terminology and guidelines for glaucoma ii - Kwaliteitskoepel

1.1 - INTRAOCULAR PRESSURE
Normal value of intraocular pressure (IOP)
The ‘normal’ IOP is a statistical description of the range of IOP in the population, and is not applicable to the individual
subject. There is some evidence that IOP increases by about 1 mm Hg with each decade after 40 years of age in
most Western populations, although this does not appear to occur in all populations. The IOP follows a circadian cycle
often with a maximum between 8 a.m. and 11 a.m. and a minimum between midnight and 2 a.m. This cycle is more
dependent on the sleep cycle than the daylight cycle. The diurnal variation can be between 3 and 5 mm Hgand is wider
in untreated glaucoma 1-5 .
Anesthetic effects on the IOP measurement.
The IOP measurement by applanation necessitates topical anaesthesia of the cornea, which does not affect the pressure.
However, in young children, topical anaesthesia is not sufficient and a general anaesthetic has to be given. The most used
substances are halothane (inhaled), ketamine (intra-muscular) and chloral hydrate (oral). In general, halothane lowers
the IOP, whereas ketamine can cause a transient rise in IOP. Under ketamine the IOP is usually about 4 mm Hg higher
than under halothane. Oxygen given during the anaesthesia has a hypotensive effect and carbon-dioxide a hypertensive
effect. Succinylcholine can produce a transitory IOP increase of about 15 mm Hg. Nitrous oxide causes a slight increase
in IOP 6-9 .
Normal IOP in children.
The IOP increases by about 1 mm Hg per 2 years between birth and the age of 12 years, rising from 6 to 8 mm Hg
at birth to 12 ± 3 mm Hg at age 12.
Normal IOP in adults and elderly.
Normal IOP values are often based on population samples that may not be fully representative in their age distribution,
as in the Framingham study where the mean age was 65 years and the mean IOP 16.5 mm Hg.
Cornea
Corneal characteristics that can affect the IOP measurements are corneal thickness, curvature and hydration 2,10-20 . The
condition of the cornea should be considered both cross sectionally when comparing individuals or groups, and longitudinally
when evaluating any patient. See next page.
Other artifacts
A tight collar or tie, Valsalva’s manovrer, holding breath, a lid speculum or squeezing the lids can all falsely increase
the IOP reading 21 .
Tonometry
The principle of the method of tonometry is based on the relationship between the intraocular pressure and the force
necessary to deform the natural shape of the cornea by a given amount. The deformation can be achieved by indentation,
as with the Schiøtz tonometer, or by applanation, as with the Maklakoff and the Goldmann tonometers 2 .
Although the pressure measured is external to the eye, the term used is “intraocular pressure” 7 .
Method of measurement
The most frequently used instrument is the Goldmann applanation tonometer, mounted at the slit lamp. The method
involves illumination of the biprism tonometer head with a blue light obtained using a cobalt filter and applanation of
the cornea after applying topical anaesthesia and fluorescein in the tear film. The scaled knob on the side of the instrument
is then turned until the hemicircle of fluorescent tear meniscus visualized thorough each prism just overlap.
Goldmann’s original equation is based on the Imbert- Fick law and assumed that: the cornea had a constant radius of
curvture, the rigidity was the same in all eyes, the globe was spherical, aqueous would not mave away from the AC
during measurement. This adds to the expected inter and intra observer variability 20 .
Ch. 1 - 3 EGS