Tonometry is used for

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Here is a complete answer based on authoritative textbook sources:

Tonometry

Tonometry is used to measure intraocular pressure (IOP) - the fluid pressure inside the eye. It is the primary clinical method for detecting and monitoring glaucoma.

Primary Purpose

Tonometry detects increased intraocular pressure, which is the main risk factor for glaucoma - a group of conditions that cause optic nerve damage and progressive visual field loss. Glaucoma is the third most common cause of blindness, and most blindness from it is preventable if detected early.

Normal IOP: approximately 10-21 mm Hg. An IOP >22 mm Hg is suspicious; >30 mm Hg carries significant glaucoma risk.

Indications for Tonometry

1. Glaucoma Screening - in high-risk individuals:

Age >45 years (>35 years in African Americans)
Family history of glaucoma (4-9x increased risk)
African American or Hispanic (age >60) ethnicity
Diabetes mellitus
Hypertension
Myopia or hyperopia (decreased visual acuity)
Central corneal thickness <0.5 mm
Prior use of corticosteroids, mydriatics, phenothiazines, or sympathomimetics

2. Diagnostic Workup:

Ocular pain (especially with signs of angle-closure glaucoma: red eye, fixed mid-dilated pupil, halos, nausea/vomiting, frontal headache)
History of visual field loss

3. Baseline IOP Determination:

Uveitis/iritis (can develop open- or closed-angle glaucoma)
After blunt ocular trauma (hyphema causes acute IOP rise)
Orbital fracture (IOP >22 mm Hg or >3 mm Hg difference between eyes suggests ocular injury)

Types of Tonometry

Type	Method	Notes
Applanation (Goldmann)	Measures force needed to flatten a fixed corneal area	Gold standard; requires slit lamp
Non-contact / Air-puff	Puff of air deforms cornea; measures rebound time	Good screening tool; no eye contact needed
Impression (Schiotz)	Measures depth of corneal indentation by a fixed weight	Standard for primary care/emergency settings
Tono-Pen (handheld)	Portable applanation device	Useful in urgent/emergency settings
Radial artery applanation tonometry	Sensor over radial artery	Used in anesthesia for continuous BP waveform monitoring (a separate, non-ophthalmic application)

Diagnostic Combination for Glaucoma

Tonometry alone is not sufficient - patients with glaucoma can have normal IOP, and not all patients with elevated IOP have glaucoma. The most sensitive and specific approach combines:

Tonometry (IOP measurement)
Fundoscopic examination (optic disc assessment)
Visual field testing

Contraindications

Active eye infection (use Tono-Pen with disposable tip or non-contact tonometry)
Corneal abrasion
Suspected penetrating ocular injury (pressure on the globe can cause extrusion of intraocular contents)

Sources: Pfenninger and Fowler's Procedures for Primary Care, 3rd ed.; Kanski's Clinical Ophthalmology, 10th ed.

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