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SAMJ VOLUME 69 , MARCH 1986 309

Tonometry in generaluse in early detectionopen-angle glaucomaH. L. E. KONIG

practIce -of primaryIts

SummaryThe intra-ocular pressure of 1078 general practicepatients aged over 45 years was measured with theSchiotz tonometer. Primary open-angle glaucomawas found in 29 patients (2,7%) and ocular hypertension in 40 (3,7%). The technique of using thetonometer is described and a plea is made for generalpract it ioners to make more use of it in screeningtheir patients for glaucoma.SAfrMed J 1986; &9: 309-311.

Primary open-angle glaucoma is the second major cause ofblindness in developed countries,1 and is by far the mostcommon cause of preventable blindness. 2 The disease occursin 2% of all people over the age of 45 years1 and is morecommon in females and blacks. 2 Patients over the age of 60years are most liable to develop the disease} and nearly 80% ofregistrations of blindness due to open-angle glaucoma occur inpeople over the age of 70 years.4 In the RSA over 100000people suffer from the disease.s It is thus not rare, bu t is adisease progressive with age leading ultimately to blindness.The disease is asymptomatic, which makes the diagnosisdifficult. From personal experience and communications withother general practitioners it is evident that primary openangle glaucoma is rarely diagnosed in general practice. To beable to diagnose glaucoma, the general practitioner should beaware of its prevalence and know which patients are especiallyat risk of developing the condition. The ability to use theSchiotz tonometer as well as an understanding of the pathology. of glaucoma are essential for its early detection.

Primary open-angle glaucomaGlaucoma is a condition producing visual field loss of acharacteristic type in that peripheral precedes central fieldloss, there is excavation or atrophy of the op tic nerve headcalled cupping and usually, but not invariably, elevated intraocular pressure.6 Glaucoma is not a distinct disease entity bu texists in many forms with different causes, clinical courses andtreatments.In open-angle glaucoma the aqueous humour has free accessto the trabecular meshwork, the drainage apparatus in theanterior chamber angle. The course of the disease is slow and

16 Burger Street, Standerton, TvlH. L. E. KONIG, M.B. CH.B., M.F.G.P. (S.A.), M.PRAX.MED.

insidious with few or no acute intervening episodes unti lfailing vision and ultimate blindness result. The diagnosis isdifficult to make and a screening test such as routine intraocular pressure measuring is invaluable.Not all patients who have raised intra-ocular pressure willdevelop glaucoma. The number of people in the populat ionover 45 years with abnormal intra-ocular pressures is approximately 10%,1 many more than the number who have frankglaucoma (approximately 2%).1 There are thus two distinctclinical entities: ocular hypertension (persons with raisedintra-ocular pressure only) and primary open-angle glaucoma(raised intra-ocular pressure, cupping of the optic nerveheadand visual field loss). Most patients with ocular hypertensionwill not develop glaucoma, even if not treated,7 and in somethe eyes may even become normotensive.S The incidence ofpatients with ocular hypertension who develop glaucoma is5 - 7%.7 It is also probable that patients with glaucoma havegraduated from the ocular hypertensive group over a period oftime. 1An isolated finding of raised intra-ocular pressure is no t anindication for ini tiat ing treatment and such patients shouldundergo a thorough ophthalmological evaluation.

Measurement of intra-ocular pressureThe process of measuring the intra-ocular pressure is knownas tonometry. The Schiotz indentation tonometer is a convenient, inexpensive and easy-to-care-for instrument. In the

USA it is used widely to perform routine cl inical measurements.} It is the ideal instrument for the general practitioner.The tonometer is placed over the eye, and the plunger isallowed to inden t the cornea un ti l the pressure in the eye issufficient to overcome the gravitational force exerted againstthe cornea by the weight of the plunger.6 The extent to whichthe plunger indents the cornea is shown on a scale by a simplelever-arm indicator needle. The lever arm magnifies the motionof the plunger 20-fold. The scale readings merely indicate thedepth to which the plunger indents the eye. A conversion tableis required to convert them into corresponding mmHg intraocular pressure (Figs I and 2).9The Schiotz tonometer has one liability. By indenting thecornea, the plunger causes an alteration in the steady state ofthe eye. Aqueous humour in the anterior chamber is displaced,causing alteration in the shape of the eye. I f the eyeball has anelastic or distensible scleral coat, a larger amount of fluid canbe displaced than if it has a rigid or less distensible coat. Inthe former case, the plunger would travel much fur ther andindicate a lower intra-ocular pressure than might be true,while with extremely rigid walls it might give a falsely highreading. The accuracy of the instrument is thus l imited by the

elastic properties of the eye, as well as by variations in thecurvature of the cornea.6,9Contraindications for tonometry.1O These are: (I) cornealor conjunctival infection; (il) corneal injury; and (iil) markedcorneal distortion, i.e. conical or badly scarred corneas.

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A -

310 SAMT DEEL 69 1 MAART 1986

G o ~ " ' ' ' ~' - " - . - .-........- ...- ,."" ..."" ..."".. .... '" ., ".,..., ... .., '" 11.19.. ,., ... .U '" UU . ... ...." 'u ." '" .uu to . .. ~ .., . ~ ; ; '"..,.. ... .... 4' ,.... "' .. .., ..,.. . .... . ..U ..., ". ,u '"t ". >I ' ,U 'U... .... "' ". ..," ,,, '" 77' ...,U "' ". ,.. ..,.. 'I> ,,. ", ..,.., .. " , .,.. ... 1>' .U ,..U " .... ... '"O. " .., ... 'U.., .. ... .,. ,,.... " 'A n. ,., I11' "! ., TU ,.,

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Fig. 1. Schiotz tonometer with conversion table.

Fig. 2. Working mechanism of the Schiotz tonometer.

Survey of patients in a Standertongeneral practicePatients and methodsI was shown how to use the Schiiitz tonometer at St John's Eye

Hospital, Johannesburg, and then made sure that the measurementsI was obtaining were accurate by measuring the intra-ocularpressure for every patient I referred to an ophthalmologist forwhatever reason the morning before he was seen. I t took about 3months or 15 referrals before I was convinced that the readingswith the Schi iiu tonometer were comparable to the ophthal mologist's tonometer (usually the Goldman applanation tonometer)readings.Then I undertook a survey in my practice by measuring theintra-ocular pressure in all patients over 45 years of age whoattended for treatment of general complaints. Patients were askedto agree to a test of eye pressure; the nature and purpose of theexamination being explained to them. No one refused. Although ithas been s tated that glaucoma screening should only be done inpatients older than 60 years i f any real cost benefit is to be

derived/I I I felt that I needed the tonometry practice so the costbenefit aspect did nor play such a large part in my decision. TodayI test rout inely only those patients over the age of 60 years andthose under 60 years i f there is any special reason to suspectdisease.The Schiiitz tonometer was checked before use for accuracy atzero on the metal tes t block suppl ied with the instrument! Thefootplate was sterilized by passing it briefly through a flame andallowing to cool, ' or immersing i t in a solution of chlorhexidinegluconate. Day-to-day cleanliness should be maintained by frequentwashing with warm sterile water and drying with cotton, since it isimportant that the plunger should move freely and that thecurvature of the footplate is not altered by foreign material.The patient is asked to lie on the examination couch and thecornea is inspected for any contraindicat ions to tonometry.Ophthalmoscopy is performed on each eye and the condition ofthe optic disc and cup noted. Both eyes are anaesthet ized with adrop of oxybuprocaine 0,4% (Novesin Wander; Restan Laboratories) and the patient asked to look at a Spot marked on theceiling above his head. Both eyes should be kept wide open. Whileholding the eyelids open, the examiner should make sure he isonly applying pressure to the orbital rim and never to the globe,and never pulling the lids so that they dis tort the globe.9 Thetonometer is held just above the cornea without touching it forseveral seconds, until the patient is under the impression that thetest is actually being performed, and until he has relaxed his initialand normal apprehension. The tonometer is then lowered gentlyuntil it rests on the centre of the cornea. The weight of thetonometer can be varied until the lever-arm indicator needle givesa reading of between 5 and 8 units on the scale. This is in themiddle range at which the tonometer functions most accurately.The weight used for most eyes with normal pressure is the 7,5 gone (5 g and 10 g weights are also supplied with the instrument).The indicator needle should show the ocular pulse, and thefootplate should at no time touch the lid margins.- The reading isthen taken and the intra-ocular pressure read off the conversiontable. Finally, as a prophylactic measure against any possibleinfection, I% chloramphenicol eye drops are instilled into eacheye.'The procedure is safe - in a series of 40000 patients examinedby Schiiitz tonometry in the USAI2 no case of trauma or infectionwas reported.

ResultsFrom 1978 to 1983 the intra-ocular pressure of I 078 patients overthe age of 45 years was measured; there were 642 women and 436men in the s tudy group. Intra-ocular pressure above 22 mmHgwas found in 73 people (6,8%), their age groups are given in TableI. These 73 patients were then referred to a private ophthalmologistor to an ophthalmological unit at a provincial hospital; 4 werefound nor to have raised intra-ocular pressure (5,5% error). Ofthese false-positive results I patient was in the 51 - 55-year-oldage group, 2 in the 56 - 60-year group and only I was over 65years. This was surprising since more false-positive results wereexpected in the older age groups as scleral rigidity becomes anincreasing problem in Schiiiu tonometry with advancing age.

Of the 69 patients with increased intra-ocular pressure 29 werefound to have primary open-angle glaucoma (2,7% of all patientsexamined, 42% of patients with raised intra-ocular pressure), their

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TABLE I. AGE DISTRIBUTION OF PATIENTS WITH INTRA-OCULAR PRESSURE> 22 mmHg

Age (yrs)

TABLE 11. AGE DISTRIBUTION OF PATIENTS WITH OPEN-ANGLE GLAUCOMA

Age (yrs)

TABLE Ill. AGE DISTRIBUTION OF PATIENTS WITH OCULARHYPERTENSION

Age (yrs)

No.% of total

No.% of total

No.% of total

45-505

0,46

45-50

45-505

0,46

51-558

0,74

51-551

0,09

51-556

0,55

56-60171,57

56-602

0,18

56-60131,20

61-65211,94

61-65111,02

61-65100,92

65222,04

65+151,39

65+6

0,55

SAMJ VOLUME 69 1 MARCH 1986 311

have been diagnosed . A further 40 were found to be ocularhypertensives and needed further follow-up. I t is thereforeimportant for doctors in the front line of medical care such asgeneral practitioners to be able to recognize both ocular hypertension and glaucoma. Recognition depends on the ability tomeasure intra-ocular pressure and to diagnose the abnormalityof the optic nervehead by ophthalmoscopy.t These are relativelyeasy procedures, take a minimum of time and can be carriedout singlehandedly during normal consultation hours.The general practitioner or primary care doctor alone hasthe opportunity to examine a high p ropor tion of the olderpatients in society and enjoys a personal relat ionship with hispatien ts which will readily induce them to submit to anexamination of their eyes even if they have noticed no eyesymptoms. 3 The genera l pract it ioner can thus make a worthwhile contribution to the problem of diagnosis and detection

of glaucoma at a stage at which treatment may be effective inpreventing further deterioration of vision. The use of theSchiotz tonometer in his pract ice will also provide him withincreased job satisfaction and patient gratitude.' I f one can diagnose primary open-angle glaucoma in the

first affected eye it is probably true to say, if the appropriatetreatment is commenced, that in the majority of cases, thepatient will have adequate vision in his working life.'13

REFERE 'CES

age groups are given in Table 11. The remaining 40 patients (3,7%of all patients examined) were regarded as ocular hypertensivesand will be regularly monitored, their age groups are given inTable Ill.The findings in this study correlate well with those of Phillips3who examined 300 patients over 60 years of age and found anincidence of primary open-angle glaucoma of 2,3% and of ocularhypertension of 2,6%. The percentage of patients in my studypopulation with abnormally high intra-ocular pressures was 6,4%,lower than the approximately 10% quoted by Luntz. t

DiscussionDuring the 4 years of this survey 29 pa ti ent s were found tohave primary open-angle glaucoma. These would not otherwise

1. Luntz MH. The glaucomas. S AfrJ Hasp Med January 1978, p. 2.2. Soli DB, Phillips AJ. Update on glaucoma managemenl. Drug Ther 1979; 5:88-100.3. Phillips MA. Early detection of chronic simple glaucoma in general practice.J R Call Gm Pract 1977; 27: 601-604.4. Perkins ES. Blindness from glaucoma and the economics of prevention.Tram Ophthalmol Sac UK 1978; 98: 293-295.5. Cenrral Stati tical Services. Census 80. Pretoria: Governmenr Printer, 1982.6. Wilensky JT. Glaucoma. In: Peyman GA, Sanders OR, Goldberg MF, eds.Principles and Practice of Oplllhalmology. Philadelphia: WB Saunders, 1980:671-674; 683-685.7. Kolke r AE, Becker B. Ocular hypertension or early glaucoma. ArchOphthalmol 1977; 95: 585-587.8. Leading Article. Ocular Hypertension. Br MedJ 1978; I: 1230.9. Chandler PA, Grant WM. Glaucoma. Philadclphia: Lea & Febiger, 1979:14-15,79.10. Miller D. Ophthalmology - The Essentials. Boston: Houghton Mifflin, 1979:46, 121, 123.11. Ross AK. Organization of a glaucoma screening in general practice.J R CallGm Pract 1968; 15: 358-362.12. Perkins ES. Glaucoma in its sociological aspecl. Tram Ophthalmol Sac UK1968; 88: 375-395.13. Morgan OG. The early clinical diagnosis of glaucoma. Trans Ophthalmol SacUK 1958; 78: 471-492.