Cornea Flashcards
How is the cornea different to the sclera?
transparent
Describe the refraction power of the cornea?
HIGHEST REFRACTION POWER IN THE EYE: 40D
What is the average diameter of the cornea in the adult and newborn?
10-13MM IN ADULTS AND 9.5-10.5MM IN NEWBORNS
What is the thickness of the cornea?
535um
Where does the cornea receive its nutrients from?
anteriorly - tear film
posteriorly - aqueous humour
Describe the sensory supply of the cornea?
CNV1 via long ciliary nerve
What are the layers of the cornea?
Epithelial layer Bowman layer Stromal layer Descemet membrane Endothelium
Describe the epithelial layer of the cornea
STRATIFIED NONKERATINIZED SQUAMOUS EPITHELIUM
high regenerative potential to injury due to the presence of limbal epithelial stem cells
Where are palisades of Vogt more prominent?
superior and inferior limbus
Describe the bowman layer
Avascular layer (no regeneration potential) that contains collagen fibres and terminates at the limbus
Describe the stromal layer
thickest corneal layer
continuous with the sclera at the limbus
mainly made up of keratocytes and regularly orientated collagen fibrils (type 1 collagen)
it can undergo scarring
no regenerative potential
Describe the descent membrane
An elastic layer containing type IV collage fibres
Describe the endothelium
Function by pumping excess fluid from the stroma to keep the cornea dehydrated to maintain its transparency
Describe bacterial keratitis
common and sight threatening
more common in contact len wearers (soft > rigid), especially in overnight wear / poor hygiene
What are the common organisms in bacterial keratitis?
P aerguinoa (most common cause in contact lens wearers)
S aureus and strep
Describe the features of bacterial keratitis
unilatera sudden-onset pain, redness and photophobia with associated discharge and dVA
purulent / mucopurulent discharge
circumcorneal injection
white infiltrates, epithelial and stromal involvement
anterior chamber cells and hypopyon in severe keratits
What are some complications of bacterial keratitis?
Corneal perforation, esp if:
N gonorrhoea
Corynebacterium diptheriae
H influenzae
How are fungi stained?
sabouraud agar and gram/giemsa stain
How are acanthamoeba stained?
non-nutrient agar with e coli
How is bacterial keratitis investigated?
corneal scrapings for microbio
how is bacterial keratitis managed?
topical broad spec ABx BEFORE lab results return
usually fluoroquinolones (ofloxacin) are typically used
Which patients is fungal keratitis common in?
AIDS
diabetics
immunocompromised
what fungus is more common in those with ocular trauma, esp with plant / tree branch?
filamentous
aspergillum and fusarium
What are specific signs in candidda infections?
small ulcer with an expanding infiltrate in a collar stud formation
What are specific signs in filamentous keratitis?
feathery branching-like infiltrate pattern
what investigations can aid in fungal keratitis?
confocal microscopy
how is fungal keratitis managed?
natamycin drops for proven filamentous infection
variconazole or amphotericin B drops for proven candida
What is a risk factor for acanthamoeba keratitis?
improper lens hygiene (showering / swimming in contact lenses)
What are the features of acanthamoeba keratitis?
pain out of proportion with clinical signs,
photophobia
blurred vision
later disease: perineural infiltrates and ring shaped strmal infiltrates can be seen
what are the investigations for acanthamoeba?
corneal scraping and / or confocal microscopy to identify the presence of amoebic cysts
how is acanthamoeba keratitis treated?
topical polyhexamethylene biguanide or
chlorhexidine
What additional condition does HSV corneal infection lead to?
blepharoconjunctivitis
where does HSV remain after infection?
in the trigeminal ganglion
Which layers of the cornea can keratitis affect?
epithelial, stromal or endothelial layers of the cornea
Describe the presentation of patients with reactivated HSV keratitis?
pain
dVA
lacrimation
foreign body sensation
What are the signs of HSV keratitis?
superficial punctate keratitis which causes a stellate (star-shaped) erosion which later becomes a dendritic ulcer - seen with fluoroscein
reduced corneal sensation
What is disciform keratitis?
this usually occurs from HSV antigen hypersensitivity, not reactivation
insidious onset