Glaucoma - Anterior Chamber Anatomy

Question 1

Which methods are used to assess anterior chamber angle?

Answer 1

Assess anterior chamber angle via: Van Herick’s, Smith’s technique (useful if opacities e.g. scarring or arcus senilus at angle and don’t have access to gonio lens) or Gonioscopy (gold standard, used in HES to classify all glaucoma, closed angle & OHT pxs)
Production & drainage of aqueous are interlinked – problems occur when there is a differential between these two. Gonioscopy determines if aqueous can be adequately drained

Question 2

What are the investigations of the anterior chamber angle?

Answer 2

• Van Herick’s or Gonioscopy acceptable when referring
  o Refer irrespective of other signs refer if Van Herrick’s technique grade 2 (but refer to local guidelines) as patient is at significant risk of developing acute closed angle glaucoma
  o Using Gonioscopy, if ≥270 degrees of posterior pigmented trabecular meshwork is not visible.
   If majority of that pigmented trab meshwork is not visible then aqueous not able to be drained via that route – px at risk of angle closure
  o A narrow angle does not mean the patient definitely has CAG

Question 3

Why is it important to view the angle?

Answer 3

• To determine if angle is open or closed
• To determine which structures are visible in the angle
  o Hence how adequately aqueous is being drained?
  o Look for posterior pigmented trab meshwork as that is where aqueous is drained
• To determine if angle structures are normal/abnormal
  o Any neovascular vessels? Too much pigmentation?
• To grade angle in order to classify glaucoma patients
• To find signs which may be suggestive of a risk factor for the development of secondary glaucoma?

Question 4

What are the errors with Van Herick’s method?

Answer 4

• Potential errors in Van Herick’s Grading due to:
  o Room illumination not reduced
  o Px fixation is not stabilised in primary position (distance) – get them to look at your ear
  o Magnification too low – if mag not high enough then cannot grade angle
  o Wide optic section (Reduced brightness of slit lamp)
  o Illumination angle not 60o
  o Reading not taken at limbus – take it from first point where it splits
  o Peripheral corneal lesions such as pterygium or arcus senilus – can overestimate depth of angle in these cases
• 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

Question 5

Describe Smith’s Technique to view anterior chamber angle?

Answer 5

• 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

Question 6

How much of angle structure can be seen on slit lamp?

Answer 6

• Cannot see the angle using a slit lamp:
  o Difficulty in viewing the iridocorneal angle is due to the critical angle of refraction at corneal/air interface
  o Goniolens replaces eye/air interface & critical angle is eliminated by steeply curved outer surface of the lens

Question 7

What is the normal angle structure?

Answer 7

• I - Iris
• Can – Ciliary Body (tends to be pigmented, level of pigmentation varies between pxs)
• See – Scleral Spur (white tissue)
• The – Trabecular Meshwork (1st pigmented trab meshwork, then non-pigmented trab meshwork)
• Line – Schwalbe’s Line (most anterior structure in angle)
• Picture: inferior angle most open and most pigmented

Question 8

What are iris processes?

Answer 8

• Small, usually tenuous extensions of anterior iris surface that insert at level of scleral spur & cover ciliary body to a varying extent – they are lacy in appearance
• Present in around a third of normal individuals, most prominent in brown eyes and in children – present in 20-35% of normal individuals
• Not to be confused with Peripheral anterior synechaie which can insert more anteriorly and are more substantial/broader (won’t be lacy structure

Question 9

Describe peripheral anterior synechiae (vs iris processes)?

Answer 9

• Can occur in uveitic glaucoma
• When inflammation present in iris tissue, it becomes sticky and adheres to other structures
• Much broader, not lacy

Question 10

Describe ciliary body in angle?

Answer 10

• Sits between the peripheral iris and the scleral spur
• Can be pink, brown or slate grey – can depend on iris colour but not always case
• Its 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
  o Not visible in some eyes due to physiological anterior iris insertion
  o Plenty of wide-open angles may only be able to see down to scleral spur
• May be irregular in appearance – means angle is slightly wider open at points than others
• Pic: relatively heavily pigmented anterior chamber angle
  o Dark iris tissue at bottom of image
  o Then go up - change in texture – band beyond peripheral iris is the dark band of ciliary body base

Question 11

Describe scleral spur in angle?

Answer 11

• Scleral spur is most anterior projection of the sclera and the site of attachment of the longitudinal muscle of the ciliary body (pulls on scleral spur when muscle is activated & can open trab meshwork more widely)
• On gonioscopy it can be seen posterior to pigmented trabecular meshwork and anterior to ciliary body base
• Appears as a narrow white band
• Pic: just above dark band of ciliary body can see narrow white band of scleral spur then above that is another pigmented band

Question 12

Describe trabecular meshwork in angle?

Answer 12

• Most important place where majority of aqueous leaves the eye – minority of aqueous will leave through uvealscleral route at ciliary body face
• Sits between scleral spur & Schwalbe’s line
• 90% of aqueous leaves via trabecular meshwork
• Flow is pressure dependent
  o Flow increases as IOP increases
  o For aqueous to exit eye by this route, IOP must be higher than episcleral venous pressure
  o At pressures below episcleral venous pressure (8to15mmHg), all aqueous outflow must be via non-conventional routes
• Anatomy:
  o Uveal layers of trab meshwork: large holes which easily allow aqueous to flow from anterior chamber into meshwork
  o Corneal scleral meshwork: holes are slightly smaller and more densely packed
  o Juxtacanilicular tissue: forms barrier at Schlemm’s canal – most of the barrier to aqueous outflow comes from this tissue
• Anterior portion bordering Schwalbe’s Line is non-pigmented & non-functional
• Posterior, functional portion borders scleral spur, level of pigmentation in this portion varies from pale grey to dark brown – important portion – this allows drainage of aqueous
  Angles vary enormously in terms of pigmentation

Question 13

Describe Schlemm’s Canal in angle?

Answer 13

• Generally not visible in most eyes
• Positioned at base of scleral sulcus, most often not visible during gonioscopy
• Not a rigid structure, therefore at high IOP the canal collapses & resistant to aqueous outflow increases
  o Pressure dependent system
  o Won’t remove any aqueous if pressure is less than that in the episcleral venous system
  o Longitudinal muscle of ciliary muscle can open Schlemm’s canal by pulling on scleral spur
   This is the method through which cholinergic drugs can decrease the resistance to outflow via this route

Question 14

Describe Schwalbe’s Line in angle?

Answer 14

• Boundary (change in curvature) between trab meshwork & corneal endothelium
  o Between peripheral corneal tissue and anterior chamber angle
• Can be some pigment settling in this area due to steeper curvature than scleral sulcus (peripheral anterior chamber angle)
  o Pic: most anterior very fine line you can see