Measurement Problems

A number of problems can arise when measuring visual acuity, such as:

• The patient has forgotten his/her glasses or has taken contact lenses out (What to do – 1)
• The patient doesn’t understand English or can’t read (What to do – 2)
• The patient is unable to see the top line of the chart (What to do – 3)
• The patient can’t open his/her eye due to photophobia or involuntary closing of the eyelid (blepharospasm) (What to do – 4)

What to do – 1

Use a pinhole occluder. The pinhole only allows parallel rays of light through, which pass straight through the cornea and lens without bending. Therefore, an unaltered image is presented to the retina. If acuity improves with the pinhole, the visual impairment is refractive and not due to an intrinsic eye or neurological disorder.

Learning bite

Use of a pinhole occluder corrects for any refractive error and enables a true visual acuity to be measured.

What to do – 2

Alternative eye testing charts are available which use either:

Fig 1: Shape orientated chart, e.g. illiterate E chart	Fig 2: Pictures chart

What to do – 3

If acuity is so poor that the patient cannot see the top line of the chart, then assess at 3m, ensuring that this is documented as 3 / x.

If no letters are seen at the shorter distance, the next step is to assess ability to count fingers (recorded as CF).

If finger counting fails, then assess ability to perceive hand motion (HM).

Finally, if unsuccessful with the above, assess ability to perceive light (LP).

What to do – 4

Instillation of local anaesthetic will often provide relief with an anterior eye problem, enabling the eye to be opened and acuity assessed.

Occasionally, treatment must precede visual acuity assessment, e.g. eye washout in chemical eye contamination.