Brit. jt. Ophthal. ( I 97 I ) 55, 538

Response to mydriatics in the African

V. P. EMIRU

Department of Ophthalmology, Makarere University Medical School, Kampala, Uganda

It is common clinical experience that the pupil in the African dilates slowly in responseto mydriatics. Full mydriasis can be obtained in about 40 minutes in the Asian orEuropean, but takes much longer in the African. This difference in response has beenobserved in the same clinic at the same time and using the same drug. Scott (I945)noted the need for stronger mydriatics to dilate the pupil in Negroes. There must be anintrinsic peculiarity in the African iris, and it has been shown that this is not due to faultyinnervation (Scott, I945).An obvious peculiarity of the African iris is the abundant pigmentation. The present

study has revealed that the relatively slower response of the pupil to mydriatics in the Afri-can is indeed related to pigmentation, and that the pupil in the albino African behavesdifferently. The exact mechanism by which pigmentation influences mydriasis is notclear, and anatomical and physiological studies have been carried out in an attempt tofind a possible explanation.

Methods

(a) The rate of dilation of the pupil was compared in Africans, Europeans, and albino Africans,using homatropine 4 per cent. and phenylephrine 4 per cent. The subjects were studied in the sameroom in the same natural daylight. Phenylephrine was instilled into the light conjunctival sac andhomatropine into the left conjunctival sac in all cases. An equal number of drops was instilled fromthe same bottle and the diameter of the pupil was measured at io-min. intervals using a ruler.

(b) Comparative biomicroscopical studies were made on the African, European, and albinoAfrican irides.

(c) A comparative histological study was made on the African and European irides.

Results

(a) Dilation of the pupilFull mydriasis was obtained after about 40 minutes in the Europeans, but in the dark-skinned Africans dilation to the same extent could not be obtained even after 6o minutes.In the albino Africans dilation of the pupil was most brisk. There was no differencebetween the sexes, but dilation was a little slower in the older age group. Dilation startedwithin 5 minutes in the Europeans and albino Africans, but in the dark Africans it startedafter about Io minutes. Mydriasis started earlier with homatropine, but the degree ofdilation was about the same with both drugs after 30 to 40 minutes. In the dark Africansthe effectiveness of both drugs was reduced in the same proportion in that the final degreeof dilation was the same. The results are shown in the Table (opposite) and in Fig. I(overleaf).

Received for publication December I I, 1970Address for reprinits: P.O. Box 7072, Kampala, Uganida, East Afiica

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Response to mydriatics in the African 539

Table Dilation of pupil at io-min. intervals. Diameter of the pupil (lower figures in eachcolumn are those of the left pupil)

Age (yrs) SexInitial

Dark 24 MAfrican

31 M

17 M

30 F

26 F

17 M

30 F

31 M

65 M

45 F

37 F

24 M

43 M

35 M

Mean

Albino 12 FAfrican

23 F

8 F

41 M

Mean

European 31

54

49

52

30

M

M

M

M

F

Mean

IO

3 33 33 33 33 33 33 33 32 22 33 33 32 22 22 22 32 22 22 22 22 22 22 22 22 22 22 22 2

2-36 2.43

3 33 52 32 33 33 4

3 4

2271 3.57

2 32 43 43 52 32 42 32 43 43 5

2-40 3'90

Race

Pupillary diameter (mm.)

Time (min.)

6o

8866677576668666567576456667

6*2I

50

7765566555457566467456356667

5.54

68

40

6655465445456556456445355567

500

689989

7

8oo

89

20

3433343434342

434343434233334

3.32

476647

5

5.57

4656454557

50IO

30

4544454434343456354445244456

4.I4

588868

6

7.00

6767665678

6-40

99

7

7-8

77677 89 9

7'50 8-5

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540 V. P. Emiru

10

K~~~~~~8 .

E KE 7 x oo

4f 5XOX X FIG. I Rate of dilation of the pupil by 4perocent. homiatropine in European, Aftrican, and

4 - 49 co 0 a(1,nalbino African

Do/ -0 Dark African2 - >

Response to mydriatics in the African 541

0 /

FIG. 2 Photomicrograph of African iris. x 23 FIG. 3 Same African iris near the root. x I 15

fA A

.... -: ,::..: "A... .. Al aFIG. 4 Photomicrograph of Euro-pean iris. X 23

FIG. 5 Same European iris near theroot. x 115

4V.* . il jkI .-I

'.., .

II,I i 'If .*I

4:.- 0d.. *r a

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V. P. Emzru

effect on the iris. From the aqueous humour the drug is absorbed by the iris surface.Gregersen (i960) has demonstrated that, in addition to the crypts, there are iris poresbetween cells in the anterior border layer. These openings communicate with tissuespaces within the iris stroma, which consist of clefts of Fuchs and tubular spaces surroundingthe endothelial cells of the iris vessels (Gregersen, I959). There is therefore a definitechannel-system within the iris stroma through which aqueous humour passes freely in andout through the iris pores and crypts. In the African, the anterior border layer of the irisis thicker with denser iris chromatophores and has fewer crypts. It is also probablethat there is more stroma within the iris and hence fewer tissue channels. The absorptionof a mydriatic from the aqueous fluid would be much slower than in an iris with morenumerous and larger crypts and pores. Thus the onset of mydriasis starts later in thedark African iris.The effect of phenylephrine, which acts on the dilator pupillae muscle, occurred later

than that of homatropine, which acts on the more superficial sphincter pupillae muscle.Phenylephrine has to traverse a much longer distance through the tissue spaces. Theresponse to adrenergic stimulators is thus more delayed than that of cholinergic blockingagents. Once absorption had taken place the effect of both drugs appeared to be thesame.

Comparison of histological features may not give reliable results. Individual variationsin the architecture of the iris of any given eye are so unique that they have led to the cultsof "irido-diagnosis" and "oculo-diagnosis". The thickness of the iris, for instance,varies according to the degree of dilation of the pupil. Immediately before death, thepupil dilates, but after death it gradually decreases in size to an average diameter of about3.5 mm. Fixatives diminish the size of the pupil still further to about 2 to 3 mm. (Duke-Elder and Wybar, I96I). It has also been shown that there is considerable individualvariation in the thickness of the iris. The average thickness when the pupil is semi-dilated is o03 to o-6 mm. at the collarette and o0os mm. at the iris root. Although reliablecomparative values cannot therefore be given, biomicroscopic study leaves the impressionthat the African iris is more compact, with possibly more stroma. This is further borneout by the frequent observation that iridodonesis is less common in aphakia in the African.It is also possible, as it has been noted before (McNair, I95I), that the dark iris is thickerthan the light iris.

If the African iris is thicker and more compact, it would be less easily compressed, andthis would partly explain its sluggish dilation, in that it may not be able to exert as muchforce as a lightly-pigmented iris is able to develop. Mapstone (1968) made a mathematicalanalysis of the mechanics of pupillary block in closed-angle glaucoma, which demon-strated the various forces exerted by the iris as the pupil dilated. These forces may bedifferent in the African and could perhaps explain the rarity ofacute closed-angle glaucomain the African. Further study of this feature seems to be indicated.

Summary

The dark African pupil dilates more slowly in response to a mydriatic than that of thelighter-skinned races or albino Africans. This difference seems to be related to the deeperpigmentation and more compact architecture of the dark African iris, with fewer tissuechannels. It is possible that the African iris may not be able to exert the force which maylead to pupillary block, such as lightly-pigmented iris is able to develop.

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Response to mydriatics in the African 543

I should like to acknowledge the useful suggestions which I have received from Prof. E. S. Perkins, Instituteof Ophthalmology, London, and to thank Prof. N. Ashton, Department of Pathology, Institute of Ophthal-mology, London, for supplying a specimen of European iris.

References

DIKVE-ELDERI, S., and WYBAR, K. C. (I96I) "System of Ophthalmology", vol. 2, p. I69. Kimpton,London

GREGERSEN, E. (I959) Acta ophthal. (Kbh.), 37, 199(I960) "Studies on the Spongy Structure of the Human Iris and its Imbibition with the

Aqueous Humour". Munksgaard, CopenhagenMcNAIR, S. S. (I95I) Amer. J. Ophthal., 34, 70MAPSTONE, R. (I968) Brit. J. Ophthal., 52, 19SCOTT, J. G. (1945) Ibid., 29, 12

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