Intraocular Pressure

Aka: Intraocular Pressure, IOP, Schiotz Tonometry, Anterior Chamber Depth, Tonometry, Tonometer, Applanation Tonometry, Goldmann Tonometer, Tono-Pen, Pneumatonometer, Air-Puff Tonometry, Non-contact Tonometry, Digital Tonometry, Applanation Tonometer

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II. Indications

  1. Glaucoma
  2. Iritis
  3. Ocular Hypotony

III. Physiology

  1. Aqueous Humor
    1. Ciliary body produces Aqueous Humor
    2. Aqueous Humor flows through pupil and into anterior chamber
    3. Trabecular meshwork drains Aqueous Humor into the venous system
  2. Intraocular Pressure
    1. Intraocular Pressure is a balance between Aqueous Humor production (ciliary body) and drainage (trabecular mesh)
    2. Intraocular Pressure may normally vary with Blood Pressure and respiration
    3. Introcular pressure typically decreases at night

IV. Techniques: Applanation Tonometry - Intraocular Pressure measurement

  1. Background
    1. Applanation devices measure the amount of pressure needed to flatten a small portion of the Cornea
    2. Apply Topical Eye Anesthetic to Cornea before testing
  2. Goldmann Tonometer
    1. Slit-lamp based device that offers most accurate Intraocular Pressure measurement
  3. Portable electronic Applanation Tonometers (e.g. Tono-Pen, Accupen)
    1. Accurate Tonometry alternative to the Goldmann Tonometer
  4. Non-contact Tonometers (air-puff)
    1. Least accurate Tonometry method, but no Topical Anesthetic needed
  5. Schiotz (older, manual method, rarely used in U.S.)
    1. Requires a skilled, practiced clinician to use accurately
    2. Scale <3.5 (5.5g plunger) recheck with 7.5g plunger
    3. Scale <6.0 (7.5g plunger) recheck with 10g plunger
    4. Scale <8.5 (10g plunger) recheck with 15g plunger
    5. Scale <11.0 (15g plunger)

V. Interpretation

  1. Normal Intraocular Pressure
    1. Intraocular Pressure 8 to 21 mmHg
  2. Elevated Intraocular Pressure (Glaucoma)
    1. Intraocular Pressure > 21 mmHg
  3. Decreased Intraocular Pressure (Ocular Hypotony)
    1. Intraocular Pressure <5 mmHg

VI. Exam: Less accurate methods to estimate Intraocular Pressure

  1. Eye pressure applied through upper Eyelid
    1. Firmer eye suggests a higher Intraocular Pressure
  2. Anterior Chamber Depth
    1. Penlight illumination of anterior chamber
    2. Normal
      1. Illumination from lateral side lights entire iris
    3. Abnormal (increased pressure)
      1. Iris is bowed forward and nasal side is in shadow
      2. Nasal iris is not illuminated

VII. References

  1. Trobe (2012) Physician's Guide to Eye Care, AAO, p. 28

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