The Night Vision Threshold Test (NVTT): A Simple Instrument for Testing Dark Adaptation in Young Children by Burris Duncana, Louise Canfieldb, Brent Barbera, John Greivenkampc, Francis O. Oriokotd, and Florence Naluyindad aDepartment of Pediatrics, College of Medicine and Children’s Research Center, College of Medicine, University of Arizona, USA bDepartment of Biochemistry, College of Medicine, and cThe College of Optical Sciences, University of Arizona, 1501 N Campbell Avenue, Tucson, AZ 85724, USA dDepartment of Pediatrics and Mwanamugimu Nutrition Unit, Makerere Medical School, Kampala, Uganda Summary It is estimated that 41 per cent of the population aged under 5 in the developing world has an inadequate vitamin A dietary intake resulting in increased morbidity and mortality. Half a million children go blind each year as a result of vitamin A deficiency. Thirteen and a half million have night blindness, the first sign of vitamin A deficiency. Unfortunately, there is no simple, sensitive and inexpensive means to identify the child who has marginal levels of vitamin A and thus institute means to prevent their development of severe deficiency. A low cost, simple, easy-to-use instrument designed to detect a young child’s ability to adapt to darkness was tested in children admitted to the Mwanamugimu Nutrition Unit at Makerere Medical School in Kampala, Uganda. Despite the severe degree of malnutrition found in these children, Night Vision Threshold Test results and serum retinol levels were related (r ¼ 0:41, p < 0:05). Further efficacy trials for this instrument are planned at community sites in Nepal. Introduction relatively inexpensive, their distribution is expensive and UNICEF estimates that 231 million children under the a means to target those communities where there is a age of 5 years or 41 per cent of the entire population documented significant prevalence of vitamin A defi- under 5 years old in the developing world have an ciency would be economically sound. However, all inadequate dietary intake of vitamin A. Thirteen and a currently available methods for the assessment of a half million children have night blindness, the first population for vitamin A deficiency have serious limitations: some require serum and expensive analysis clinical sign of vitamin A deficiency. Three million 3 children have xerophthalmia and 500 000 have severe of retinol levels; some lack sensitivity and specificity, 1 and others require considerable cooperation and are eye damage resulting in blindness. Vitamin A is the 4–10 most important cause of preventable childhood blindness inappropriate for young children. and represents one-third of all cases of childhood A history of night blindness remains the most blindness world-wide. More than one-half of these frequently used and the most reliable indicator for the earliest and mildest expression of clinical xerophthal- children die within a few months of becoming blind. In 11 addition, children with inadequate vitamin A status have mia. The history of night blindness, however, is based a 23 per cent greater risk of dying from common on the person’s perception; it does not determine the childhood illnesses, such as gastroenteritis, respiratory degree or severity of dark adaptation and its accuracy is diseases and measles.12 The efficacy of vitamin A highly questionable in the infant or very young child. supplementation for the young child with vitamin A A reliable, inexpensive, non-invasive, rapid test for deficiency in decreasing morbidity and mortality from assessing the prevalence of vitamin A deficiency in measles and measles-associated diarrhoeal, and respira- infants and young children is needed. Such a test would tory illness has been documented2 prompting vitamin A provide the means for the selection of at-risk individuals distribution programmes as a current strategy of the or communities to receive vitamin A supplementation WHO and UNICEF. Although vitamin A capsules are and nutrition educational programmes. It could be used for monitoring the effectiveness of vitamin A interven- tion programmes designed to restore vitamin A status in individuals and in their communities. In addition, most Correspondence: B. Duncan, Department of Pediatrics, Room 3335, University Medical Centre, 1501 N Campbell Avenue, previous techniques have targetted severely vitamin A Tucson, AZ 85724, USA. E-mail <[email protected]. deficient children, i.e., those who have developed or are edu>. developing clinical symptoms of vitamin A deficiency. 30 q Oxford University Press 2000 Journal of Tropical Pediatrics Vol. 46 February 2000 B. DUNCAN ET AL. This group represents about 2 per cent of the population 30 cm. A green LED is used to match the spectral of most developing countries. Unfortunately, the effects response of the eye. Since the light output of an LED is a of severe vitamin A deficiency are not reversible and function of the electrical current through the LED, the children at this stage often die despite supplementation. illuminance of the projected spot can be easily varied by There are, however, many more children with marginal connecting the LED to a battery through a series of vitamin A stores (0.35–0.7 mmol/l). In this group, resistors. A 12-position rotary switch was used allowing vitamin A deficiency is completely reversible. More the projected illuminance to vary from about 400 mlux importantly, vitamin A supplementation significantly down to about 0.1 mlux. Each switch position reduced the protects this group of children against respiratory and illuminance by a factor of about a half. A digital ammeter gastrointestinal infections which together are responsible with numeric display on the outside of the case is provided for a very large number of deaths of children world- to measure the electrical current through the LED and to wide.1 Thus, this study was designed to evaluate a monitor the light output while the system is in use. technique for identifying children with marginal vitamin A deficiency so that interventions can be Testing compliance implemented prior to the development of severe vitamin A game was designed and adapted after the method used A deficiency. Specifically, this study was designed to by audiologists for testing the hearing of young children. test the efficacy of an inexpensive, non-invasive, easy- In the case of hearing, a sound stimulus is activated in to-use instrument, the Night Vision Threshold Tester one corner of a sound-proof room and the hearing-abled (NVTT), to evaluate infants and young children for their child will turn to the place where the sound originates. In ability to adapt to darkness or for the presence of night the game we designed to test for dark adaptation; after blindness; the first clinical sign of vitamin A deficiency. sitting in a dark room for 10 min, a light is directed to one corner of the room and scanned from one side to the other. If the child can see the light, he/she should follow Materials and Methods it and light perception can be determined by observing Instrument design the child’s head turn in response to the movement of the The design of the instrument is shown in Fig. 1. Its use is light. based on the premise that the function of the rods in the Twenty-four children between the ages of 7 and 45 retina, the eye’s ability to adapt to darkness, is a reflection months attending the Pediatric Clinic at the University of of the body’s retinol stores. The instrument is contained in Arizona were evaluated to determine if young children a plastic box approximately 12 × 18 × 5 cm. A light- would play this ‘game’. Four (17 per cent) could not be emitting diode (LED) is imaged and magnified by a tested because of inattention or being afraid of the dark. simple lens to project a round uniform circle of light on The remaining children (83 per cent), 13 (65 per cent) of the wall of a darkened room. The system was designed for whom were less than 24 months of age, co-operated with operation at 3 m that produces an illuminated spot of about the test procedure. Fig. 1. The Night Vision Threshold Tester (NVTT). Journal of Tropical Pediatrics Vol. 46 February 2000 31 B. DUNCAN ET AL. Fig. 2. Light perception of subjects with retinitis pigmentosa. Results of SSTDA as compared with light intensity from the NVTT instrument. Standardization of the NVTT Dietary information Nine subjects between the ages of 8 and 61 months with Both a 24-h and a 7-day food frequency dietary history, retinitis pigmentosa (RP) were tested for night blindness including an estimate of serving size, were obtained with the NVTT and the results compared with those from the child’s mother by the field research team. A obtained using the more sophisticated SST Dark qualitative analysis of the dietary data was performed to Adaptometer (SSTDA). RP is a condition that affects describe the dietary pattern for the study population. the rods of the retina and in its early phases causes night blindness. Each eye of nine subjects was tested Determination of the night vision threshold individually. Twelve of the 18 eyes tested were not The child sits on the mother’s lap in a completely dark able to detect light from the highest intensity emitted room facing a wide strip of white paper attached to the from the SSTDA but could detect the light beam when wall 3 m in front of the subject. The observer stands shown through the brightest light of our instrument, as it behind the mother and child and, while waiting for dark was far brighter than the brightest light emitted from adaptation to occur, discusses the importance of vitamin the SSTDA. The varying levels of light intensity detected A and how to enrich the child’s diet with it.