scotopic, or rod, vision, and precise information about the photochemical

Transcription

1 256 J. Physiol. (I94) IOO, I :6I I I AN INVESTIGATION OF SIMPLE METHODS FOR DIAGNOSING VITAMIN A DEFICIENCY BY MEASUREMENTS OF DARK ADAPTATION BY D. J. DOW AND D. M. STEVEN From the Department of Biochemistry, Oxford (Received 12 May 1941) MEASUREMENTS of the speed and amount of dark adaptation have been used for a number of years to estimate the level of vitamin A nutrition of persons, and to detect states of deficiency. All such tests are based on the fact that vitamin A is commonly the critical factor in the cure of nutritional night-blindness in man and animals [Fridericia & Holm, 1925; Tansley, 1931; Wald, Jeghers && Arminio, 1938; Wald & Steven, 1939; Hecht & Mandelbaum, 1939, 1940], and also on the part played by it in the photochemical cycle of visual purple in the rod cells of the retina [Wald, 1935]. Night-blindness is characteristically a defect of scotopic, or rod, vision, and precise information about the photochemical processes of the retina, in which vitamin A is involved, is at present available for the rods only. Many of the methods that have been devised in recent years for the clinical diagnosis of vitamin A deficiency are inadequate, since certain precautions that are necessary in the present state of knowledge of visual processes have been ignored. The aim of many of these clinical tests has been to obtain convincing results rapidly, and therefore the measurements of visual threshold have been made during the early stages of dark adaptation, following light adaptation for various periods to various brightnesses of illumination. A method that has commonly been used, which may be referred to as the 'recovery time' type of test, is to measure the time in seconds that the subject requires in total darkness in order to perceive a test field of constant brightness, following a short period of light adaptation [Pett, 1939; Goss, Farmer & McFarlane, 1941; Steele, 1940; Haines, 1938; Gridgeman & Wilkinson, 1938; Jeans & Zentmire, 1934]. An abnormally long recovery time is taken to

2 VITAMIN A DEFICIENCY AND DARK ADAPTATION 257 indicate a state of vitamin A deficiency, and if the result of therapy is to shorten the time appreciably, the diagnosis is said to be confirmed. In tests of this type the particular part of the retina stimulated is rarely held constant in any way, so that any given reading may' be obtained by stimulation of any part of the retina. As the rod-cone composition of the retina varies greatly throughout its sensitive area, cones only being present at the fovea and almost exclusively rods at the periphery, and as the dark adaptation characteristics of rods and cones differ in important respects, it is important in all measurements to stimulate a defined area, in which the proportions of the two types of sensitive elements is known at least approximately. Measurements of the early stages of dark adaptation, following even a short period of light adaptation, mainly record the initial adaptation of the cones, and the relation of vitamin A to the photosensitive pigment of the cones is still unknown. The most sensitive single index available of the state of vitamin A nutrition is the final threshold of the rods [Hecht & Mandelbaum, 1939, 1940; Wald et al. 1938; Steven & Wald, 1941], to obtain which requires at least 30 min. of dark adaptation, and which can be measured accurately by the adaptometer method. Although cone adaptation seems also to be affected during states of vitamin A deficiency, the derangement is usually less than that of the rods, and the photochemical basis remains unknown. The following tests were therefore made in order to investigate consequently the relation between 'recovery times' and the final threshold of rod vision. EXPERIMENTAL The dark adaptation characteristics of two well-nourished subjects in good health were determined with an adaptometer. The method used was that described by Steven & Wald [1941] and Wald [1941]. Readings of the visual threshold of each subject were made on a circular retinal field, 20 27' in diameter, 60 above the fovea. This area was exposed to flashes of 1 sec. duration. The colour of the test flash was white and readings were binocular. The complete course of dark adaptation was recorded for 40 min. in darkness, following an initial period of light adaptation for 3 min. to a screen of a brightness of approximately 2000 millilamberts. The readings of visual threshold were recorded in micro-millilamberts (10-9 lamberts), and were plotted on logarithmic scale. The adaptometer, which has been described in detail by Wald [1941], consists of a box containing batteries and a control panel, and a test unit. The latter is built from a section

3 258 D. J. DOW AND D. M. STEVEN of brass tubing about 4 in. long and 2 in. in diameter. Mounted in the base is the test lamp, which is on a potentiometer circuit, and which is calibrated photometrically. This illuminates the test field of opal glass, which is at the other end of the tube, for flashes of - sec. duration, through a camera shutter mounted in the central portion. The fixation lamp is on a separate circuit, and is mounted in a small housing on top of the test unit. It shines through a pinhole aperture, over which is a deep red filter (Wratten no. 70). Attached in front of the test unit is a funnel-shaped mask, which holds the head of the subject in the correct position, 12 in. from the test field. The relative positions of the subject's eyes, the test field and the fixation point are such that the test field subtends an angle on the retina of 20 27', 60 above the fixation point. The whole test unit is mounted on an adjustable camera tripod. The test unit is connected with the box containing the batteries and control panel by a length of light, insulated wire. On the control panel are switches for the test and fixation lamp circuits, a resistance attached to a scale for varying the brightness of the test lamp, and a second resistance and voltmeter for setting the line voltage of the test circuit. To make a reading of the visual threshold the subject is seated comfortably with his face against the mask and his eyes focused on the red fixation point. The operator illuminates the test field by clicking the camera shutter. Starting with the brightness of the test lamp at a subthreshold level, he increases it by a small regular amount between each exposure until the subject reports seeing it. The time and scale reading of brightness are then noted. The increase in brightness of the test field between successive exposures is insignificant, and a high degree of accuracy is therefore obtained in each measurement of the visual threshold. After a few trial readings the great majority of subjects are found to give data of a high degree of reproducibility, 'learning' effects being practically eliminated by this method. By repeated tests during a period of several months both subjects were found to possess stable values of final rod adaptation which were identical, namely log mul. During part of this period they received a daily supplement of approximately 9000 i.u. of vitamin A, and these values may therefore be considered their minimal rod thresholds. On certain days the adaptometer was modified to give a test of the 'recovery time' type. The subject was light-adapted to the screen of brightness 2000 millilamberts for a period of 30 sec., and the number of seconds required subsequently in total darkness to perceive the test field of 20 27' diameter, and of a constant brightness of 3-5 log m,ul., was measured. As in the majority of such tests no attempt was made to fixate the fovea of the subject. Readings were made in groups of three, with a rest of 3 min. between each reading, over a period of 4 weeks. No 'learning' effect was detected in either subject. It was found that D. S. required consistently a longer period to perceive the test field than D. D., and that the range of variability of both subjects was considerable. The results of fifty successive readings from these two subjects are analysed in Table 1. The difference between the mean 'recovery times' is considerably greater than the sum of the standard errors, and may therefore

4 VITAMIN A DEFICIENCY AND DARK ADAPTATION 259 be considered to show a real difference in the initial speeds of dark adaptation of these two subjects under these conditions. TABLE 1. Comparison of fifty successive 'recovery time' readings for two subjects Recovery time in seconds Standard Standard Subject Mean Max. Min. deviation error D. S. 31* TF6*35 T D. D. 20* T9-60 -T1359 It was considered possible that the speed of dilatation of the pupils during the first few seconds of dark adaptation might 'be a limiting 0..Q 0. o 0 *t -5 qr S 20 Minutes in darkness Fig. 1. Subject D. D. The result of one experiment showing the course of dark adaptation of a retinal field 20 27' in diameter, following 3 min. light adaptation at 2000 millilamberts, recorded without a fixation light (filled-in circles) and with the test field fixated 6 above the fovea (empty circles). factor, which would account for the difference between the 'recovery times' of the two subjects, and perhaps also for the large variabilities. The pupil diameters were measured therefore in minimal red light with

5 260 D. J. DOW AND D. M. STEVEN a Cruise Scotometer immediately after each reading. Both subjects gave a diameter of 5-6 mm. Independent measurements on D. S. showed that the pupils had opened to this diameter after sec. in darkness, at the time when D. D. commonly gave the 'recovery time' reading. A difference in the speed of dilatation of the pupils cannot therefore be the factor determining the difference in the 'recovery times' of these two subjects. The effect of the lack of a fixation point was investigated in the following way. The course of dark adaptation of both subjects was recorded by the adaptometer method for a period of 40 min. in darkness, following a 3 min. period of light adaptation, first without any fixation point and then with the test field fixated 6 abcve the fovea. Both subjects gave the same type of results at two sittings made on different days. The results of a single experiment on D. D. are shown in Fig. 1. It is obvious that the recording of dark adaptation is more accurate when the test field of the retina is fixated. The data follow a more steady course, and there is less variability between successive readings. It will be noted also that the readings made without a fixation light indicate in general a poorer level of dark adaptation, which is most apparent in that part of the curve where adaptation of the rods is taking place. This is probably due to the fact that many of the readings made without a fixation point are perceived on the fovea or very close to it, in which part of the retina rods are scarce or totally absent. These curves show clearly that it is important to define the area of the retina tested in order to obtain accurate data at any stage of dark adaptation. DISCUSSION It has been shown that for subjects D. S. and D. D. the 'recovery times' required to perceive a field of constant brightness is significantly different, whereas their final rod thresholds are identical and their level of vitamin A nutrition optimal. There is no ground therefore for stating that the longer 'recovery time' of D. S. indicates a poorer state of vitamin A nutrition than D. D. It is emphasized also that the initial speed of dark adaptation bears no necessary relation to the final level of adaptation of the rods, which is the most sensitive index of vitamin A deficiency, although there is probably a general correlation between these two functions [unpublished observations]. Hecht & Mandelbaum [1939] found a partial correlation of 0 44 between the rod and cone threshold values of 110 subjects in good health.

6 VITAMIN A DEFICIENCY AND DARK ADAPTATION 261 It is concluded therefore that although a test of the 'recovery time' type will probably detect a severe case of night-blindness, such tests are not sufficiently sensitive to give unequivocal data on the course of dark adaptation. This is due partly to the fact that the early stages of dark adaptation are the least affected in states of nutritional deficiency and are the most difficult to measure precisely [Hecht & Mandelbaum, 1940], and partly to the lack of a fixation point to define the position of the retinal field tested. On theoretical grounds moreover it is inadvisable to base a test for vitamin A deficiency on single readings made during that part of dark adaptation which is concerned with the cones, since the relation of vitamin A to the photosensitive pigments of the cones is obscure. In certain circumstances a 'recovery time' test may be of use. It may be important to measure the initial speed of dark adaptation of aviators, seamen or other nocturnal workers. Such tests may be of value in these cases, especially when the complete course of dark adaptation is recorded also by an accurate method, but they must be considered ad hoc tests of the speed of dark adaptation and not a method of diagnosing vitamin A deficiency. SUMMARY 1. The complete course of dark adaptation and the final level of adaptation of the rods of two well-nourished healthy subjects has been determined, and found to be the same. 2. On the other hand a 'recovery time' type of test, which has been done on each subject fifty times, shows a significant difference between them. 3. The significance of these findings is discussed in relation to vitamin A and the visual cycle, and it is concluded that tests of the 'recovery time' type are of little value for detecting states of vitamin A deficiency. We wish to thank Dr H.M. Sinclair for reading the manuscript and for many helpful suggestions. One of us, D. M. S., wishes also to acknowledge a grant from Magdalen College, Oxford, in aid of this work.

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