Academic journal article Bulletin of the World Health Organization

Visual Field Constriction as a Cause of Blindness or Visual Impairment

Academic journal article Bulletin of the World Health Organization

Visual Field Constriction as a Cause of Blindness or Visual Impairment

Article excerpt

Introduction

In 1973 a WHO Study Group on the prevention of blindness proposed five categories of visual impairment that have since been widely adopted, permitting direct comparison of blindness and visual impairment rates from various studies in different populations (Table 1) (1).

Table 1: WHO categories of blindness and visual impairment

                               In the better eye:

               WHO       Visual        Visual field
             category    acuity        constriction

Visual          1         <6/18
impairment      2         <6/60
                3         <3/60    or    <10[degrees]
Blindness       4         <1/60    or    <5[degrees]
                5         NPL(a)

(a)NPL = no perception of light.

Visual field loss is recognized as a major disability. Severe visual constriction despite preserved central vision constitutes one of the internationally agreed categories of blindness (1). It has been recommended that in areas where onchocerciasis is present visual field testing be included in the basic eye examination (2). Nevertheless, almost all reports on the prevalence of blindness have omitted field constriction, even in onchocercal areas, because such data are difficult to validate and their collection is very time consuming.

In this article we report the effect of measuring field constriction in addition to visual acuity on estimates of blindness and visual impairment rates in mesoendemic onchocercal communities in the guinea savanna of Kaduna State, northern Nigeria.

Methods

Population characteristics

As part of a large randomized, controlled trial to investigate the impact on onchocerciasis of annual mass chemotherapy with ivermectin, 34 communities in Kaduna State, northern Nigeria were selected for detailed ophthalmic screening (the criterion for selecting communities was that the prevalence of Onchocerca volvulus positive skin-snips in those aged [equal to or greater than] 20 years be [equal to or greater than] 30%). The prevalence of positive skin-snips was 71% in all the communities studied and lay in the range 39-93%. The community microfilarial load (CMFL) in those aged [equal to or greater than] 20 years in the 34 study communities was 3.23 microfilariae per mg of skin and ranged from 0.95 to 9.96. The vast majority of the households in the study communities live by subsistence farming with few cash crops grown. The habitat is guinea savanna and the vectors are Simulium damnosum s.s. and S. sirbanum (3).

Screening methods

After the free and informed consent of the participants had been obtained, an extensive ophthalmic screening examination was performed at a central location in each community by six trained ophthalmic nurses. This basic eye examination included the tests outlined below.

Visual acuity. Visual acuities were tested using single optotype E charts at 6 m in ambient outdoor light. The scale of the trial dictated that all non-essential tests be omitted to permit completion before the rains made work impossible. The following levels of acuity were recorded: 6/9, 6/18, 6/36, 6/60, 3/60, perception of light (PL) and no perception of light (NPL). Acuities of <6/9 were checked with a pinhole device and the result recorded separately.

Visual fields. Peripheral field defects were assessed using the simultaneous counting fingers test. The paracentral visual field was also assessed by confrontation using a 6-mm white target. Paracentral perception of red and red desaturation was assessed using the red-dot card test (4). If a defect was detected by any of these or other basic tests, a Friedmann analysis of the paracentral field was performed.

Methodology of tests

* Simultaneous counting fingers test in peripheral field

Eyes with visual acuities of hand movements or less were not tested. …

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