Glaucoma Flashcards
What is glaucoma
Glaucoma is caused by a disorder in the normal flow of aqueous humour. It is important to understand aqueous humour in detail.
Revise
Physiology of glaucoma
Aqueous humour supplies nutrients to the cornea and lens
- It flows from the posterior chamber to the anterior chamber via the pupil.
- It fills the anterior chamber at 0.25mL (larger than the posterior chamber)
- It is composed of 99% water and has a lower glucose and protein concentration than plasma.
Production and secretion of aqueous
Aqueous humour is formed by the non-pigmented inner epithelium of the ciliary processes at the pars plicata
The inner non-pigmented epithelium is continuous with the neural retina
Most secretion occurs by active transport which is under sympathetic control:
- Stimulation of beta-2 receptors increase secretion
- Stimulation of alpha-2 receptors decreases secretion
What structure is continous with the RPE
The ciliary body also has an outer pigmented epithelium that is posteriorly continuous with RPE
Drainage of AQH
AQH travels from the posterior chamber to the anterior chamber via the pupil (the hole in the iris centre). There are 2 main pathways for AQH to drain from the eye:
- Trabecular path (90%)
- Uveoscleral path
Compare Trabecular vs Uveoscleral drainage of AQH
Trabecular path (90%)
- Also known as the conventional route
- AQH flows through the trabecular meshwork → Schlemm’s canal → episcleral veins
- Contraction of ciliary muscle increases trabecular outflow
Uveoscleral path
- AQH passes directly outward through ciliary muscle → suprachoroidal space → choroidal veins → vortex veins
What is the Trabecular Meshwork (TM)
The TM is a band of cellular sheets which form a sieve-like structure. It is triangular in cross-section and is found at the anterior chamber angle
What are the 3 parts of the TM
- Innermost → uveal meshwork with large holes
- Middle → corneoscleral meshwork with smaller holes
- Outermost → juxtacanalicular meshwork connects TM to Schlemm canal
What area confers the largest resistance to AQH flow
The juxtacanalicular meshwork contains the narrowest spaces and confers the largest resistance to normal AQH outflow via the conventional route
Scleral Spur
A fibrous projection from the sclera.
- Longitudinal ciliary muscles insert onto its posterior surface.
- It is an important landmark in gonioscopy.
Anterior Chamber (AC) Angle
Extends from Schwalbe’s line (termination of Descemet’s membrane of the cornea) to the TM, scleral spur and ciliary body.
Clinically significant because the closure of the angle will impede AQH outflow via the conventional route.
What is the glaucoma TRIAD
Diagnosis is based on the ‘glaucoma triad’:
Raised IOP, abnormal disc, and visual field defects.
The latter 2 signs are most important.
Gold standard way to measure IOP
Goldmann tonometry
IOP normal values and variations
- Normal range is 11-21 mmHg
- Diurnal variation of up to 5 mmHg is normal.
- Peaks in the morning
- Supine positioning leads to higher IOP than sitting
- IOP measurement is affected by corneal thickness, astigmatism and the axial length of the eyeball.
Is raised IOP required to diagnose glaucoma?
NO!
Optic Disc Changes in glaucoma
What finding may raise suspicion of optic nerve damage?
Decreased nerve fibre layer (NFL) thickness readings on OCT
What investigation is used to measure Visual Fields?
Perimetry
The best test for monitoring glaucoma is Humphrey 24-2 perimetry
Why do visual field defects occur in glaucoma?
Glaucomatous visual field defects occur as a result of damage to the retinal nerve fibres
Gonioscopy
Gonioscopy is a method of visualizing the anterior chamber angle, it is important in identifying angle-closure glaucoma.
Peripheral anterior synechiae might be found in angle closure, this is the iris bowing forward and closing off the angle.
Lable the structures seen on gonioscopy
- Schwalbe’s line, 2. Trabecular meshwork (TM), 3. Scleral spur, 4. Ciliary body, 5. Iris.
List all structures seen on gonioscopy (superficial to deep)
- Posterior corneal surface
- Schwalbe’s line
- Non-pigmented trabecular meshwork
- Pigmented trabecular meshwork
- Scleral spur
- Ciliary body
- Iris
- Pupil
How much can medical treatment lower IOP in glaucoma
Medical treatment can on average lower IOP by 20-30% (5-7mm Hg)
Drugs used in glaucoma (in order of use)
- Latanoprost
- Timolol
- Brinzolamide
- Brimonidine
Exceptions to glaucoma drugs in pregnancy/peadiatrics
No eye drops are licensed for children or pregnant women. Use brimonidine in first the trimester and switch to timolol in 3rd
Biggest issue with prescribing mutiple eye drops?
Compliance