Q4 LONG ANSWER 10M ★ High-yield

Describe the assessment of visual acuity and the methods of refraction.

Ophthalmology · 3rd Professional

Visual acuity is a measure of the resolving power of the eye — the ability to distinguish two points as separate. The normal eye resolves an angle of one minute of arc
The snellen chart is constructed so that each letter subtends 5 minutes of arc at the stated distance and each stroke and gap subtends 1 minute. It is read at 6 metres, at which distance rays are effectively parallel
Recording — V = d / D, where d is the distance at which the patient reads the line and D the distance at which a normal eye should read it. 6/6 is normal; 6/60 means the patient reads at 6 metres what should be read at 60

Retinoscopy (skiascopy) is the objective determination of refractive error, requiring NO response from the patient — which makes it indispensable in children, in the uncooperative and in the intellectually disabled
Principle — light is thrown into the eye and the movement of the reflex in the pupil is observed as the streak is moved. with movement (the reflex moves in the same direction as the streak) indicates that the far point is behind the observer — hypermetropia, emmetropia, or myopia less than the working distance — and is neutralised with plus lenses. against movement indicates myopia greater than the working distance and is neutralised with minus. At neutralisation the pupil fills with a bright reflex that does not move
The working distance must be subtracted, since retinoscopy is done at a finite distance: at 1 metre subtract 1.00 D; at 2/3 metre subtract 1.50 D. Omitting this step is the classic error, and produces a systematically over-plussed result

Cycloplegic	Strength, onset, duration and use
Atropine 1% (ointment)	The most powerful cycloplegic. Applied for 3 days before refraction. Cycloplegia lasts 2 to 3 weeks. Reserved for children under 5, and especially for accommodative squint, where complete cycloplegia is essential. Risks systemic atropine toxicity — fever, flushing, delirium — so parents are told to press on the punctum and use ointment rather than drops
Homatropine 2%	Intermediate; used in children of 5 to 20 years; recovery in 1 to 3 days
Cyclopentolate 1%	The most widely used in practice; adequate cycloplegia in 30 to 45 minutes with recovery in 6 to 24 hours; suitable for most children over 5 and for adults
Tropicamide 1%	The weakest cycloplegic, though a good mydriatic; recovery in 4 to 6 hours. Adequate for adults, but not sufficient for a child

Situation	Point to observe
The malingerer or functional visual loss	Test at different distances with charts of different sizes; use a plus-minus lens combination that cancels; check the optokinetic drum.
The illiterate patient	Tumbling E, Landolt broken ring, or picture charts — and never record vision as untestable because the patient cannot read
The infant	Fixation and following; objection to occlusion of one eye; preferential looking with Teller cards; and optokinetic nystagmus
The patient with nystagmus	Test binocularly as well as monocularly, and allow the head posture that damps the nystagmus; occluding one eye worsens it
Media opacity	Assess potential acuity with the pinhole, laser interferometry or the potential acuity meter, and always record projection OF rays
Recording a presenting versus a best-corrected acuity	WHO categories and Indian blindness statistics now use presenting acuity.

Subjective refinement follows the objective result: the duochrome (bichrome) test.
The jackson cross-cylinder for refining the axis first and then the power of a cylinder; and the astigmatic fan and clock dial
Binocular balancing before the final prescription, to ensure accommodation is equally relaxed in both eyes
Always test the pinhole before refracting; if it does not improve the vision, look for disease rather than persisting with lenses
Subtract the working distance in retinoscopy; forgetting it over-pluses every prescription and is the commonest single error in the technique

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