• Assessment of intraocular pressure (IOP) is an essential diagnostic procedure for a thorough ophthalmic examination.
• It is particularly useful to distinguish between cases of uveitis and glaucoma (low or high pressure):
  • Glaucoma is a rare disease in cattle, but congenital, hereditary and secondary glaucoma have been described.
  • Glaucoma can develop following an uncontrolled uveitis.
• Types of tonometry:
  • Digital tonometry (index fingers).
  • Applanation tonometry (Tono-Pen AVIA, Tono-Pen Vet, Tono-Pen XL, Mackay-Marg tonometer, Perkins handheld tonometer.
  • Rebound tonometry (TonoVet).

Uses

• Assessment of intraocular pressure as part of examination of the eye Eye: examination - overview.

Advantages

• Digital Tonometry assists estimation of IOP and may help to differentiate between obviously soft or hard eyes.
• Instrumental tonometers are atraumatic, portable and provide accurate and repeatable estimates of intraocular pressures by exerting minimal restraint of the patient if possible.
• Treatment success can be evaluated in follow up examinations.
• Rebound and applanation tonometers have demonstrated reliability in normal bovine eyes.
• Applanation tonometers can be used in any orientation.
• Rebound tonometers do not require topical anesthesia.

Disadvantages

• All methods can be affected by blepharospasm, extraocular muscle rigidity, eyelid swelling, eyelid trauma Eyelid surgery: third eyelid flap, retrobulbar disease or enophthalmos.
• Digital tonometry (using fingers) is subjective.
• Electronic tonometers are expensive.
• Applanation tonometers require training to be used correctly and topical anesthesia is necessary.
• Rebound tonometers require the eye be directed forward to allow positioning of the tonometer parallel to the ground.
• Animals not used to be handled around the face may shy away. Any tension on the neck or a tilted head position, as well as forcibly opening eyelids can falsely increase electronic IOP readings.
• The effect of corneal surface disease on the accuracy of applanation and rebound tonometry has been determined in small animals and suggests to perform repeat measurements with the same type of tonometer.
• The position of the eye in cattle precludes using the Schiotz tonometer.

Time required

Preparation

• Topical anesthesia is required for applanation tonometry: 1-5 min for complete desensitization Local anesthesia: overview.
• Topical Tetracain can be used for local anesthesia.

• Rebound tonometers do not require administration of topical anesthesia.
• Local eyelid blocks (auriculopalpebral Auriculopalpebral nerve block) to prohibit eyelid closure may make measurements more easily obtainable and more reliable.
• Sedation may be required Sedation: overview.

Procedure

• <5 min per eye.

Decision taking

Criteria for choosing test

• Examination of the eye, especially in conditions where changes in intraocular pressure (IOP) are likely.