S.C.C.O.8/21/20151 INTRAOCULAR PRESSURE LECTURE. S.C.C.O.8/21/20152 I. DEFINITION A. the tissue...

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INTRAOCULAR PRESSURE INTRAOCULAR PRESSURE LECTURELECTURE

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I. DEFINITIONI. 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

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II. HOW IS IOP PRODUCEDII. 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)

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II. HOW IS IOP PRODUCEDII. 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

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III. FACTORS THAT III. FACTORS THAT INFLUENCE IOPINFLUENCE 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

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III. FACTORS THAT III. FACTORS THAT INFLUENCE IOPINFLUENCE IOP

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

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– 4. Race - African-Americans have a higher incidence of glaucoma than whites

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III. FACTORS THAT III. FACTORS THAT INFLUENCE IOPINFLUENCE 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

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– 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

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– 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

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– 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

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– 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

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IV. MEASUREMENT OF IOPIV. 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

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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

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– 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

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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

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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

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

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– 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

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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

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– 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)

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

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d. this leaves – Pressure = Force / Area

e. the area of the probe has a diameter of 3.06 mm

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– 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.

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– 6. Sources of Error a. improper width or position of mires b. inappropriate fluorescein levels c. unusual corneal thickness

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– 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)

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

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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

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– 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)

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

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V. IOP AND GLAUCOMAV. 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

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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

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

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