The World Health Organization (WHO) provides a comprehensive framework for classifying vitamin A deficiency (VAD), which is critical for both individual clinical diagnosis and global public health surveillance. This classification system evaluates VAD based on observable clinical signs, primarily affecting the eyes (xerophthalmia), and on biochemical markers measured in the blood. By standardizing these criteria, the WHO enables health professionals worldwide to consistently assess, monitor, and address this significant nutritional problem, particularly in low- and middle-income countries where it is most prevalent.
The Clinical Classification: Xerophthalmia
The most widely used WHO classification system for VAD relies on a series of clinical signs, collectively known as xerophthalmia. This Greek term, meaning "dry eyes," describes the progressive ocular damage caused by insufficient vitamin A. The stages of xerophthalmia, identified by specific codes, track the condition from its earliest signs to irreversible blindness.
- XN (Night blindness): This is often the earliest and most common symptom. It refers to the difficulty or inability to see well in low-light conditions. Night blindness occurs because vitamin A is essential for the production of rhodopsin, a pigment in the eye's retina that is highly sensitive to light.
- X1A (Conjunctival xerosis): This stage involves the pathological dryness of the conjunctiva, the thin membrane covering the white of the eye. Without sufficient vitamin A, the conjunctiva loses its normal moist, glistening appearance, becoming dull, dry, and thickened.
- X1B (Bitot's spots): Characterized by the appearance of foamy, white, or grayish patches on the conjunctiva. These spots are made of keratinized epithelial debris and represent a more advanced stage of conjunctival xerosis.
- X2 (Corneal xerosis): The dryness spreads to the cornea, the transparent outer layer of the eye. The cornea loses its luster and appears hazy, which can cause significant visual impairment if left untreated.
- X3 (Corneal ulceration and keratomalacia): This is a severe, late-stage sign of VAD. Keratomalacia is the softening and ulceration of the cornea, which can lead to rapid, irreversible damage. It is further divided into two sub-stages based on the extent of the ulceration:
- X3A: Ulceration involving less than one-third of the corneal surface.
- X3B: Ulceration covering more than one-third of the corneal surface.
- XS (Corneal scar): The final stage, which results from the healing of a corneal ulcer. The resulting scar tissue can cause partial or total blindness.
- XF (Xerophthalmic fundus): Refers to characteristic changes in the fundus (the inner posterior surface of the eye) that can be observed by an ophthalmologist.
Biochemical Classification: Subclinical VAD
Beyond the visible clinical signs, the WHO also defines VAD based on biochemical indicators, which can identify subclinical deficiency in individuals and assess the prevalence within a population. Subclinical VAD means the body has inadequate vitamin A stores, but the clinical eye signs have not yet appeared.
The primary biochemical marker used is the concentration of retinol in plasma or serum. The WHO has established specific cut-off values for assessing VAD in populations, typically focusing on preschool-age children, a particularly vulnerable group.
- Serum retinol < 0.70 μmol/L: This threshold indicates subclinical VAD in children.
- Serum retinol < 0.35 μmol/L: This value signifies severe VAD.
These biochemical measures are essential for public health planning, as they reveal the extent of the problem in a community before the more severe, visible signs become widespread.
Classification of Public Health Significance
The WHO also classifies VAD according to its public health significance, based on the prevalence of specific indicators within a population. This classification helps national health programs and international organizations prioritize interventions, such as supplementation and food fortification.
WHO Classification of Public Health Significance for VAD
| Indicator | Prevalence Cut-off | Public Health Significance |
|---|---|---|
| Serum or plasma retinol (<0.70 µmol/L) in preschool children | < 2% | No public health problem |
| 2–9% | Mild public health problem | |
| 10–19% | Moderate public health problem | |
| ≥ 20% | Severe public health problem | |
| Night blindness (XN) in pregnant women | ≥ 5% | Moderate public health problem |
This table provides clear, data-driven thresholds for determining the severity of VAD on a population level, guiding decision-making for public health interventions.
Comparison of Clinical and Biochemical Classification
The clinical and biochemical methods for classifying vitamin A deficiency offer different but complementary perspectives on the issue. Clinical classification relies on observable eye signs, making it a valuable tool for direct diagnosis and immediate treatment of individual patients in the field. It is particularly useful for identifying advanced stages of VAD, such as corneal damage. However, clinical signs often only manifest after a prolonged and severe deficiency, meaning they may not capture the full scope of a population-wide problem. Biochemical classification, using serum retinol levels, can identify subclinical deficiency, allowing for earlier, preventative interventions. While more costly and logistically complex to conduct on a large scale, it offers a more sensitive and comprehensive assessment of VAD prevalence within a population, which is crucial for determining public health strategies.
Prevention and Conclusion
Prevention is the most effective strategy against vitamin A deficiency. The WHO advocates for a multi-pronged approach, including dietary improvement, food fortification, and vitamin A supplementation in at-risk populations, particularly young children. Ensuring a diet rich in vitamin A sources—including green leafy vegetables, orange and yellow fruits and vegetables, and animal products like liver and eggs—is fundamental. Public health programs, guided by the WHO's classification, can effectively target their efforts to the populations most in need.
The WHO's robust classification system for vitamin A deficiency, which combines clinical observation with biochemical analysis, provides the essential framework for both individual treatment and global public health action. By detailing the progression of ocular symptoms (xerophthalmia) and setting thresholds for subclinical and public health significance, the WHO enables a standardized and comprehensive approach to addressing this preventable and devastating nutritional disorder. This systematic classification has been instrumental in focusing intervention strategies, ultimately saving the sight and lives of millions of children worldwide.
How is vitamin A deficiency classified according to WHO?
- Clinical Signs (Xerophthalmia): The WHO classifies VAD based on a spectrum of eye-related signs, from mild night blindness (XN) to severe corneal damage (X3A, X3B, XS).
- Biochemical Assessment: VAD can also be identified through low serum retinol levels, with specific cut-off points used to define subclinical deficiency in a population (<0.70 μmol/L).
- Public Health Significance: The WHO uses prevalence data of clinical and biochemical indicators to classify VAD as a mild, moderate, or severe public health problem in a given area.
- Systematic Coding: The International Classification of Diseases (ICD-11) codes VAD as 5B55 and uses decimal-based subcodes to specify the clinical manifestation, such as night blindness (5B55.0).
- Population-Specific Indicators: Different indicators and prevalence thresholds are used depending on the population, such as night blindness in pregnant women or serum retinol in preschool children.
