pseudomonas 

Q. Soft contact lens users prone to develop?

Ans. Acanthamoeba keratitis 

Investigation

It includes staining and culture investigation 

• The stains used are the following:
  • calcofluor White 
  • Acridine Orange 
  • PAS
  • Culture media: It includes non-nutrient agar with e.coli 
• PCR 
• Confocal microscopy

Treatment

• PHMB (Polyhexa Methyl Biguanide): This is a drug of choice, and is a form of eye drop. It is one hourly drug. You also add with it chlorohexidine drug, given one hourly.
• Diamedines (Propamidine Isethionate): It shows antifungal properties. 
• Neomycin - It acts only on trophozoites. It is also modality of treatment. 
• NSAIDS for pain can be used 

Interstitial Keratitis

Different Etiologies

• RP + Deafness 
• Ushers Syndrome 
• Interstitial keratitis is divided into syphilitic and non-syphilitic. 
• Syphilitic 
  • They are cognitive.
• Non-syphilitic 
  • It includes Leprosy, tuberculosis, and HSV.
  • Acanthamoeba can be present. 
  • Cogan syndrome -It is an autoimmune disease. 
• Inflammation in the stroma leads to new vascularization and bleeding. 
• The pink patch is called - the salmon patch. It is a feature of syphilis. 

Treatment

Following treatments are done for interstitial keratitis.

• Topical treatment in the form of a cream is used.
• May need systemic steroids and immunosuppressive drugs 
• Topical cycloplegic 
• Systemic penicillin has no role in syphilitic interstitial keratitis. 

Keratoconus

• Ectatic Dystrophy of cornea leading to conical protrusion.
• Autosomal dominant 
• It is a slowly progressing disease 
• Myopia + irregular Astigmatism 
• This condition happens when the cornea—the transparent, dome-shaped front surface of your eye—thins and begins to swell outward and assume a cone-like form. 
• Vision blurring and potential glare sensitivity are side effects of a cornea with a cone shape.

It occurs due to the following reasons

• Ectatic dystrophy of the cornea 
• This leads to its conical protrusion 
• Genetic disease
• Progresses slowly 
• Mode of inheritance - autosomal dominant 
• Clinical features 
  • Diminishing of vision 
  • Frequent change of glasses (in young patients)
  • In high Astigmatism, chances of uniocular diplopia 

Findings

• Fleischer’s ring - Iron deposition at the base of an epithelial layer can be seen through the slit lamp. 
• Munsun’s sign - v-shaped deformity of the lower lid on down gaze can be seen in the torch light.
• Oil droplet reflex - seen through a distant direct ophthalmoscope. 
• Retinoscopy - objective refraction method, reflex looks like a cross, also called scissor’s reflex.
• Corneal topography - Initial stage shows regular astigmatism and a bow-tie appearance.
  • Soon, this leads to an asymmetrical appearance. 
  • This progresses as irregular astigmatism and asymmetrical appearance 
  • Investigation choice - Pentacam 

Causes of Thickening of Corneal Nerves

• Leprosy -The upper respiratory tract mucosa, peripheral nerves, skin, and eyes are all impacted by the illness.
• Neurofibromatosis 1 (NF1)- A hereditary disorder that makes tumors enlarge along your nerves.
• Old age - With growing age, thickening of the endothelium and epithelial basement membranes take place.
• Interstitial keratitis in Cogan syndrome 

Treatment

• The person in case of eye infection should avoid eye rubbing.
• Use spectacles, or toric soft contact lens. Astigmatism vision correction is much easier using toric lenses. 
• If soft contact lens is replacing the irregularity with regular surface, the choice of contact lens will be rigid gas permeable lenses or RGP lenses.
• If it is a progressive case of Astigmatism, opt for Corneal collagen cross-linking with riboflavin (C3R) treatment. 
• Put on riboflavin eye drops for 30 minutes while exposed to UV rays.
• For displaced or eccentric cores use INTACS. 
• If nothing works, replace the cornea with a healthy cornea.
• Keratoplasty 
  • PK
  • DALK 

Associations of Keratoconus

• It can be either ocular or systemic.
• Systemic 
  • Can be seen in Ehler danlos syndrome. 
  • It also includes Osteogenesis imperfecta. 
  • Other examples include - Down syndrome, Marfan Syndrome.  
• Ocular 
  • It occurs in Vernal keratoconjunctivitis.
  • It includes - Atopic keratoconjunctivitis and Blue sclera. 

Keratoplasty

• Keratoplasty is the replacement of diseased cornea by donor cornea obtained from the cadaveric eyes of a donor.
• Ideally, the donor cornea must be extracted within 6 hours of the death with relaxation for up to 12 hours. 

The two methods of tissue removal are –

• Whole eye excision 
• Corneoscleral button extraction

Blood test is done to exclude the contraindications.

Contraindications for cornea donation

Tests before keratoplasty 

• Blood tests for infections, malignancies, or disease biomarkers.
• Examination of all the three layers of donor cornea.
  • One of the criteria to approve a cornea for transplantation is endothelial cell count: at least 1500 to 2000 cells/ mm^2.

Storage of cornea after removal

Classification of Keratoplasty 

2 types: 

• Penetrating keratoplasty/ full thickness keratoplasty
• Lamellar keratoplasty

Penetrating keratoplasty (PK)

• Penetrating keratoplasty is a surgical procedure in which the whole cornea, including endothelium is replaced. 
• Ideal size of the graft – 0.25 mm larger than cornea of the recipient. That amounts to ideal size of 7.5mm.
• Most common infection after PK is staph. Epidermidis.
• Indications of Penetrating Keratoplasty

Post-op complications of Penetrating Keratoplasty

• Early:
  • Infection (most common: staphylococcus epdermidis), 
  • Glaucoma  
  • Shallow anterior chamber 
  • Persistent epithelial defects 
  • Desmos membrane detachment
  • Urrets zavalia syndrome (rare) - a triad of iris atrophy, fixed dilated pupil and secondary glaucoma.
• Late:
  • Astigmatism (most common late complications)
  • Glaucoma
  • Graft rejection

Graft rejection 

Signs of graft rejection

• Corneal edema
• Keratic precipitates on the corneal graft but not on the peripheral recipient cornea
• Corneal vascularization
• Stromal infiltrates

Types of graft rejection

1. Epithelial graft rejection: Kayes dots
2. Subepithelial rejection: Krachmer spots
3. Stromal haze
4. Endothelial graft rejection: Khodadoust line

Lamellar Keratoplasty (LK) 

• Lamellar keratoplasty is a partial thickness replacement. It is of two types based on the layers involved. 

1. Anterior lamellar keratoplasty: 

• It is the replacement of the anterior portion of the cornea without disturbing the endothelium. There are two types based on the thickness of the stroma involved.
• Superficial Anterior LK: includes less than or equal to one-third of stroma replaced.
  • Done in pterygium and Limbal dermoid.
• Deep anterior LK: replaces more than one-third of the stroma.

2. Deep endothelial lamellar keratoplasty

• This type replaces the endothelium with or without a part of the stroma. It is of two types.
• Descemet stripping endothelial keratoplasty: replaces a part of stroma along with Descemet membrane and endothelium.
• Descemet membrane endothelial keratoplasty: does not involve stroma.

Keratoprosthesis

• It is a surgical procedure where a diseased cornea is replaced by an artificial cornea after one or two failed corneal transplants. It is of two types.
  • Boston keratoprosthesis
    • It is a collar button design keratoplasty and the most used type worldwide. 
  • Osteo-odonto keratoprosthesis
    • It is also called tooth in eye surgery, ideal for patients with end-stage corneal inflammatory diseases. 
    • Removal of tooth is followed by drilling a hole and fitting optics into it. 
    • After growing in patient’s cheek for a month, it is implanted in the eye.

Procedure:

Removal of the tooth from a patient or donor
↓
a lamina of tissue from the tooth is grown in the patient’s cheek
↓
a hole is drilled in the tissue and optics are filled
↓
Implantation into the patient’s eye

Corneal degeneration

• Many physiological processes and diseases lead to abnormal depositions in the cornea, causing vision impairments called corneal degeneration. The major types are as follows. 

Arcus Senilis

• It is an age-related degeneration characterized by round opacity in cornea due to lipid deposition.
• Lipid deposition in the stroma or bowman membrane starts as superior and inferior arcs and completes the circle gradually.
• Round opacity at a young age is called arcus juvenilis, caused by dyslipidaemia.

Band Shaped Keratopathy (BSK)

• Calcium deposition in subepithelial, bowman’s membrane and anterior stromal layers.

Causes

• Idiopathic (MC), generally age-related factors. 
• Ocular diseases - chronic uveitis, absolute glaucoma, hypercalcemia.
• Metabolic causes: Sarcoidosis, vitamin D toxicity, and hyperthyroidism.

Spheroidal degeneration

• Also known as Labrador keratopathy/ Climatic droplet keratopathy (CDK)/ Actinic degeneration

• Amber-coloured proteinaceous granules deposition.

Causes

• UV exposure (Common in tropical countries) – PRIMARY CAUSE
• Inflammation or infection- SECONDARY CAUSE

Treatment

• Avoid sun exposure.
• Removal of deposits through superficial keratectomy.
• Lamellar keratoplasty.

Salzmann Nodular Degeneration

• Hyaline deposits usually seen above the Bowman's membrane.

Causes

• Chronic irritation, such as trachoma, chronic blepharitis and dry eye.

Treatment

• Lubrication
• Manual or excimer superficial keratectomy.

Vortex keratopathy or cornea verticillata

• Deposition of drugs in whirl-like pattern is characteristic of vortex keratopathy.

Causes

• Chloroquine.
• Amiodarone (dose related).
• Tamoxifen.
• Indomethacin 
• Fabry's disease.

Lipid Keratopathy

• It is an irregular deposition of fat, cholesterol, and phospholipids in stroma.

Types 

• Primary lipid keratopathy → idiopathic and no vascularization.
• Secondary lipid keratopathy → causes corneal vascularization resulting from infection with herpes simplex virus or herpes zoster ophthalmicus.

And that is everything you need to know about the Cornea to scale up your Ophthalmology preparation. For more interesting and informative posts download the PrepLadder App and keep following our blog!