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
a
Department of Pediatrics, College of Medicine and Children’s Research Center, College of Medicine, University of
Arizona, USA
b
Department of Biochemistry, College of Medicine, and c The College of Optical Sciences, University of Arizona, 1501 N
Campbell Avenue, Tucson, AZ 85724, USA
d
Department 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
UNICEF estimates that 231 million children under the
age of 5 years or 41 per cent of the entire population
under 5 years old in the developing world have an
inadequate dietary intake of vitamin A. Thirteen and a
half million children have night blindness, the first
clinical sign of vitamin A deficiency. Three million
children have xerophthalmia and 500 000 have severe
eye damage resulting in blindness.1 Vitamin A is the
most important cause of preventable childhood blindness
and represents one-third of all cases of childhood
blindness world-wide. More than one-half of these
children die within a few months of becoming blind. In
addition, children with inadequate vitamin A status have
a 23 per cent greater risk of dying from common
childhood illnesses, such as gastroenteritis, respiratory
diseases and measles.12 The efficacy of vitamin A
supplementation for the young child with vitamin A
deficiency in decreasing morbidity and mortality from
measles and measles-associated diarrhoeal, and respiratory illness has been documented2 prompting vitamin A
distribution programmes as a current strategy of the
WHO and UNICEF. Although vitamin A capsules are
Correspondence: B. Duncan, Department of Pediatrics, Room
3335, University Medical Centre, 1501 N Campbell Avenue,
Tucson, AZ 85724, USA. E-mail <[email protected].
edu>.
30
q Oxford University Press 2000
relatively inexpensive, their distribution is expensive and
a means to target those communities where there is a
documented significant prevalence of vitamin A deficiency would be economically sound. However, all
currently available methods for the assessment of a
population for vitamin A deficiency have serious
limitations: some require serum and expensive analysis
of retinol levels; some lack sensitivity and specificity,3
and others require considerable cooperation and are
inappropriate for young children.4–10
A history of night blindness remains the most
frequently used and the most reliable indicator for the
earliest and mildest expression of clinical xerophthalmia.11 The history of night blindness, however, is based
on the person’s perception; it does not determine the
degree or severity of dark adaptation and its accuracy is
highly questionable in the infant or very young child.
A reliable, inexpensive, non-invasive, rapid test for
assessing the prevalence of vitamin A deficiency in
infants and young children is needed. Such a test would
provide the means for the selection of at-risk individuals
or communities to receive vitamin A supplementation
and nutrition educational programmes. It could be used
for monitoring the effectiveness of vitamin A intervention programmes designed to restore vitamin A status in
individuals and in their communities. In addition, most
previous techniques have targetted severely vitamin A
deficient children, i.e., those who have developed or are
developing clinical symptoms of vitamin A deficiency.
Journal of Tropical Pediatrics
Vol. 46
February 2000
B. DUNCAN ET AL.
This group represents about 2 per cent of the population
of most developing countries. Unfortunately, the effects
of severe vitamin A deficiency are not reversible and
children at this stage often die despite supplementation.
There are, however, many more children with marginal
vitamin A stores (0.35–0.7 mmol/l). In this group,
vitamin A deficiency is completely reversible. More
importantly, vitamin A supplementation significantly
protects this group of children against respiratory and
gastrointestinal infections which together are responsible
for a very large number of deaths of children worldwide.1 Thus, this study was designed to evaluate a
technique for identifying children with marginal
vitamin A deficiency so that interventions can be
implemented prior to the development of severe vitamin
A deficiency. Specifically, this study was designed to
test the efficacy of an inexpensive, non-invasive, easyto-use instrument, the Night Vision Threshold Tester
(NVTT), to evaluate infants and young children for their
ability to adapt to darkness or for the presence of night
blindness; the first clinical sign of vitamin A deficiency.
Materials and Methods
Instrument design
The design of the instrument is shown in Fig. 1. Its use is
based on the premise that the function of the rods in the
retina, the eye’s ability to adapt to darkness, is a reflection
of the body’s retinol stores. The instrument is contained in
a plastic box approximately 12 × 18 × 5 cm. A lightemitting diode (LED) is imaged and magnified by a
simple lens to project a round uniform circle of light on
the wall of a darkened room. The system was designed for
operation at 3 m that produces an illuminated spot of about
30 cm. A green LED is used to match the spectral
response of the eye. Since the light output of an LED is a
function of the electrical current through the LED, the
illuminance of the projected spot can be easily varied by
connecting the LED to a battery through a series of
resistors. A 12-position rotary switch was used allowing
the projected illuminance to vary from about 400 mlux
down to about 0.1 mlux. Each switch position reduced the
illuminance by a factor of about a half. A digital ammeter
with numeric display on the outside of the case is provided
to measure the electrical current through the LED and to
monitor the light output while the system is in use.
Testing compliance
A game was designed and adapted after the method used
by audiologists for testing the hearing of young children.
In the case of hearing, a sound stimulus is activated in
one corner of a sound-proof room and the hearing-abled
child will turn to the place where the sound originates. In
the game we designed to test for dark adaptation; after
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
it and light perception can be determined by observing
the child’s head turn in response to the movement of the
light.
Twenty-four children between the ages of 7 and 45
months attending the Pediatric Clinic at the University of
Arizona were evaluated to determine if young children
would play this ‘game’. Four (17 per cent) could not be
tested because of inattention or being afraid of the dark.
The remaining children (83 per cent), 13 (65 per cent) of
whom were less than 24 months of age, co-operated with
the test procedure.
Fig. 1. The Night Vision Threshold Tester (NVTT).
Journal of Tropical Pediatrics
Vol. 46
February 2000
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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
Nine subjects between the ages of 8 and 61 months with
retinitis pigmentosa (RP) were tested for night blindness
with the NVTT and the results compared with those
obtained using the more sophisticated SST Dark
Adaptometer (SSTDA). RP is a condition that affects
the rods of the retina and in its early phases causes
night blindness. Each eye of nine subjects was tested
individually. Twelve of the 18 eyes tested were not
able to detect light from the highest intensity emitted
from the SSTDA but could detect the light beam when
shown through the brightest light of our instrument, as it
was far brighter than the brightest light emitted from
the SSTDA. The varying levels of light intensity detected
by each eye of the other twelve subjects (six eyes) had a
linear relationship with the amount of light emitted from
our instrument and that from the SSTDA (Fig. 2).
Malnourished children
Forty-eight children between the ages of 6.7 and 67
months were enrolled in the study in Uganda. Fortytwo had been admitted to the Mwanamugimu Nutrition
Unit, Makerere Medical School and Mulago National
Referral Hospital in Kampala and the other six were
recruited from an ophthalmology clinic in a neighbouring community. Demographic information and clinical
and dietary histories were obtained from the mothers.
The children were examined, anthropometric measures
taken and the dark adaptation test was done as described
below. A blood sample was obtained and the child was
given a vitamin A supplement.
32
Dietary information
Both a 24-h and a 7-day food frequency dietary history,
including an estimate of serving size, were obtained
from the child’s mother by the field research team. A
qualitative analysis of the dietary data was performed to
describe the dietary pattern for the study population.
Determination of the night vision threshold
The child sits on the mother’s lap in a completely dark
room facing a wide strip of white paper attached to the
wall 3 m in front of the subject. The observer stands
behind the mother and child and, while waiting for dark
adaptation to occur, discusses the importance of vitamin
A and how to enrich the child’s diet with it. After 10 min
in absolute darkness, the tester then turns on the light
source from the NVTT and directs it at the white strip of
paper to the child’s right or left and scans the light from
one side to the other. Observing movement of the child’s
head is facilitated by a small cap decorated with ‘glowin-the-dark’ stars that the child wears. The intensity of
the light is decreased until the child no longer follows the
light and that level of intensity is noted as the level of
dark adaptation for the child.
Vitamin A supplementation
Each child was administered the contents of a capsule
containing vitamin A as retinyl palmitate in oil supplied
by the Helen Keller Foundation. The dosage was 100 000
International Units (IU) for infants between 6 and 12
months and 200 000 IU for the older children.
Journal of Tropical Pediatrics
Vol. 46
February 2000
B. DUNCAN ET AL.
Sample collection and preparation
Two to three ml of blood was obtained by venipuncture
and transferred to glass tubes. Blood was also drawn
from one of us (BD) on each sample day and served as a
control. The blood was centrifuged and the serum
removed and stored in a ¹408C freezer with a back-up
power source until it could be transported to the laboratory
in Tucson, Arizona for analysis. Serum samples were
precipitated with ethanol to remove protein and the lipids
were extracted from the supernatant with hexane as
previously described.10
Results and Discussion
Demographics
Twenty-six of the 48 children were males and 22 were
females. The average age was 21.9 months (range 6.7–
67.3 months). Fourteen (29 per cent) were less than 12
months; 18 (37.5 per cent) were between 13 and 24
months; nine (18.7 per cent) were between 25 and 36
months; and seven (14.6 per cent) were more than 36
months of age. Ten (21.8 per cent) of the 48 were the
only children in the family, although three of the 10 were
only children as a result of sibling deaths. There were
three sets of twins but in only two were both twins in the
malnutrition unit. There were two other families each of
whom had two siblings in the unit and were included in
the study. Thus four of the 44 families had two children
each in the malnutrition unit. Fourteen (29 per cent) were
either twins or had younger siblings in the family.
HPLC analysis
Hexane extracts were evaporated to dryness under N2
and re-suspended in 150 ml of the mobile phase,
[methanol:tetrahydrofuran (THF),(90:10 v/v) containing
0.25 g/l butylated hydroxytoluene (BHT)]. The extract
(50 ml of the 150 ml sample) was injected onto a YMC
(Morris Plains, NJ) reversed phase C18 column using an
IBM auto sampler (Model LC/9050 SE). Samples were
eluted isocratically at a flow rate of 1.7 ml/min using a
Waters model 510 pump. The HPLC effluent was detected
at 325 nm using a Milton Roy programmable detector,
model SM 4000 controlled by a Maxima 820 Chromatography Workstation (Warters Associates, Milford, MA).
Automated integration of vitamin A on all chromatograms was verified manually by the technician. Where
baseline resolution could not be achieved, tangent skimming analyses were performed to remove bias. Using an
injection volume of 50 ml, limits of detection (defined as a
peak with a signal:noise ratio $3) for retinol was
30 nmol.
Anthropometrics
Only two of the 48 children were above the National
Center for Health Statistics’ 50th centile weight for age
while 77 per cent (37/48) were below the 5th centile and
64 per cent (31/48) were at or below the first centile.
One-third (16/48) of the children had oedema which
inflated the weight data. Only four of the 48 were above
the 50th length for age centile while 71 per cent (34/48)
were below the 5th centile which reflects long-standing
undernutrition or stunting. Forty-two per cent (20/48) of
the children were below the 5th centile weight for length.
Twelve (25 per cent) of the 48 children were still
receiving at least a portion of their nutrition from
breastmilk. Of the 36 who were no longer being
breastfed, 14 per cent had been weaned before the 6th
month and 39 per cent were no longer breastfeeding by
their first birthday. Seventy-two per cent (26/36) had
been weaned before 18 months and 94 per cent were no
longer receiving breastmilk at 24 months of age.
Quantitation
The HPLC was calibrated at the onset of the study
using standard curves established from authentic retinol
[National Institutes of Standards and Technology (NIST)]
and the same curve was used for standardization
throughout the study.
Dietary analysis (Table 1)
Nutritional intake for this group of children reflects
their severe undernourished condition. The 7-day food
frequency information reveals a monotonous diet consisting primarily of milk, corn and bananas. Although,
Data analysis
Descriptive statistics and correlations were performed
using Microsoft Excel 5.0 (Microsoft Corp.)
Table 1
Categories of children’s 7-day food consumption
Type of food categories eaten by the children
Servings per weeka
<1
1–5
6–10
11–15
16–20
> 20
a
Vegetables
Fruits
Seeds
Breastmilk
Milk
Meats, eggs, fish
4
14
12
10
3
5
9
19
11
5
1
3
11
23
6
6
0
0
37
3
7
0
0
0
6
14
25
3
0
0
5
26
10
5
1
1
Reported by mothers using a 7-day food frequency instrument.
Journal of Tropical Pediatrics
Vol. 46
February 2000
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B. DUNCAN ET AL.
Fig. 3. Correlation between dark adaptation and serum retinol levels in children whose retinol level was < 0.7 mmol/l.
most of the children received at least one serving of milk
each day, almost two-thirds (31/48) received less than
one serving of either meat, eggs or fish each day and
about the same percentage received no more than one
serving of vegetables (31/48) nor more than one serving
of fruits (29/48) each day.
Relationship of NVTT and serum retinol concentration
(Fig. 3)
For the group, serum retinol concentrations and NVTT
values were related (r ¼ 0:3) but the correlation did not
reach significance ( p < 0:12). However, when NVTT
values with retinol levels in subjects whose serum retinol
concentrations were less than 0.35 mmol/l, the correlation
was stronger (r ¼ 0:41, p < 0:05). These data indicate
that the NVTT may be a valuable field method for
detecting children with marginal levels of vitamin A
deficiency. Because serum retinol is not a reliable
predictor of vitamin A status, the present data do not
allow us to evaluate directly the efficacy of the NVTT for
predicting the vitamin A status of an individual. For this
work, more sensitive indicators such as the mRDR13,14
are needed. Studies to evaluate this relationship are
currently in progress.
The present data indicate that the NVTT is not suitable
for evaluating the severely malnourished child. These
data are consistent with the poor response of these
children to external stimuli due to the malaise accompanying malnutrition. In addition, severely malnourished
children often present with clinical symptoms consistent
34
with vitamin A deficiency and management recommendations include the administration of large doses of
vitamin A on admission to the clinic or hospital.
In summary, in preliminary trials, an inexpensive,
simple-to-use instrument designed to measure a young
child’s ability to adapt to darkness, or night vision,
correlates with serum retinol concentrations. This
instrument shows potential for identifying children
with marginal vitamin A stores and thus offers the
opportunity to institute the means to prevent their
developing severe vitamin A deficiency.
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