Ophthalmology
Visual Acuity Assessment
Visual acuity testing for MBBS and NEET-PG: Snellen chart, pinhole test, refractive errors and their lenses, presbyopia, refractive surgery and amblyopia, mapped to NMC codes OP1.
MedNext Academy | 3 min read
Visual Acuity Assessment
Visual acuity testing for MBBS and NEET-PG: Snellen chart, pinhole test, refractive errors and their lenses, presbyopia, refractive surgery and amblyopia, mapped to NMC codes OP1.
This chapter teaches the ordered clinical assessment of vision, from recording Snellen distance acuity down to perception of light, and the use of the pinhole to separate refractive from organic causes. It then covers correction of the refractive errors, the principles of refractive surgery, and the recognition of amblyopia within the critical window of visual development.
High-yield: Visual Acuity Assessment
- Distance visual acuity is recorded as a Snellen fraction where the numerator is the test distance of 6 metres and the denominator is the distance at which a normal eye reads that line, so 6/6 is normal.
- If the largest Snellen letter is not seen at 6 metres, the patient is walked forward and acuity is recorded as 3/60, 2/60 or 1/60 by the distance at which the top letter is read.
- When the top letter is not read even at 1 metre, testing moves to counting fingers, then hand movements, then perception and projection of light.
- The pinhole test removes the effect of refractive error, so an improvement in acuity through a pinhole points to an uncorrected refractive cause rather than to media or retinal disease.
- Myopia is corrected with a concave (minus) lens, hypermetropia with a convex (plus) lens, and astigmatism with a cylindrical lens.
- Presbyopia is the age-related loss of accommodation from lens hardening and is corrected with a plus lens for near work, usually starting around the age of 40.
- The near point of accommodation recedes with age, which is why reading difficulty is the first symptom of presbyopia.
- A relative afferent pupillary defect, detected by the swinging torch test, signals a lesion of the optic nerve or extensive retina rather than a media opacity.
- Amblyopia is reduced best-corrected acuity in a structurally normal eye and must be treated before the age of about 7 to 8 while visual development is still plastic.
- The commonest causes of amblyopia are strabismus, anisometropia and stimulus deprivation such as a dense congenital cataract.
- Occlusion or patching of the better eye is the mainstay of amblyopia therapy, forcing use of the amblyopic eye.
- LASIK reshapes the corneal stroma under a flap to correct refractive error, and a stable refraction with adequate corneal thickness is required before surgery.
- Keratoconus must be excluded by corneal topography before refractive surgery, as an ectatic cornea is a firm contraindication to LASIK.
- Colour vision is screened with Ishihara pseudo-isochromatic plates, and acquired colour desaturation, especially for red, is an early clue to optic nerve disease.
Refractive errors and their correction
- **Myopia:** Image focuses in front of the retina. Corrected with a concave (minus) lens.
- **Hypermetropia:** Image focuses behind the retina. Corrected with a convex (plus) lens.
- **Astigmatism:** Unequal corneal curvature. Corrected with a cylindrical lens.
- **Presbyopia:** Age-related loss of accommodation from around 40. Corrected with a near plus lens.
NMC competencies in this chapter
- **OP1.1:** Physiology of vision and the visual pathway
- **OP1.2:** Refractive errors and their correction
- **OP1.3:** Technique of visual acuity testing
- **OP1.4:** Principles of refractive surgery
- **OP1.5:** Strabismus, amblyopia and visual development
- **OP8.5:** Optic nerve and visual pathway disease
- **OP9.4:** Avoidable blindness and the NPCBVI programme
Frequently Asked Questions
What does 6/6 vision mean?
It is a Snellen fraction: at a test distance of 6 metres the patient reads the line a normal eye reads at 6 metres. It represents normal distance acuity, not perfect or the best possible vision.
Why is the pinhole test done?
The pinhole blocks peripheral rays and reduces the effect of refractive error. If acuity improves through the pinhole, the cause is likely an uncorrected refractive error rather than media opacity or retinal disease.
Until what age can amblyopia be treated?
Amblyopia responds best while visual development is still plastic, up to about 7 to 8 years. Treatment, usually patching the better eye, should start as early as possible for the best outcome.
How is presbyopia different from hypermetropia?
Presbyopia is the age-related loss of accommodation affecting near vision from around 40, while hypermetropia is a refractive error present at any age. Both are corrected with plus lenses, but presbyopic correction is for near work only.
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