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INTRAOCULAR PRESSUREMOHAMED FAROUK KAMELMRCS EdAIR FORCE HOSPITALThe term glaucoma refers to a group of diseases that have in common a characteristic optic neuropathy with associated visual field loss for which elevated IOP is one of the primary risk factors.In most individuals the optic nerve and visual field changes seen in glaucoma are determined by both the level of IOP and the resistance of the optic nerve axons to pressure changes.In all the glaucomas, accurate assessment of the IOP is of great importance and is really the cornerstone in any discussion of glaucoma either a diagnostic or a therapeutic standpoint.Although an isolate pressure reading does not either establish the diagnosis of glaucoma or guarantee its proper management, control of the IOP is the single most significant measure available in the prevention of ocular damage from glaucoma.

Definitions:IOP is the pressure exerted by the intraocular contents on the partially elastic corneosclera.IOP maintains the eyeball in such a shape to perform its optical function.By convention, IOP is measured in millimeters of mercury (mm Hg).General agreement has now been reached that, no clear line exists between safe and unsafe IOP.Screening of Glaucoma based solely on IOP > 21 mm Hg may miss up to half of the people with glaucoma in the screened population.

Statistically:IOP, like many biologic parameters, varies in the population as a whole.Pooled data from large epidemiologic studies indicate that the mean IOP is approximately 16 mm Hg with 2 standard deviation on either side of this giving a normal IOP range 11 21 mm Hg.In children: 6-8 mmHg at birth increase by 1 mmHg /2years till 12 yearsIn the elderly the mean IOP is higher particularly in women, and the standard deviation is greater than in younger individuals. This means that normal IOP in elderly women may range up to 24 mm Hg.It is estimated that 4-7% of the population over the age of 40 years have IOP > 21 mm Hg without glaucomatous detectable damage (OHT)HOW IS IOP PRODUCED? Aqueous is produced by the ciliary processes into the posterior chamber ---> flows to anterior chamber --->Exits the eye by two different routs :trabecular (conventional) route: accounts for approximately 90% of aqueous outflow.Uveoscleral (unconventional) route : accounts for approximately 10% of aqueous outflow.Some aqueous also drains via the iris. (flow rate = 2.5 microliters/min)

3 factors determine the IOP:The rate of aqueous humour production.Resistance to aqueous outflow across the trabecular meshwork-schlemms canal system.The level of episcleral venous pressure.

FACTORS THAT INFLUENCE IOP Despite the relative constancy of IOP, there are factors that contribute to variations in IOP.Long Term1. Genetics - relatives of individuals with open-angle glaucoma are more likely to have high IOP2. Age - IOP increases with increasing age3. Sex - IOP's equal in the age range 20 to 40, after menopause women have higher IOP's4. Race - African-Americans have a higher incidence of glaucoma than whites

FACTORS THAT INFLUENCE IOPShort Term1. Diurnal variation - 3 - 6 mm Hg change in 24 hr period ( Change probably related to aqueous production and not drainage ) ; > 10 mm Hg change is pathogenic2. Sitting - going from a sitting to a lying position results in an increase in IOP which is even greater in glaucoma patients3. Total Body Inversion - causes an increase in IOP by as much as 15 mm Hg

FACTORS THAT INFLUENCE IOP4. Blinking - raises IOP briefly5. Exercise - decreases IOP6. Blepharospasm - increases IOP7. Coughing - increases IOP8. Blood pressure - some people believe that there is a link between blood pressure and IOP but no clear evidence9. General anesthesia - decrease IOP (important exceptions: ketamine, succinylcholine which may raise IOP])10. Alcohol - decreases IOPFACTORS THAT INFLUENCE IOP11. Cannabis - decreases IOP12. Tobacco - increases IOP13. Cholinergic Stimulating Agents (i.e., pilocarpine and echothiophate) - decrease IOP by increasing the aqueous outflow14. Adrenergic Stimulating Agents (i.e., epinephrine, propine, iopidine, alphagan) - lower IOP by enhancing aqueous outflow15. Adrenergic Blocking Agents (i.e., timolol and betaxolol) - decrease IOP by decreasing aqueous production16. Carbonic anhydrase inhibitors (i.e., diamox, trusopt, azopt) - decrease aqueous production17. Prostaglandins (i.e., xalatan, rescula, travatan, lumigan) - increase uveoscleral outflow

Measurement of the IOP There are two methods for measuring the IOP:Manometry.Tonometry:A. Manometry1. Cannulate the anterior chamber and directly measure the pressure2. Can not be done on humans3. The original method used to measure IOP

B. Tonometry (indirect measurement of IOP) The pressure inside a flexible sphere can be estimated by using fixed forces to create measurable deformation of the wall, or by using variable forces to produce predetermined deformations of the sphere wallDigitalInstrumental

Tonometry Why?Because.IOP is the only modifiable risk factor in GlaucomaMost frequently examined parameter in the follow up of a glaucoma patient.

Digital tonometry:

Although digital tonometry often yields incorrect impression of the ocular tension, a pathologically high or low tension is readily appreciated.The the sclera can be palpated through the upper lid above the tarsal plate. The tips of the two index fingers rest lightly upon the upper eyelids, one finger is kept still, the other is made to indent the globe lightly downwards and inwards as if to the center of the globe.It is a substitute for instrumental methods when they are not available or when infections preclude instruments or after giving the local anaesthesia.

Instrumental tonometry:

It estimates the resistance offered to deformation of the walls of the globe. For this purpose, tonometers may either measure the depth of impression produced upon the ocular wall by a given force represented by a plunger which has a small bearing area indenting it (Indentation tonometry) or they may record the force necessary to flatten an area of the cornea (Applanation tonometry)

1. Indentationa. the older of the 2 methods to measure IOP in humansb. involves measuring the indentation of the cornea resulting from a given weightc. the Schiotz tonometer is an indentation tonometerd. the weight of the tonometer displaces fluid in the eye and thus affects the IOP measurement2. Applanationa. only flattens a small portion of the cornea so does not displace a large amount of aqueous.b. better accuracy than indentationc. the NCT and the Goldmann tonometers are examples

C. Schiotz Tonometer1. a plunger of a known weight pushes on the cornea - thus result depends on ocular rigidityThe greater error arises from variations in ocular rigidity, if it is highIt is important to prevent foreign material from drying within the footplate because this affects the movements of the plunger.

2. Advantages of Schiotz tonometrya. small and easily transportedb. inexpensive c. does not require electricity

3. Disadvantages of Schiotz tonometrya. not extremely accurate - ocular rigidity dependent and instrument scale markings are not detailedb. requires anesthetic for most patientsc. assumes everyones epithelium is 0.05 mm thickd. technique can produce abrasionse. best if patient in a reclining positionf. placing the tonometer on the eye changes the IOP

D. Goldmann Tonometry1. The most accurate method for IOP measurement2. Readings within 1 - 2 mm of actual IOP3. Flattens a small portion of the cornea4. Theorya. 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 forced. this leaves Pressure = Force / Areae. the area of the probe has a diameter of 3.06 mmCentral Corneal Thickness:Thicker Corneas: Artificially elevated IOPThinner Cornea: Artificially decreased IOP

Correction nomogram:525 Micron1 -3 mm hg per 40 micron deviation.

. Procedurea. instill fluorescein into the tear layerb. view fluorescein pattern with the blue light on the slit lampc. doubling prism in place to split the view in halfd. image of the split circle must be lined upe. pressure = the number on the drum times 10 in mm Hg. Sources of Errora. improper width or position of miresb. inappropriate fluorescein levelsc. unusual corneal thickness

Figure 1Excess corneal applanation (IOP lower than tonometer reading)

Figures 2 and 3Insufficient corneal applanation (IOP higher than tonometer reading)

Figure 4Correct endpoint corneal applanation (IOP equals tonometer reading)

. Advantages of Goldmann Tonometrya. highly accurate and reliable (procedure does not influence IOP)b. accepted norm for IOP measurementc. easy to performd. not very expensive Disadvantages of Goldmann Tonometrya. requires anesthesiab. can result in an abrasionc. must sterilize instrument after each used. not portable

Periodic calibration check recommended: at least twice yearly

Perkins applanation tonometer: uses Goldmann prism adapted to small light source.It is hand- held and therefore can be used in bed-bound or anaesthetized patient.

Tono-pen: hand-held electronic contact tonometer. Its main advantage is the facility to measure IOP in eyes with distorted or oedematous corneas , through a bandage contact lens and in supine patients.

Noncontact Tonometer1. Achieves corneal flattening by an air jet of calibrated, increasing force2. Corneal flattening is detected by a photo cell3. From the known force of the air jet and the dimensions of the air jet the pressure is calculated4. 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. Particularly useful in children and other non-compliant patient groups.

CCT independent IOP Measurements:Ocular Response Analyzer: Recently developed pneumotonometer which measures IOP whilst attempting to compensate for corneal biomechanical propertiesCorneal HysteresisLess influenced byCorneal thickness and resistance

CCT independent IOP Measurements:Pascals Dynamic Contour Tonometry

Dynamic contour tonometry (DCT) is a novel method which uses principle of contour matching instead of applanation. This is designed to reduce the influence of biomechanical properties of the cornea on measurement. These include corneal thickness, rigidity, curvature, and elastic properties. It is less influenced by corneal thickness but more influenced by corneal curvature than the Goldmann tonometer

SterilizationRecommendation (HIV, HSV, and adenovirus): wipe tip clean and disinfect tip only with bleach (1:10 dilution x 5, changed once daily).Alternative is 3% H2O2, changed at least twice daily (affects tip less than bleach or ETOH).Alternative #2: wiping tip with 70% ETOH

Sources of error in tonometryOcular/Periocular anomalies:Lid, muscle, orbit malformation, infilteration, or congestion.Corneal anomalies: thickness, scarring, oedema.Absence of aqueous free space behind cornea.Abnormal scleral rigidity (indentation tonometry) .Patient-induced:Lid squeezingBreath holding, constrictive clothing.Eye/head movement.Unsuspected/unreported drug effects (usually lower IOP) e.g. recent ethanol ingestion, marijuana, systemic beta blockers.Recent exercise (usually lower IOP) may be higher in pigmentary glaucoma.

Sources of error in tonometryInstrument error:Poor maintenance, cleaning.Out of caliberation.Operation error:Failure to consider/observe any of the above.Applying pressure to the lids.Using inappropriate fluorescein concentration. Failure to record time of the day/

Target IOPThe "target" IOP should be considered as the "optimum" IOP for the eye in question, and would be set at a level designed to minimise the risk of disease progression. Unfortunately, in an individual patient, there is no way of knowing the IOP level that is required to do this.However, the lower the presenting IOP and the more severe the damage at presentation, the lower the "target" IOP. Conversely, a high presenting IOP (e.g. over 40 mmHg) in conjunction with evidence of early damage may result in an initial target IOP of 20 mmHg or less. The status of the fellow eye and the patients life expectancy are useful guides.Target IOPRecent evidence has suggested that IOP may rise significantly during sleep, and that this rise may be mirrored by the IOP rise that occurs after lying supine. The decision to alter medical therapy, or consider surgery to achieve the target IOP should not be taken without taking into account all available IOP data. This is particularly important if the proposed therapy carries significant risks. Similarly, a target IOP may be modified in the light of field and optic disc data acquired during the course of monitoring the patient.

Normal tension glaucomaApart from the finding of an IOP that does not exceed 21 mmHg, this form of open angle glaucoma may be indistinguishable from its higher pressure variant.It has been shown that central corneal thickness (CCT) may be thinner in some NTG patients compared with normal controls. This suggests that true IOP may be greater than measured IOP in some individuals with NTG.As with POAG, it is important to be sure that the visual function and the optic disc appearance equates with a diagnosis of glaucoma, as the differential diagnosis may include space occupying intracranial lesions. Where doubt exists, the appropriate investigations should be performed.IOP in special conditionsIn pregnancy: IOP show an average decrease of 1.5mmHg during pregnancy. Post-LASIK : stromal edema or interface fluid on slit lamp exam, increase in pachymetry measurements, steepening of corneal topography, or inappropriately low IOP measurements.Contact lens wearer : Small amounts of corneal edema probably cause an overestimation of IOP. Remove CL 2 hrs before tonometry

Recommendation

An ideal tonometer must make accurate and repeatable measurements of IOP without affecting the pressure or without harming the eye.As Goldmann applanation tonometry is the most reproducible, it is recommended for IOP measurement in patients with healthy corneas .Applanation tonometer tips should be disinfected between each patient .

Recommendation

Measurement of CCT, preferably with ultrasonic means, should be performed on all patients with glaucoma and ocular hypertension. The variance from the mean in a given population may under- or overestimate the true value of IOP in a given individual, and may influence the risk of an individual with ocular hypertension converting to glaucomaRecommendation

Hand-held devices such as Perkins/Kowa, Tono-Pen, and hand-held noncontact tonometers are useful in children, and in those unable to come easily to the slit lamp (e.g., obese, bedridden, with postural difficulties)When the cornea is scarred, irregular or oedematous , it is useful to have a tonometer that does not depend on an optical end point for its measurement (e,g, The Mackay-Marg tonometer and The Pneumotonometer.40Recommendation

Record the time of all pressure readings.A single normal reading, particularly if taken in late afternoon, may be misleading and it may be necessary to take several readings at different times of day (phasing).In clinical practice phasing during the morning hours may be sufficient because 80% 0f patients peak between 8.00 a.m. And noon.When changing therapy, schedule the next appointment at the same time of the day.When not changing therapy, vary the time of appointments to help reveal the circadian pattern.If a consistent peak time is found, most appointments should be scheduled at or near this time.

Finally ----Among patients with glaucomatous optic neuropathy, there are some patients for whom pressure is not an important risk factors, other individuals have increased pressure but their glaucoma continues to progress even with maximal lowering of IOP ( pressure is only a partial risk factor). A third highly sensitive group exists for whom IOP is virtually the sole causative factor for the development of their glaucomatous optic neuropathy.Thank You