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.
The shape of the cornea
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.
The third layer, the thickest layer, the majority part includes - stroma.
Consists of collagen lamellae.
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
Strongest and toughest layer.
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.
Single layer of polygonal cells.
Does not regenerate.
Cornea is the most densely innervated tissue in body.
Sensory supply: Ophthalmic division of trigeminal → nasociliary nerve → long posterior ciliary nerve → supplies cornea
Long posterior ciliary nerve forms 3 plexus:
Intra epithelial plexus
Sub epithelial plexus.
Neurotrophic keratitis- Due to fifth nerve lesion, we don’t get neuropeptides → leads to keratitis.
Neuroparalytic keratitis - Due to seventh nerve palsy → lagophthalmos → exposure keratitis.
Physiology of Cornea
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.
If <500 cells/mm square leads to
Bullous keratopathy – Sign of corneal decompensation
Any activation of anaerobic glycolysis leads to lactic acid formation. This will lead to metabolic acidosis, which will lead to inhibition of sodium potassium pump. This leads to corneal oedema.
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.
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.
Steep cornea, rings are closer and viseversa.
Name of instrument is Orbscan.
3-D image of both anterior and posterior cornea.
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).
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 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.
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.
Specular microscopy - To examine endothelial cells in high magnification.
Corneal Guttata - Empty spaces on specular microscopy, seen in Fuchs endothelial endoscopy.
Cochet –Bonnet aesthesiometer
It has a nylon filament of 60mm which could be decreased to 5mm.
Retract at 0.5cm step till the patient feels contact . Shorter the length lesser the sensation.
This should be done in each quadrant of cornea and also compared with other eye.
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.
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
It is a device used to measure corneal thickness.
Image description - Pachymeter - Shows the thickness of the cornea.
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
H - Homophilous
S - Shigella
Any superficial vascularisation and some degenerative change
Blurring of vision
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?
Ulcus serpens also called hypopyon corneal ulcer
Exotoxins Collagenic effects ? Perforation
TE ? Healing ? Corneal opacity
Infection by virulent organisms ? replaced by inflammatory exuadtes ? Pseudocornea
Corneal Scraping ? Kinura Spacula
AFB – Acid fast stain
Cooked Meal broth
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)
If there is any infective epithelial ulcer, use of bandage is avoided, and secondly, no use of steroids.
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.
Yeasts → Candida albicans
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
Q. What is the most common cause of orbital cellulitis
Dry/ rough ulcer
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.
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
Subconjuctival injection of Fluconazole
Intra cameral (inside the chamber)
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.
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.
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.
Viralkeratitis - It is the Infection of the cornea
Diabetes- It reduces the corneal nerve fibers.
Chronic degenerative condition of the cornea, like - band-shaped keratopathy
Absoluteglaucoma - A condition when the entire eye loses all visibility.
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’srule- 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.
Chronic – Eye Disease
Recurrent – Infection
Decreased Corneal sensations.
Diagnosis is mostly clinical
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.
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.
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.
Happens in soft contact lens users.
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?
Q. Soft contact lens users prone to develop?
Ans. Acanthamoeba keratitis
It includes staining and culture investigation
The stains used are the following:
Culture media: It includes non-nutrient agar with e.coli
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
RP + Deafness
Interstitial keratitis is divided into syphilitic and non-syphilitic.
They are cognitive.
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.
Following treatments are done for interstitial keratitis.
Topical treatment in the form of a cream is used.
May need systemic steroids and immunosuppressive drugs
Systemic penicillin has no role in syphilitic interstitial keratitis.
Ectatic Dystrophy of cornea leading to conical protrusion.
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
Mode of inheritance - autosomal dominant
Diminishing of vision
Frequent change of glasses (in young patients)
In high Astigmatism, chances of uniocular diplopia
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
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.
Associations of Keratoconus
It can be either ocular or systemic.
Can be seen in Ehler danlos syndrome.
It also includes Osteogenesis imperfecta.
Other examples include - Down syndrome, Marfan Syndrome.
It occurs in Vernal keratoconjunctivitis.
It includes - Atopic keratoconjunctivitis and Blue sclera.
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.
Recent studies showed that the tissue can be safely extracted about 24 hours of death. But the timeline of within 6 hours is the best, between 6 and 12 hours is safe, and between 12 to 24 hours is allowable. .
Contraindications for cornea donation
Death due to an unknown cause
CNS infections or diseases
Most haematological malignancies
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/ mm2.
Urrets zavalia syndrome (rare) - a triad of iris atrophy, fixed dilated pupil and secondary glaucoma.
Astigmatism (most common late complications)
Signs of graft rejection
Keratic precipitates on the corneal graft but not on the peripheral recipient cornea
Types of graft rejection
Epithelial graft rejection: Kayes dots
Subepithelial rejection: Krachmer spots
Endothelial graft rejection: Khodadoust line
Cornea is immune privileged
Corneal graft rejection is uncommon as they are less prone to adverse immune reactions due to avascularity, absence of lymphatic structures, minimal MHC expression, and no HLA role.
Gender-biased graft compatibility
A cornea from a male donor can be used only for male patients, while cornea from a female donor is compatible with both.
Lamellar Keratoplasty (LK)
Lamellar keratoplasty is a partial thickness replacement. It is of two types based on the layers involved.
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.
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.
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.
It is a collar button design keratoplasty and the most used type worldwide.
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.
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
Many physiological processes and diseases lead to abnormal depositions in the cornea, causing vision impairments called corneal degeneration. The major types are as follows.
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.
Unilateral arcus senilis
Commonly, arcus senilis is bilateral affecting both the eyes. But if the opacity occurs in a single eye, it indicates a contralateral carotid artery stenosis.
Band Shaped Keratopathy (BSK)
Calcium deposition in subepithelial, bowman’s membrane and anterior stromal layers.
Metabolic causes: Sarcoidosis, vitamin D toxicity, and hyperthyroidism.
How do sarcoidosis, vitamin D toxicity and hyperthyroidism cause BSK?
These conditions are multisystem disorders that increase the concentration of calcium in blood and other organs. As they persist, calcium aggregation leads to the granule depositions in the affected organs, including eyes.
Also known as Labrador keratopathy/ Climatic droplet keratopathy (CDK)/ Actinic degeneration
Amber-coloured proteinaceous granules deposition.
UV exposure (Common in tropical countries) – PRIMARY CAUSE
Inflammation or infection- SECONDARY CAUSE
Avoid sun exposure.
Removal of deposits through superficial keratectomy.
Why is spheroidal degeneration called actinic degeneration?
Actin present in the cornea gets denatured loosing its native structure and aggregate to form droplets. They also appear to replace Bowman’s membrane. Hence, it is also known as actinic degeneration. The primary cause of this is exposure to UV light.
Salzmann Nodular Degeneration
Hyaline deposits usually seen above the Bowman's membrane.
Chronic irritation, such as trachoma, chronic blepharitis and dry eye.
Manual or excimer superficial keratectomy.
Vortex keratopathy or cornea verticillata
Deposition of drugs in whirl-like pattern is characteristic of vortex keratopathy.
Amiodarone (dose related).
Chlorpromazine deposits on endothelial cells. (it does not cause vortex keratopathy)
It is an irregular deposition of fat, cholesterol, and phospholipids in stroma.
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!
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