investigations

Last reviewed 06/2022

The angle formed by the iris and cornea is normally hidden from direct observation by the overlapping of the sclera at the limbus. The angle may be viewed microscopically once the refraction of the cornea is cancelled by applying a dome-shaped contact lens - for a hand-held microscope - or a Goldmann lens - for a slit-lamp microscope.

In an open angle, the end of Descemet's membrane in the cornea appears as a pale line marking the anterior insertion of the trabeculae. In a narrow angle, only Schwalbe's line and a glimpse of the trabeculae are visible. In a wide angle, the root of the ciliary body may be seen.

Many techniques enable measurement of intra-ocular pressure. The three most popular ones are:

• Goldmann applanation tonometer - requires topical anaesthesia and fluorescein but has a high level of accuracy. The force required to flatten a small area of cornea is measured by a prism fitted to a tonometer.
• "air-puff" tonometer - the cornea is flattened using a small puff of air. The procedure does not require anaesthesia but is not very accurate.
• Schiotz tonometer - the weight required to indent the cornea is measured and converted to a pressure value. It requires topical anaesthesia but is portable.

NICE suggest that with respect to the diagnosis of chronic open angle glaucoma (COAG):

• at diagnosis offer all people who have COAG, who are suspected of having COAG or who have OHT all of the following tests:
  • visual field assessment using standard automated perimetry (central thresholding test), repeated if necessary to establish severity at diagnosis
  • optic nerve assessment and fundus examination using stereoscopic slit lamp biomicroscopy, with pupil dilatation
  • IOP measurement using Goldmann applanation tonometry (slit lamp mounted)
  • peripheral anterior chamber configuration and depth assessments using gonioscopy
  • central corneal thickness (CCT) measurement

Notes:

• intraocular pressure (IOP) measurement using Goldmann applanation tonometry (slit lamp mounted)
  • actual value > 21mmHg
    • artefactually high IOP levels are seen in greater central corneal thickness
    • artefactually low IOP levels are seen in lesser central corneal thickness
  • diurnal variation in IOP is greater than 4 mmHg (2)

Reference:

• (1) NICE (September 2017). Glaucoma: diagnosis and management of chronic open angle glaucoma and ocular hypertension
• (2) The College of Optometrists 2009. Glaucoma (primary open angle) (POAG).