INTRAOCULAR PRESSURE (IOP)

     John Grigg
     Senior Lecturer
     Department of Ophthalmology, University of Sydney
     johng@eye.usyd.edu.au
 
Intraocular Pressure - What is it?
 
  • The vertebrate eye is a fluid filled spheroid having a flexible and partially elastic wall
  • maintenance of a stable shape is necessary for optical performance
  • IOP = tissue pressure of intraocular contents
  • IOP is maintained in a narrow range by aqueous production matching outflow
 
Diagnosis - Intraocular Pressure
 

 
IOP measurement
 

 

 

  
  • Applanation tonometry is gold standard
 
IOP measurement
 

 

 

  
  • Schiotz indentation tonometry - portable not as accurate  

  • Tonopen - pressure transducer  
The Blue Mountains Eye Study - Distribution of IOP
 

 
Why 21 is considered normal
 

 
Why is 21 considered "normal"?
 
  • 21 mmHg used as screening pressure not "abnormal" but damage much greater at pressures higher than 21

  • chosen because it made search for glaucoma damage more efficient
 
Role of IOP
 
  • Ocular hypertension
    - high IOP associated with damage

  • "Normal tension" glaucoma
    - "normal" IOP associated with damage
 
Does IOP Cause Damage?
 
  • Damage
    - common with increased IOP
    - usually in eye with increased IOP
  • asymmetrical IOP
    - eg trauma
  • truly "low" IOP rare with glaucomatous damage
 
The Blue Mountains Eye Study: Prevalence of glaucoma at each IOP level
 

 
IOP and Optic Nerve Damage
 
  • Dose response relationship
    - IOP 16-19
       • risk > 2 times
    - IOP > 23
       • risk > 10 times
 
Baltimore Eye Study - Prevalence of POAG in relation to screening IOP
 

 
Why Damage with "Low" IOP ?
 

  • In population IOP < 22 mmHg is More common than IOP > 22 mmHg
  • risk of glaucoma with IOP <22 is approx 1/6 of patients with IOP >22 (2.8 compared to 12.8 see Baltimore Eye study table)
  • however, 20 times more people have IOP <22 therefore, adjusted risk of glaucoma is 1:1
  • 50% with glaucomatous damage have "normal" IOP
 
IOP and Optic Nerve Damage
 
  • IOP
    - Is associated with nerve damage
    - Is a risk factor for nerve damage

  • IOP is a causal risk factor
 
Glaucoma and Myopia
 
  • Myopia is associated with POAG

  • Myopia is a risk factor for POAG

  • It is not a causal risk factor
 
Normal Aqueous Flow
 

 
Anterior Chamber Angle Anatomy
 
  • Normal aqueous pathway

  • Presumed alteration in flow in open angle glaucoma
 

  

 

 
Anterior Chamber Angle Anatomy
 

    Diagrams from "Glaucoma - A Colour Manual of Diagnosis and Treatment" by Jack J. Kanski, James James A. McAllister and John F. Salmon. Butterworth-Heinemann Publishers, a division of Reed Educational & Professional Publishing Ltd

 
Identification of angle structures
 
  • Ciliary body band:
    The most posterior structure appears pink to dull-brown to slate-grey in colour. Its width depends on the position of the iris insertion; tending to be narrower in hypermetropic eyes and wider in myopic eyes. The angle recess represents the dipping of the iris as it inserts into the ciliary body.

  • Scleral spur:
    This is the most anterior projection of the sclera and the site of attachment of the longitudinal muscle of the ciliary body. Gonioscopically, the scleral spur is situated just posterior to the trabeculum and appears as a narrow dense, often shiny, whitish band. It is the most important land-mark because it has a relatively consistent appearance in different eyes.

  • Trabecular meshwork:
    This extends from the scleral spur to Schwalbe's line. The posterior functional pigmented part lies adjacent to the scleral spur and has a greyish-blue translucent appearance. The anterior non-function part lies adjacent to Schwalbe's line and has a whitish colour. Trabecular pigmentation is rare prior to puberty.

  • Schwalbe's line:
    This is the most anterior structure and appears as an opaque line. Anatomically it represents the peripheral termination of Descemet's membrane and the anterior limit of the trabeculum.

 
Shaffer grading system
 

  • The examiner grades the angle according to the visibility of the various angle structures.
  • Shaffer grading system provides a method of comparing the widths of different chamber angles.
  • System assigns a numerical grade to each angle with associated anatomical description, the angle width in degrees and implied clinical interpretation.
 
Shaffer grading system
 
  • Grade 4 (35 - 45 degrees)
    - widest angle characteristic of myopia and aphakia in which the ciliary body can be visualised with ease.
  • Grade 3 (25 - 35 degrees)
    - an open angle in which at least the scleral spur can be identified.
  • Grade 2 (20 degrees)
    - moderately narrow angle in which only the trabeculum can be identified;
    - angle closure is possible but unlikely.
  • Grade 1 (10 degrees)
    - very narrow angle in which only Schwalbe's line and perhaps also the top of the trabeculum can be identified;
    - angle closure is not inevitable but the risk is high.
  • Grade 0 (0 degrees)
    - a slit angle in which no angle structures can be identified;
    - angle has the greatest danger of closure.
 

  

 

 
Shaffer grading system
 

  • An "occludable" angle is considered to be an angle in which the pigmented trabecular meshwork is not visible without indentation or manipulation in three quadrants of the circumference.  

     

 
Angle closure glaucoma