Eye (1991) 5, 138-144 Nyctalopia With Normal Rod Function: A Suppression of Cones by Rods F. M. FALCAO-RBIS,* C. R. HOGG, T. E. FRUMKESt and G. B. ARDEN London Summary 1\venty-nine patients with exaggerated rod-cone interaction are described. All were referred because they appeared to suffer from night blindness. ERG and EOGs were performed but were normal. However, investigation with a modified dark-adapt­ ometry technique showed that in these patients cone flicker thresholds rise consider­ ably more during dark adaptation than is normal, and this is sufficient to explain the symptoms. In one case, the condition appears familial. Many patients report their symptoms begin in early adult life and slowly get worse, but we have no objective evi­ dence of progression. Symptoms of impaired visual adaptation are ocular or systemic abnormality. In some, the known as hemeralopia and nyctalopia. Hem­ complaint is hysterical, but in others there eralopia or 'day blindness' is characterised by seems to be a true difficulty in seeing at night a defective cone function. Nyctalopia or and in these pathophysiological disturbance 'night blindness', is a common symptom in must be sought. The dark adaptation curve is many conditions which affect rods. It is the the usual way of showing changes in the sensi­ hallmark of retinitis pigmentosa, although it is tivity of the eye.1 Following intense light not unusual to find RP patients without sub­ adaptation, threshold is determined at vari­ jective nyctalopia, even in adult life. It also ous times in the dark. Log thres�old light occurs, in a non progressive form, in the vari­ intensity, plotted against time of dark adap­ ous forms of congenital stationary night blind­ tation gives a curve with two distinct ness. Other causes include extreme myopia of branches. First, the cones recover sensitivity more than 12-15 dioptres, which is associated over one or two minutes. At this time the rods with retinal degeneration. Endstage glau­ are still very insensitive and threshold remains coma results in nyctalopia, when the remain­ more or less constant for 9 to 12 minutes. Fol­ ing field is so small that rod sensitivity is lowing this, continuing recovery of the rods reduced. Nyctalopia due to vitamin A defi­ causes a second slower decrease in threshold ciency is, today, associated with chronic which lasts for 30-40 minutes. For clinical alcoholism and malabsorption syndromes. work, preadapting and stimulus conditions However, it has been recognised for a long are standardised, and commonly only white time that many patients complaining of not light is used and a position 10 to 20 degrees seeing well at night fail to demonstrate any from fixation is tested. To separate out rod From: 'Department of Ophthalmology, Oporto Medical School. Av Prof Hernani Monteiro, Oporto, Portugal. tDepartment of Psychology. CUNY, New York, USA. From: Department of Clinical Ophthalmology, Institute of Ophthalmology, and Electrodiagnostic Department, Moorfields Eye Hospital. Correspondence to: Electrodiagnostic clinic, Moorfields Eye Hospital, City Road, EC1 V 2PD London, UK (Prof. Arden) . NYCTALOPIA WITH NORMAL ROD FUNCTION 139 from cone contributions, one can use red and description of this syndrome, we have tested blue light. However, because the scotopic for it routinely and in this paper report on 29 system becomes extremely sensitive as time cases in whom the exaggerated rod-cone goes by, at the end of dark adaptation, the rod interaction was the only abnormality to system responds to red flashes which are so explain their disability. dim that they cannot be detected by the cone system. At the end of dark adaptation, to get a Material and Methods response derived solely from the excitation of Patients were seen at the Electrodiagnostic cones, it is necessary to exploit another differ­ Clinic, Moorfields Eye Hospital, to investi- ence between the classes of receptors, and it is . gate what had been considered to be a night convenient that rods cannot follow rapid blindness problem or to clarify situations alternation of light and dark. A flicker of where a particular finding on the fundoscopy 15 Hz or greater, especially if the stimulus or dubious family history raised suspicion of a wavelength is> 600 nm, must be perceived by heredo degenerative disorder. They were cones. Thus, the time course of dark-adap­ investigated by standard clinical techniques tation of cones and rods can be measured sep­ such as EOG6 and the ERG,7 the latter using arately using red flickering light and single an extended protocol. In addition, dark flashes of short wavelength light. One there­ adaptometry had been carried out by a com­ fore could expect that after the rod-cone puterised instrument, which enables cone and break (the knee in the dark adaptation curve) rod thresholds to be determined separately. cone flicker threshold would remain constant until the end of dark-adaptation. This is not EOG the case: at around the time of the rod-cone Electrodes were attached to the skin over­ break the cone flickerthreshold begins to rise. lying the bony margins of the orbit opposite This phenomenon reflectsa suppressive inter­ the lateral and medial canthi. A ground elec­ action between the scotopic and photopic ret­ trode was placed on the forehead. Patients inal systems.2-4 made a number of voluntary horizontal eye The interaction, however, is absent, if the movements between two fixation spots, sep­ frequency of the red flicker is decreased as is arated by 30 degrees. This was repeated every clearly seen in Figure 1 which shows a dark­ minute. Readings were taken for 12 minutes adaptation curve in a person who has normal with lights off and 8 minutes with lights on. rod-cone interaction. Under our test condi­ Potentials were amplified in a Medelec MS6, tions the increase in flicker threshold during with a bandpass of 0.5-50 Hz and printed. the rod phase of dark adaptation is about The ratio light peak/light trough 100 was X 0.6-1 log unit. Some perons however, have an calculated. exaggerated effect, ie the flicker threshold rises by more than 1 log unit. 5 In such persons, ERG it is common to findthe complaint of nyctalo­ Earth and reference electrodes were 10 mm pia, even though the rod threshold is perfectly silver-silver chloride discs which were placed normal. On close questioning, such persons on the forehead. The active electrode was a describe a diminution in their visual abilities gold foil electrode placed in the lower fornix at night, but the functions they lose are those usually without anaesthesia. A Grass PS22 mediated by cones, not by rods, and specific­ stroboscope was used to back illuminate a ally, they complain of difficulty in driving. rectangle of 3 mm thick opal Perspex placed They may complain of glare, but frequently 4 cm from the patient's nose. The diffusionof say that they cannot see the kerb markings or light through the perspex presents a uniform traffic lights. The difficulties are so great that field of so large an extent that it can be con­ none of these patients will drive at night. By sidered a Ganzfeld. Between the stroboscope contrast patients with real nyctalopia (eg and the diffuse screen a number of coloured caused by retinitis pigmentosa) are reluctant or neutral filters were inserted, which to stop driving at night even when advised to together with the controls on the stroboscope do so by their ophthalmologist. Since the first unit provided a range of light intensities over F. M. FALCAO-REIS ET AL. 140 SUPPRESSIVE ROD-CONE INTERACTION NORMAL OBSERVER 6 RED FLICKER (15 Hz) 0 BLUE FLICKER ... RED (APPEARANCE) • BLUE (APPEARANCE) o o -1 6 00 6 6 6 6 !:;, !:;, !:;,6 !:;, o 0 6!:;,6 - - -- DO� !:;, ---- ------------------------------!:;, �� £5� 6� !:;, ..... -2 CJ 0...J o ..... o ...J0 -3 0 0 0 J: ... ... en o ILl -4 a:: J: I- o 00 - 5 • o • • - 6 0 10 20 30 40 TIME (MINUTES) Dark adaptation curves obtained from a subject showing normal rod-cone interaction. For full description Fig. 1. see text. Note that after the'knee' the red flickerthreshold rises. The dashed line shows the threshold expected in the absence of rod-cone interaction. 4.4 logs units. Records were obtained with a added computer control was used. The tech­ Medelec MS 6 polygraph system, which nique has been described elsewhere.8 Since includes an artifact rejection system. Poten­ that publication, the green LEOs have been tials were amplified 10,000, with a band­ replaced by blue (Siemens, peak emission X pass 0.16 Hz to 1.6 KHz. Between 16 and 64 480 nm) and an infra-red TV camera has been responses were averaged. Patients' pupils installed to monitor patient fixation. A one were dilated with one per cent cyclopentolate, degree red (660 nm) or blue (480 nm) stimu­ and they were dark adapted at least for 20 lus can be placed anywhere in the visual field. minutes with dark-room goggles. A further Rod-cone interaction was measured at 10° time, 10 min, was spent in complete dark­ nasally in the horizontal meridian. A fixation > ness before recordings began. spot was provided by a dim red LED. A series of intensities of blue, red and white Patients' pupils were dilated by instillation flasheswere given. The repetition rate varied of one per cent cyclopentolate and light with the flash intensity but was never more adapted by examining them for 30 seconds frequent than lIsec. The a- and b-waves, with the full intensity of an indirect ophthal­ oscillatory potentials and cone flicker moscope with a 200 lens, a technique pre­ responses were analysed. The large range of viously described.