INTRAOCULAR PRESSURE LECTURE

I. DEFINITION A. the tissue pressure of the ocular contents B. about 15 mm Hg but does fluctuate (15.5 +/- 2.57) C. normal range of pressures: 10.5 - 20.5

II. HOW IS IOP PRODUCED A. Aqueous produced by the ciliary processes into the posterior chamber ---> flows to anterior chamber and vitreole chamber ---> bulk of fluid flows out the angle (trabecular outflow) of the anterior chamber (flow rate = 2.5 microliters/min)

II. HOW IS IOP PRODUCED 1. resistance to flow at the lens-iris interface 2. resistance to flow at the angle of the anterior chamber a. pressure in the eye increases until the force pushing fluid out of the eye (i.e., the eye pressure) results in the same amount of fluid leaving the eye as is produced and enters the eye B. Aqueous exchanged with the retina, lens, ciliary body, iris and cornea

III. FACTORS THAT INFLUENCE IOP A. Long Term 1. Genetics - relatives of individuals with open-angle glaucoma are more likely to have high IOP 2. Age - IOP increases with increasing age

III. FACTORS THAT INFLUENCE IOP 3. Sex - IOP's equal in the age range 20 to 40, after menopause women have higher IOP's

III. FACTORS THAT INFLUENCE IOP 4. Race - African-Americans have a higher incidence of glaucoma than whites

III. FACTORS THAT INFLUENCE IOP B. Short Term 1. Diurnal variation - 3 to 6 mm Hg change in 24 hr period; > 10 mm Hg change is pathogenic a. Change probably related to aqueous production and not drainage

III. FACTORS THAT INFLUENCE IOP 2. Sitting - going from a sitting to a lying position results in an increase in IOP which is even greater in glaucoma patients 3. Total Body Inversion - causes an increase in IOP by as much as 15 mm Hg

III. FACTORS THAT INFLUENCE IOP 4. Blinking - raises IOP briefly 5. Exercise - decreases IOP 6. Blepharospasm - increases IOP 7. Coughing - increases IOP 8. Blood pressure - some people believe there is a link between blood pressure and IOP but no clear evidence 9. General anesthesia - decrease IOP 10. Alcohol - decreases IOP

III. FACTORS THAT INFLUENCE IOP 11. Cannabis - decreases IOP 12. Tobacco - increases IOP 13. Cholinergic Stimulating Agents (i.e., pilocarpine and echothiophate) - decrease IOP by increasing the aqueous outflow 14. Adrenergic Stimulating Agents (i.e., epinephrine, propine, iopidine, alphagan) - lower IOP by enhancing aqueous outflow

III. FACTORS THAT INFLUENCE IOP 15. Adrenergic Blocking Agents (i.e., timolol and betaxolol) - decrease IOP by decreasing aqueous production 16. Carbonic anhydrase inhibitors (i.e., diamox, trusopt, azopt) - decrease aqueous production 17. Prostaglandins (i.e., xalatan, rescula, travatan, lumigan) - increase uveoscleral outflow

IV. MEASUREMENT OF IOP A. Manometry 1. Cannulate the anterior chamber and directly measure the pressure 2. Can not be done on humans 3. The original method used to measure IOP

IV. MEASUREMENT OF IOP B. Tonometry 1. Indentation a. the older of the 2 methods to measure IOP in humans b. involves measuring the indentation of the cornea resulting from a given weight c. the Schiotz tonometer is an indentation tonometer d. the weight of the tonometer displaces fluid in the eye and thus affects the IOP measurement

IV. MEASUREMENT OF IOP 2. Applanation a. only flattens a small portion of the cornea so does not displace a large amount of aqueous. b. better accuracy than indentation c. the NCT and the Goldmann tonometers are examples

IV. MEASUREMENT OF IOP C. Schiotz Tonometer 1. a plunger of a known weight pushes on the cornea - thus result depends on ocular rigidity a. concept of ocular rigidity developed by Friedenwald

IV. MEASUREMENT OF IOP b. log Pt - log Po = EVc log Pt1 - log Pt2 = E(Vc2 - Vc1) Pt = tonometric pressure Pt1, Pt2 = tonometric pressures with different plunger loads Po = IOP before tonometry E = coefficient of ocular rigidity Vc1, Vc2 = volume of corneal indentation corresponding to Pt1 and Pt2 c. average value of ocular rigidity is 0.0215, the slope of the line joining the plots of 2 nomogram readings

IV. MEASUREMENT OF IOP 2. Advantages of Schiotz tonometry a. small and easily transported b. inexpensive (about $100) c. does not require electricity

IV. MEASUREMENT OF IOP 3. Disadvantages of Schiotz tonometry a. not extremely accurate - ocular rigidity dependent and instrument scale markings are not detailed b. requires anesthetic for most patients c. assumes everyones epithelium is 0.05 mm thick d. technique can produce abrasions e. best if patient in a reclining position f. placing the tonometer on the eye changes the IOP

IV. MEASUREMENT OF IOP D. Goldmann Tonometry 1. The most accurate method for IOP measurement 2. Readings within 1 - 2 mm of actual IOP 3. Flattens a small portion of the cornea

IV. MEASUREMENT OF IOP 4. Theory a. the cornea is covered with a tear layer which exerts a surface tension (force in towards the cornea) b. a probe applied to the cornea is acted against (a force pushing out from the cornea) by the corneal thickness and elasticity (the bending force)

c. if the area of the probe is of the proper size then the force from the surface tension will cancel the bending force

IV. MEASUREMENT OF IOP d. this leaves Pressure = Force / Area e. the area of the probe has a diameter of 3.06 mm

IV. MEASUREMENT OF IOP 5. Procedure a. instill fluorescein into the tear layer b. view fluorescein pattern with the blue light on the slit lamp c. doubling prism in place to split the view in half d. image of the split circle must be lined up e. pressure = the number on the drum times 10 in mm Hg.

IV. MEASUREMENT OF IOP 6. Sources of Error a. improper width or position of mires b. inappropriate fluorescein levels c. unusual corneal thickness

IV. MEASUREMENT OF IOP 7. Advantages of Goldmann Tonometry a. highly accurate and reliable (procedure does not influence IOP) b. accepted norm for IOP measurement c. easy to perform d. not very expensive (about $1000)

IV. MEASUREMENT OF IOP 8. Disadvantages of Goldmann Tonometry a. requires anesthesia b. can result in an abrasion c. must sterilize instrument after each use d. not portable

IV. MEASUREMENT OF IOP E. Noncontact Tonometer 1. Achieves corneal flattening by an air jet of calibrated, increasing force 2. Corneal flattening is detected by a photo cell 3. From the known force of the air jet and the dimensions of the air jet the pressure is calculated

IV. MEASUREMENT OF IOP 4. The higher the IOP the longer it takes to flatten the cornea (i.e., if IOP = 17 mm Hg, flattening takes 10.5 msec; if IOP = 36 mm Hg, flattening takes 24 msec) 5. Advantages of NCT a. no anesthesia required b. fairly reliable for pressures in the normal range (less reliable as the pressure increases) c. quick and easy to perform d. does not touch the eye (can use on diseased eyes)

IV. MEASUREMENT OF IOP 6. Disadvantages of NCT a. expensive (over $5000) b. not accurate at high pressures c. patient apprehension (puff) raises IOP d. not easily portable

V. IOP AND GLAUCOMA A. If the IOP is high enough for a long enough time, the patient will lose visual field in a characteristic pattern B. If the angle is totally blocked the pressure rises to 55 or 60 mm Hg and field loss is seen in a few hours with total loss in a few days

V. IOP AND GLAUCOMA C. In open angle glaucoma the pressure is more often in the 20's or 30's 1. this pressure can be tolerated for months before damage occurs 2. sometimes patients with normal pressure develop glaucomatous changes

V. IOP AND GLAUCOMA D. To diagnose glaucoma usually need: 1. high IOP 2. visual field defects 3. cupping at the disc