Tonometry
Tonometry is the measurement of the pressure inside the eye (intraocular pressure). It is the main test for glaucoma, and it is the only risk factor for glaucoma that can be treated. Pressure is determined by the balance between the production of aqueous humour by the ciliary body and its outflow through the trabecular meshwork and the uveoscleral route.
The most used method is applanation tonometry, which measures the force needed to flatten a fixed area of the cornea (3.06 mm in diameter). It is done at the slit lamp, after a drop of topical anaesthetic and fluorescein, and a prism is brought gently into contact with the cornea. The examiner turns a dial until the inner edges of two fluorescein semicircles meet.
Other methods are non-contact (air puff) tonometry, which measures the time taken for a puff of air to flatten the cornea, rebound tonometry, which measures the movement of a light probe that touches the cornea and requires no anaesthetic, handheld applanation devices, dynamic contour tonometry, and indentation tonometry. Palpation of the eye through the lid gives a rough estimate only.
The test takes 1 to 2 minutes. Normal pressure is 10 to 21 mmHg, with a mean of about 15 to 16 mmHg. About 1 in 3 people with glaucoma have pressure in the normal range at diagnosis (normal tension glaucoma), and about 4 to 10% of people with a pressure over 21 mmHg have glaucoma damage. Each 1 mmHg reduction in pressure lowers the risk of progression of glaucoma by about 10%.
Indications.
- Screening for glaucoma in adults, particularly with a family history, African or Asian ancestry, over 40 years, diabetes, myopia, or long term steroid use.
- Diagnosis and monitoring of glaucoma and ocular hypertension, and response to treatment.
- Acute red eye, pain, headache with halos and nausea (acute angle closure), and uveitis (high or low pressure).
- Eye trauma, hyphaema, and after eye surgery (cataract, glaucoma, vitreoretinal), and after laser or injection treatment.
- Patients on corticosteroids by any route, and those with steroid response.
- Suspicious optic discs or visual field defects, and pre-operative assessment.
- Thyroid eye disease and other causes of raised venous pressure.
Not indicated by contact methods when there is an open globe injury, or active corneal infection (non-contact methods or palpation may be used).
Contraindications and precautions.
- Suspected or known open globe injury: no contact with the eye, and no pressure.
- Active corneal infection, ulcer, or marked epithelial defect: contact methods are avoided because of spread of infection and damage.
- Adenoviral conjunctivitis: strict hygiene and disinfection of the prism, or disposable prisms.
- Allergy to topical anaesthetic or fluorescein.
- Severe corneal oedema, scars or irregular astigmatism reduce accuracy.
- Patients who cannot sit, or who cannot keep still: rebound or handheld devices.
- Contact lenses are removed before applanation tonometry, since fluorescein stains soft lenses.
Preparation and technique.
The patient is seated at the slit lamp, with the head against the rest, and is asked to look straight ahead and keep both eyes open. A drop of local anaesthetic with fluorescein is placed in the lower fornix, and the cobalt blue light is used. The prism is advanced slowly until it touches the central cornea, and the dial is turned until the inner edges of the two semicircles just touch.
The reading is taken at the point of the pulse of the eye, and is read directly in mmHg from the dial. The test is done for each eye, repeated if the pattern is wrong, and the time of day is recorded since pressure varies by 3 to 6 mmHg over 24 hours, with the highest values often in the early morning. Rebound and non-contact readings are taken in three tests and averaged.
