Week 10 - Gonioscopy Flashcards
What does the aqueous humour do?
• The aqueous humor supplies nutrition and removes waste from the clear structures in the anterior eye - the cornea and the lens
• The balance between aqueous production and outflow determines the intraocular pressure
The ciliary body contains:
• Longitudinal muscles
- pull on the scleral spur
- open the trabecular meshwork
• Circular muscles
- accommodate the lens
• Forms part of the blood-aqueous barrier
• Forms hyaluronate for the vitreous
• Ciliary epithelium
- makes aqueous
• Circular epithelium
- makes aqueous
• In the ciliary processes
- There are ~80
Aqueous Humour:
• Made from plasma
• In the non-pigmented epithelium of the ciliary body
Made by:
1. Ciliary body stroma
- Highly vascular
2. Pigmented epithelium
3. Non-pigmented epithelium
4. Aqueous humour
Aqueous Humour production:
• Active transport (secretion)
• Ultrafiltration
• Diffusion
Aq humour production: Active transport:
• Active transport (secretion)
- energy expended
- sodium driven into the posterior chamber, water follows
- responsible for moving ascorbate and other large charged molecules
Aq humour production: Ultrafiltration
• Ultrafiltration
- passive
- via micropores in cell membrane
- movement along gradients
- pressure dependent
Aq humour production: Diffusion
• Diffusion
- passive
- lipid soluble materials
- driven by concentration gradients
Aqueous humour vs plasma
• Aqueous has virtually no protein
- Protein in the aqueous is seen as flare
• Aqueous has 10 - 50 x the levels of ascorbate
• Other differences are minor
Aqueous Outflow:
• Primarily through the trabecular meshwork into the episcleral venous system
- Pressure dependent
•Also through the ciliary body face and iris root into the suprachoroidal space
- Pressure independent
How can you assess the anterior chamber angle?
• Van Herick’s
• Smith’s Technique
• Gonioscopy
Van hericks or gonioscopy acceptable when referring:
• Refer irrespective of other signs refer if Van Herrick’s technique grade 2 as patient is at significant risk of developing acute closed angle glaucoma
• Using Gonioscopy, if ≥270 degrees of posterior pigmented trabecular meshwork is not visible.
Why is it important to do gonioscopy when viewing the angle?
• To determine if the angle is open or closed
• To determine which structures are visible in the angle
• Hence how adequately aqueous is being drained?
• To determine if angle structures are normal/abnormal
• To grade the angle in order to classify glaucoma patients
• To find signs which may be suggestive of a risk factor for the development of secondary glaucoma?
Why cant just Van Hericks be used?
Potential errors in Van Herick’s Grading due to:
• Room illumination not reduced
• Fixation in not stabilised in primary position (distance)
• Magnification too low
• Wide optic section (Reduced brightness of slit lamp)
• Illumination angle not 60°
• Reading not taken at limbus
• Peripheral corneal lesions such as ptergium or arcus senilus
• And it only tells you if the angle is open or closed, it doesn’t tell you anything about the appearance of the angle structures
Smiths technique
• Used when no gonioscopic lens
• Assesses anterior chamber depth by assessing the depth of the central anterior chamber
• Therefore useful in cases where there is an obscuration/opacity in the peripheral cornea making Van Herick’s difficult
• NOT listed as an acceptable way to refer patients under the SIGN glaucoma guideline
Why cant you see the angle using a slit lamp?
• The difficulty in viewing the iridocorneal angle is due to the critical angle of refraction at the corneal / air interface.
• The goniolens replaces the eye / air interface and the critical angle is eliminated by the steeply curved outer surface of the lens.
Normal Angle Structure:
• I - Iris
• Can - Ciliary Body
• See - Scleral Spur
• The - Trabecular Meshwork
• Line - Schwalbe’s Line
Iris processes:
• Small, usually tenuous extensions of the anterior iris surface that insert at the level of the scleral spur and cover the ciliary body to a varying extent
• Present in around a third of normal individuals, most prominent in brown eyes and in children
• Not to be confused with Peripheral anterior synechaie which can insert more anteriorly and are more substantial/broader
Ciliary Body
• Sits between the peripheral iris and the scleral sour
• Can be pink, brown or slightly grey
• It’s width depends on the position of iris insertion and tends to be narrower in hyperopes than myopes
• The angle recess represents the posterior dipping of the iris as it inserts into the ciliary body
• It may not be visible in all eyes
• May be irregular in appearance
Scleral Spur
• The scleral spur is the most anterior projection of the sclera and the site of attachment of the longitudinal muscle of the ciliary body
• On gonioscopy it can be seen posterior to the pigmented trabecular meshwork and anterior to the ciliary body base
• Appears as a narrow white band
The Trabecular Meshwork
• Sits between the scleral spur and Shwalbe’s line
• 90% of aqueous leaves via the trabecular meshwork
• Flow is pressure dependent
• The anterior portion bordering Schwalbes line is non pigmented and non-functional
• The posterior, functional portion borders the scleral spur, the level of pigmentation in this portion varies from pale to dark brown
Schlemm’s canal
• Positioned at the base of the scleral sulcus, most often not visible during gonioscopy
• Not a rigid structure, therefore at high intra ocular pressure the canal collapses and resistance to aqueous outflow increases
Schwalbe’s line
• Boundary between the trabecular meshwork and the corneal endothelium
• Can be some pigment settling in this area due to steeper curvatu than scleral sulcus
Principle of Gonioscopy:
• To view the angle during gonioscopy we need to overcome total internal reflection
• To methods
- Direct visualisation of the angle
- Indirect visualisation of the angle
Direct Gonioscopy: Advantages:
Direct lenses
• Patients lie supine
• Give a direct stereoscopic, panoramic view of the angle
Advantages
• Good magnification (1.5X)
Easy orientation for the observer
• Possible to simulaneously compare both eyes
• For high magnification need an illuminated loupe or a slit lamp
• Can be used in bedbound patients
• Very little corneal distortion
• Wide field of view for teaching