Br J Ophthalmol: first published as 10.1136/bjo.65.11.754 on 1 November 1981. Downloaded from British Journal ofOphthalmology, 1981, 65, 754-759 The pupillary light reflex in normal subjects C. J. K. ELLIS From St Thomas's Hospital, London SE] SUMMARY In 19 normal subjects the pupillary reflex to light was studied over a range of stimulus intensities by infrared electronic pupillography and analysed by a computer technique. Increasing stimulus intensity was associated with an increase in direct light reflex amplitude and maximum rate of constriction and redilatation. Latency from stimulus to onset of response decreased with increas- ing stimulus intensity. The normal range for each of these parameters is given and the significance of these results in clinical pupillary assessment discussed. The technique of infrared pupillometry' has allowed PUPILLOMETRY the normal pupillary response to light to be studied in A Whittaker Series 1800 binocular infrared television detail. Lowenstein and Friedman2 have shown that pupillometer was used in this study. All recordings in response to light the pupil constricts after a latent were made in darkness with no correction for refrac- period and that the length of this latent period, the tive error. The eyes were illuminated from a low- copyright. amplitude of the response, and the speed of the intensity, invisible infrared source and observed by pupillary constriction are dependent on the stimulus means of a closed circuit television system sensitive to intensity employed. These findings have subse- infrared light. The pupils were displayed on television quently been confirmed.3" monitor screens providing instantaneous feedback of Borgmann6 gave 95% confidence limits in defining the quality of the pupil images. The pupillometer the effect of age on reflex amplitude and constriction system incorporated circuitry that recognised the and redilatation velocity. There has been no study in maximum vertical pupillary diameter and placed a which a statistical analysis of the normal pupillary white crescent on one edge of the pupil. The correct light reflex over a range of stimulus intensities has positioning of this crescent confirmed that measure- http://bjo.bmj.com/ enabled the definition of the normal light reaction. ments of pupillary diameter would be accurate and The introduction of computer analysis of averaged without interference from eyelids or iris. The pupillary reflexes has allowed a more accurate estima- maximum vertical pupillary diameter of both eyes tion ofselected parameters ofthe pupillary reaction to was displayed on a dual strip chart recorder and also light. This technique has been used in a study of recorded on magnetic tape for computer analysis. anisocoria in normal subjects.7 This study reports the The maximum vertical pupillary diameter was methodology and results from a series of 19 normal measured by the pupillometer system by counts of the on September 30, 2021 by guest. Protected subjects studied over a range ofstimulus intensities by number of television lines intersected by the recog- infrared pupillometry and defines a normal range in nised pupil. This system incorporated 625 lines, and these subjects. the estimated precision in measurements of pupil diameter was approximately 0-03 mm. The pupil- Materials and methods lometer imaged the pupil every 20 ms. The pupillometer was calibrated by recording 2 CONTROL SUBJECTS model pupils the diameters of which were measured Nineteen healthy subjects were investigated by infra- with a travelling microscope to be 2-20 mm and red pupillometry. They were aged 18 to 48 years, and 7*90 mm. These 2 diameters were chosen as they lay 12 were female. Their corrected visual acuity was at either end of the range ofpupil diameter commonly normal in every case. In one case there was anisome- encountered. The 2 measured diameters were given tropia of uncorrected vision. to the computer. In addition 2 standard voltage cali- Correspondence to Dr C. J. K. Ellis. King's College Hospital. Den- brations approximately equivalent to the 2 model mark Hill, London SE5. pupil diameters were recorded on the data tape. As 754 Br J Ophthalmol: first published as 10.1136/bjo.65.11.754 on 1 November 1981. Downloaded from The pupillary light reflex in normal subjects 755 LRtt J A U61 At (a) Fig. I (a) Train of 10 pupillary responses elicited by stimulation of to I the right eye. Downward deflections 4-1 indicate decrease in pupillary diameter. Stimuli are delivered DATE STINULUS every 8 seconds. (b) Example of 6 A0E 26 R SibE computer analysis ofthe averaged responsefor (a). 2 '4 6; 616L FINALE S SEC pupillary 4.- Da. 3 F/ 5. 6 For explanation see text. -2 - S RESPONSES 7 savYSo LEFT RIONT copyright. NAmNITUPE MN 37 .3S 4 6 If 2 RESTINo mN 69 6.o (b) Is 7 4-1 LATENCY NS 367 274 'I- R6 - 2' -3 -3. 6 RATEmxNNS . 1. isS1 1. 23 2 4 -6- '1I'5 4- -2 http://bjo.bmj.com/ a' -4 - 2 4 6 616 part of the calibration programme the computer gave was adopted with the head supported on a chin rest numerical values for these standard voltage cali- and the eyes fixating a distant, dim red light by means brations. These values could be compared on every of a mirror system. The surroundings were quiet, at a on September 30, 2021 by guest. Protected occasion recordings were made. This allowed constant temperature, and the technique was fully detection offluctuations in tape speed orpupillometer explained to the subject before recording. gain at any recording session throughout the period of A 100 ms white light stimulus converging to a the study. In addition the electrical calibrations were 2-0 mm beam at the midpupillary point, was delivered repeated during the recording session before each run 7°31' temporal to the visual axis every 8 seconds, of pupillary reflexes was elicited. In this way the cali- thus impinging on the nasal retina. Two Sylvania bration was repeated every time the tape recorder tungsten concentrated arc lamps were used as the was started. light source giving an intensity of approximately 550 The output from the pupillometer was recorded on candles/m2 at the pupil. The stimulus length was a 4-channel Yasec CD 110 cassette data tape recorder adjusted on the pupillometer photostimulator which with separate channels for each eye, stimulus events, controlled the 2 Uniblitz electronic shutters, move- and a reference channel to compensate for variations ments of which gave the stimulus almost square-wave in tape speed during replay. characteristics. Movements of the shutter were also All subjects were dark-adapted for 30 minutes in recorded on the stimulus channel of the data tape. red goggles before recording. A comfortable position The intensity of the 2 stimulus lamps was regularly Br J Ophthalmol: first published as 10.1136/bjo.65.11.754 on 1 November 1981. Downloaded from 756 C. J. K. Ellis direct light reflex latency (ms) amplitude (un) 500 r. 3 r 400 2 300 V I 200 - 0 1 2 3 4 5 5.3 stimulus intensity (log units) 100 L Fig. 2 The relationship between direct lightreflex amplitude and stimulus intensity (Y= -0 12+0 66 X -0 04 X2). The 2 3 4 5 53 lines indicate the mean and 95% confidence limits. The black dots are the valuesfrom the left eye ofthe 19 subjects. stimulus intensity (log units) Fig. 3 The relationship between latency and stimulus intensity (Y=445-7-22-9 X +76-2 X2). The lines indicate measured with a photometer to ensure equal stimulus the mean and 95% confidence limits. The black dots are the intensity in the 2 eyes. Six light intensities were valuesfrom the left eye ofthe 19 subjects. employed, variation in stimulus intensity being copyright. achieved by filtering the 2 stimulus lamps with neutral amplitude, and latency from stimulus to onset of density filters. Between 6 and 10 stimuli were de- response. The second graph shows rate of change of livered at each level of light intensity. pupillary diameter in mm/s against time in seconds. A pause between increments of stimulus intensity From this plot derive the maximum rates of constric- allowed dark adaptation to be maintained. Care tion and dilatation. was taken to ensure that the nonstimulated eye was kept in darkness. If stray light spread to this eye, STATISTICAL ANALYSIS recordings were repeated with the nonstimulated eye The relationships between the measured parameters http://bjo.bmj.com/ patched. of the light reflex were expressed mathematically by linear regression analysis or second order polynomials COMPUTER ANALYSIS as appropriate, by means of standard techniques. The The data tape was replayed into a Varian V72 digital normal range was defined as the mean+2 SD. computer with compensation for variations in tape speed by utilising a reference input channel. Before Results processing, the analog output of the pupillometer was converted to digital form. The computer was pro- In all subjects the averaged pupillary response was of on September 30, 2021 by guest. Protected grammed to average the pupillary responses at each similar shape (Fig. lb), showing that pupillary con- level of stimulus intensity and to give numerical striction occurred after a latent period after stimula- values for several parameters of this averaged tion. After constriction, pupillary redilatation response. An example of the computer print-out is occurred, and the prestimulus resting diameter was shown in Fig. 1. In (a) the paper record of a train of 10 reached. The rate of pupillary constriction increased pupillary reflexes elicited by stimulation of the right rapidly to a maximum and then decreased progres- eye is shown. In (b) the computer analysis of the sively until the redilatation phase ensued. Similarly average of these responses is shown.