ACTA O P H T H A L M O L O G I C A 1958

From the Ophthalmic Clinic of the University of Helsinki. Head: Professor Mauno Vannas, M. D .

SENSITIVITY OF THE CORNEA IN ARCUS SENILIS:’)

BY

Henrik Forsius

The purpose of this study was to investigate whether arcus senilis corneae which, as is well-known, increases with the years, causes decreased sensitivity of the cornea with rising age. This factor is readily distinguished from others affecting the sensitivity, as a marked arcus corneae is sometimes observed also in young persons.

The peripheral parts of the cornea are innervated by conjunctival nerve threads emanating from plexus paramarginalis superficialis, whereas the central parts of the cornea are mainly innervated from nervus ciliaris, and penetrate into the mid layer of the cornea, gradilally finding their way to its surface. The conjunctival area is 11/z mm broad (Attias b), but great individual diver- gences are noted: even the centre of the cornea may sometimes be innervated from conjunctival nerve plexa (Reiser). These nerves run in the superficial layers of the cornea. According to Thomas, the nerves penetrate to a depth of ‘ i s of the parenchyma, as assessed from the posterior part, and an abundance of anastomoses appear between the superficial conjunctival nerves and the deep ones. Owing to the numerous anastomoses, the centre of the cornea does not loose its sensitivity until incisions are made along the periphery by a cut of 300 degrees in length (Rexed & Rexed).

Attias a) examined a large series regarding the corneal innervation and its changes in the aged. The corneal conjunctiva, the tissue containing an abundance of blood vessels leading from the conjunctiva on to the surface of the cornea. increases with age. When the arcus increases in breadth, the corneal con- junctiva follows suit. I t thickens the cornea, it contains no fat, and the vasculari- zation consists of intact vessels. Occasional nerves with fatty degeneration are seen in the cornea of young persons, even. In the aged, they appear regularly especially in the limbus and in the region of the arcus; this is a constant phenomenon. The fat drops in the nerves are largest when they pass the arcus.

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*) Received October 28th 1957.

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The nerves emanating from the conjunctiva do not become impregnated with fat until they reach the region of the arcus.

Investigations on the sensitivity of the cornea at various ages were first published by Kriickmann who observed that the sensitivity was reduced in the elderly. Zobel seemed to think that the sensitivity increased at the age of 15-50, after which it decreased. Jalavisto, Orma & Tavast were unable to verify this observation. They noted a decreasing sensitivity at all ages, and more rapidly so after the age of 45. A similar observation seems to appear from Boberg-Ans' diagram of the sensitivity in different age groups. The physiological variation is great, however. Attias a) observed no decreased sensitivitv in the central parts, but, in the aged with intense arcus, examined by him, lie noted decreased sensitivity in the periphery of the cornea. More- over, in tests on intelligent persons with extensive arcus, his observations of reduced sensitivity were not infrequent. Zobel, already, observed that the changes in sensitivity were more pronounced peripherally in the aged, and Abe mentions decreased peripheral sensitivity in two cases of arcus juvenilis, and in three cases of arcus senilis. Potvin noted a sector-shaped area of de- creased sensitivity. This area always corresponded to a fine line in the margin which was highly impregnated with cholesterin.

Controversial reports on the decreasing sensitivity of the cornea in the elderly have been published.

Patho-anatomically, Attias a) revealed fatty degeneration of the corneal nerves. This he considered to be the cause of the reduced sensitivity of the arcus. Cantonnet maintains that the sensitivity is reduced due to fatty degene- ration of the glandula ciliaris. In their discussions on why the sensitivity de- creases with age, Jalavisto, Orma & Tavast concluded that there were several concurrent reasons. The fibrous tissue thickens and becomes impregnated with fat due to which greater power is required to form such a deep depression in the cornea that the irritation is perceived by the nerve endings and, in addition, there are senile changes in the central nervous system.

The sensitivity of the cornea may be measured by many methods. For testing areas of such a small size as the arcus, the hair or thread methods are the only possible means. This method was introduced by Frey, in 1894. But air of varying temperature and humidity affects the rigidity of the hairs and conse- quently their power, and with time, they become brittle. Therefore Boberg- Ans introduced the use of nylon thread (Brulon Monofilament 240, diameter 0.0051 1" = 0.1 1 mm. Imperial Chemical Industries Ltd.). H e gives a detailed description of the advantages of nylon thread in his monography.

In the present study, Boberg-Ans' technique, using nylon thread, was fol- lowed. The simple apparatus that I have used consists of two metal tubes running parallely inside each other, and the length of the nylon thread may be increased to maximum 63 mm by shifting one of the tubes. When m. p. was

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less, the thread was held vertically, and the free thread used and measured after the examination. For detailed examination the studies were sometimes per- formed under the microscope with X 10 enlargemeunts, but generally the patient was seated and observed through a binocular lupe during the examination.

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The answers were given in length of thread. The diagram shows the pressure in mg, correlated on analyses scales. Persons with intense arcus were selected for the investigation. At evaluation of the sensitivity, a source of error arises in that the examiner is dependent on the patient’s power of observation and interest. A sensitive person becomes irritated as soon as the eyelid is lifted. Others, again, are readily affected by suggestion and say that they can feel the thread in spite of the point of the thread not having touched the cornea. For these reasons it was therefore found necessary to sort out the material. Despite repeated intervals between the examinations, the irritation threshold was often found to be reduced during the studies. The results are given as averages of acceptable measurings.

The intensity of the arcus was divided into four groups. 1) Mxrked arcus, when the fat penetrates all layers of the corneal parenchyma at least at some point, 2) weak arcus, 3) microscopic arcus, which is not seen without bio- microscope, 4) no arcus. The material was subdivided into two groups: persons under 40 years, and persons over 40 years.

Persons under 40 years (Arcus juwnil is) Eight persons with marked arcus were examined. The average age in this

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group was 29.4 years. The minimum perceptibility in the corneal centre was 90 mni (52-140). In the corneal periphery, which included a 2 mm broad area from limbus cornea, the average sensitivity was 50 mm. The average for each eye separately was obtained by measuring several points around the periphery, outside the arcus, and immediately inside it. The control group included 8 persons without arcus or with arcus visible only in the cornea microscope. The average age of these persons was 30.4 years. Their minimum perceptibility in the centre of the cornea was 102 mm (70-140) and in the periphery 60 mm, thus a somewhat higher value.

Persons over 40 years (Arciis senilis) The sensitivity in all persons over 40 years was less, compared with the

youngcr age groups. On study of elderly persons, comparison was made with persons with marked arcus and those of the same age with microscopic or weak arcus. In the group of intense arcus, the minimum perceptibility in mid cornea was 66 mm and in the periphery 36 mm. The group consisted of 10 persons, average age 56.3 years. In the control group the values were 76 mm in the centre and 40 mm in the periphery, calculated from the average value for 11 per.;ons (59.6 years).

In a case of intense arcus, the sensitivity was reduced on the arcus as com- pared with other points a t the same distance from limbus. This is clearly evi- denced especially when the arcus forms a well demarkated crescent, as in the case in Figure 1 . Increased sensitivity was generally noted on proceeding from the linlbus, radially inward to the centre.

In the senile type of arcus, the lucid zone was generally covered by the corneal conjunctiva, and the sensitivity test of this area of the cornea gave the same value or a somewhat higher one than on the conjunctiva; this value was not included in the collocation. In juvenile arcus, the pan- nus-like membrane had not penetrated on the cornea. I obtained, in two cases, higher sensitivity values outside, than inside the arcus; the sensitivity on arcus was less than either inside or outside it. In two other cases the sen- sitivity on arcus was less than outside it. A sector-shaped Teduced sensitivity in partial arcus was found in five persons of various ages. In eight other cases in which a difference could be made between an intense section of the arcus and less marked areas - no sector-shaped reduced sensitivity areas on the cornea, corresponding to the intense arcus, could be observed, in spite of several of these persons having excellent perceptibility. I n 1 1 , or almost all in whom such a crescent was distinguished from other section of the periphery, the sensitivity was less on the fatty arch than on the corresponding free areas.

Figure I shows the Ieft eye of a man, aged 32. The arcus was distinctly sector-shaped but undcr the microscope the crescent could be followed around the 'periphery, which is alwa) s the case both in primary and secondary arcus. This patient was a particularly

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Fig. 1.

good subject for observation. It was distinctly seen that the sensitivity was reduced where the arcus was more marked. In the upper part of the lucid zone the minimum perceptibility was 30 mm, on arcus 37, and inside arcus 40. At the lower crescent the threshold value for the crescent was 37, peripherally 37; medially of arcus 48; in the centre of the cornea 52 mm. Sector-shaped reduction of the sensitivity was observed as far as to the pupillary area.

In normal persons, the sensitivity of the cornea in the vertical plane was often worse than in the horizontal plane. This is seen also in young persons without arcus. The reduced sensitivity on arcus which most often occurs in straight upward or downward line, is however seen in cases such as presented in Figure 1 where arcus is temporally downward.

Figure 2. This patient was a woman aged 7 1 , whose serum-cholesterol level was 375 mg per cent. In her left eye, lipoid keratitis had been formed on the basis of maculae after a corneal affection in her youth, and there was an extremely strong double arcus in her other eye. The sensitivity on arcus was 25-27 mm. The sensitivity on the corneal conjunctiva, which penetrated far on to the cornea, was 12 mm and on the conjunctiva 10 mm. In the central parts of the cornea, the sensitivity was 40, and increased constantly independently of the lucid zones. Extraction of cataract was performed, the incision being made peripherally of the arcus. Healing of the wound was uneventful.

Fig. 2.

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Fig. 3.

The patient described in Figure 3 was a man, aged 74. His serum-cholesterol value was 32s mg per cent, and he had lipoid keratitis in one eye. In the other (Fig. 3) an extremely marked arcus crescent was seen. This arcus has been reproduced in a previous paper by the present author (1954, Fig. 6). Now perilimbal loss of tissue (,>Rinncn bildung<<) had occurred in the lower part in nasal direction. The sensitivity was highly increased in this depression. When the sensitivity on the arcus crescent was fiist examined with a nylon thread and the depression then touched with the same thread, the patient reacted by expressing pain. Evidently the nerve endings lacked their protecting epithelial cover. The arcus crescent was somewhat narrowed due to the depression which had completely corroded the arcus tissue. The condition must very probably be considered a sequelae to the penetration of fat into the cornea.

The youngest patient examined was 17 years of age. She was not included in the series as she had only a weak arcus. Her cholesterol level in serum was 422 a t the age of 14, and now 220 mg per cent. The arcus was somewhat thinned out but quite distinct. The sensitivity was poor. The minimum pre- ception was 75 mm in the centre, on arcus 33 mm, and the average in the periphery 40 mm.

The blood cholesterol of these patients was repeatedly examined. All young persons included in this series had raised total cholesterol levels in the blood. As previousIy described (Forsius), the cause of intense arcus in young persons is most often the high fat values in serum which lead to fatty deposits in the corne'i. The values obtained suggest that the corneal fatty infiltration causes reduced corneal sensitivity, mainly on arcus, but also in all other parts of the cornea - in young persons as well as in the elderly - but the difference is not great. T o conclude that the only reason for reduced corneal sensitivity, as is :howl1 to be a fact in increasing age, lies in the arcus is improbable, however, as, for instance, in one person the sensitivity was greater in the eye in which the arcus was more marked. The sensitivity was good in young persons with microscopic arcus. The abundance of anastomoses between the nerves from the conjunctiva and those penetrating into the cornea are evidently able to maintain normal sensitivity. Variations within the innervation may well ex- plain the individual differences.

SUMMARY

Eighteen persons with intense arcus lipoides (juvenilis and senilis) in whom the fat had penetrated all layers of the corneal parenchyma were studied by means of a nylon thread (0.1 1 mm) with respect to corneal sensitivity. Nineteen persons without arcus - or in the older age groups, those with microscopically visible or weak arcus - were used for comparison. The results show that the sensitivity is as a rule decreased on arcus and sometimes 'there is a sector- shaped reduction as far as the region of the pupilla, and that the minimum

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perceptibility was decreased both peripherally and in the centre of the cornea in arcus cases, both in persons over 40 years (average age 56.3 years) and under 40 years; average age 29.4 (8 persons with marked arcus).

REFERENCES

Abe, Y.: Acta SOC. Ophth. Japon. 1937: 41. Attias, G.: Graefes Arch. Ophth., 1912: 81.: 405. Attias, G.: Graefes Arch. Ophth., 1912: 83: 207. Boberg-Ans, J.: Om corneasensibiliteten med sserligt henblik p i kliniske undersegelses-

metoder. Otto Tellers bogtrykkeri, Hellerup, Danmark 1952. Cantonnet, A.: Prat. med. frc., 1936: 1 7 : 359 (cit. Potvin). Forsius, H . : Arcus senilis corneae. Acta ophth., Suppl. 42. 1954. Frey, M . von: Ber. Sachs. Ges. Wiss.: 1894: 185: 283 (cit. Boberg-Ans). Jalavisto, E., Orma, E. & Tavast, M . : Acta Phys. Scand., 1951: 23: 224. Kriickmann, E.: Graefes Arch. Ophth., 1895: 41: 20. Potvin, M.: Bull. SOC. Belg. Ophth., 1938: 7 7 : 160. Reiser, K . A.: Arch. f . Augenhk., 1935: 109: 251. Rexed, B. & Rexed, U.: Brit. J. Ophth. 1951: 35: 38. Thomas, C. 1.: T h e Cornea, Charles C. Thomas Publisher, Springfield, U. S. A., 1955. Zobel, H. : Graefes Arch. Ophth. 1938: 139: 668.

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