A pachymeter is a device for measuring the thickness of the front of the eye. This instrument uses an ultrasonic probe to take 25 measurements in less than one second and a small inbuilt computer to collate the results giving an instant reading of thickness to within 5 microns ( I micron is one thousandth of a millimetre).
The average central corneal thickness is 545 microns and most tonometers used to measure pressure in the eye (intra ocular pressure) are calibrated on this basis. This results in an under estimation of pressure in someone with a thin cornea; and over estimation in someone with a thick cornea. A good analogy to this is if a bicycle tyre were pumped up to 25 psi then it would feel fairly soft but a tractor tyre with the same pressure would feel much harder due to the walls of the tyre being thicker.
Why does this matter? In patients who have suspicious changes in appearance to their optic nerve or visual field but apparently normal intra ocular pressure, the optometrist may wrongly assume they have a lower risk of progressing to glaucoma and hence observe them less frequently. Conversely in patients where the measured pressure is high and who may just have a thicker than average cornea, the practitioner may be over cautious causing unnecessary inconvenience and anxiety to the patient.
The use of a pachymeter therefore gives us a ‘true’ indication of intra ocular pressure by taking into account the patient’s corneal thickness.
Observation of the appearance of the optic nerve still remains the most important factor in detecting glaucoma, but pachymetry adds to the range of diagnostic tools at our disposal in further assessing the likelihood of changes in the appearance of the optic nerve being due to the patient developing glaucoma.
Test Chart 2000
This is a computerised version of the old light box test chart which everyone associates with an ‘eye-test’; but it allows far more flexibility in the way it is used.
We are able to present pictures, check the eyes are working together and even randomise the presentation of letters – so no more learning the bottom line!
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Digital Clinical Imaging
We now have cameras which allow us to photograph both the inside and outside of the eye. This helps us to monitor any changes in the appearance of the eye and its structures, which may give us an early indication of any disease process. Images are useful when seeking second opinions and referring patients as well as proving invaluable as a teaching tool to educate both patients and ourselves.
Cornea Topographer
This instrument produces a 3D image or colour contour map of the shape of the front of the eye. This is particularly useful when fitting more complex contact lenses such as bifocal or toric gas permeable lenses or ortho-k. Orthokeratology relies on reshaping the cornea to correct vision, so being able to monitor tiny changes in shape is essential. The shape of the cornea is also altered in some eye diseases such as keratoconus and this technique can assist in early diagnosis.
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The corneal topographer |
The normal corneal contour |
The cornea in keratoconus |
Humphrey Visual Field Analyser
This is the gold standard field analyser used in most hospitals and research establishments around the world. It allows a variety of testing strategies enabling early detection of visual field defects. This is also one of the instruments used in the detection of glaucoma. The DVLA require the testing of drivers visual fields when diseases such as strokes, diabetes or glaucoma may impair their vision. A binocular field testing programme called Esterman fields is used for this purpose.
Images captured with our digital equipment
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