UNIT 305 Visual Acuity and Pharmacological Dilation

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UNIT 305Visual Acuity and Pharmacological
Dilation

• By
• Jenny Gaffney Jean Macpherson

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What is Visual Acuity

• Test of visual performance.
• An assessment of the patients ability to see
  detail.
• Distance vision constitutes the initial part of
  the screening examination.
• It acts as a baseline for future examinations and
  comparison of previous readings.
• It is a requirement prior to administering
  mydriatic drops.

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Disadvantages of Measuring Visual Acuity

• Can cause worry for patient if vision has
  deteriorated from last test.
• This may be simply due to patients blood sugar
  running high at screening.
• The test is only as accurate as the person
  performing the test and this may vary from
  screener to screener.
• May cause embarrassment if patient is unable to
  read.

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• Different Methods and
• Equipment Available for Testing Visual Acuity

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Snellen Chart

• Probably the most commonly used.
• Suitable for anyone who can recognise the
  alphabet and co-operate.
• Working at different distances. Various
  illuminations, reverse chart with mirror.
• A major disadvantage is that letter size is not
  equal and appears overcrowded.
• Limited accuracy in patients with poor vision.

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E Chart

• Can be used for children or people with literacy
  problems.
• Set out like the Snellen chart with lines of Es
  pointing in different directions.
• Patient holds a letter E and mimics the position
  of the letter on the chart.
• Patients must be able to co-operate.
• Not very accurate as there can only be a
  combination of 4 different directions.

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Bailey Lovie logMAR Chart

• The gold standard test available in different
  distances. It is the most accurate method
  preferred by the National Screening Programme in
  Scotland.
• Consists of lines of letters, the same number on
  each line 5 which are equally spaced with an
  equal mix of difficulty.
• Patients must be able to recognise the alphabet.

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Kay Picture Test Chart

• It can be used for children and people with
  communication problems.
• It consists of simple drawings on cards which
  decrease in size that have to be matched by the
  patient.
• It is not very accurate, can be time consuming
  and may require 2 members of staff.

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Sheridan Gardiner Test

• Used for children and people with speech
  problems.
• A portable test.
• Consists of letters on a card which are held up
  and must be matched from a master card which the
  patient holds.
• There are only 7 letter combinations, it is not
  illuminated and can be time consuming.

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Corrected Visual Acuity

• Can be achieved with up to date distance glasses,
  bi-focal, vari-focal, single vision lenses or
  pinhole correction. NOT READING GLASSES
• Enables us to determine if vision loss is due to
  a simple refractive error rather than eye
  disease.
• Any reading result worse than 6/9 (Snellen) or
  more than 0.2 (Logmar) requires pinhole testing
  using an occluder.

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Effect of Pinhole Testing on Visual Acuity

• Allows us to obtain the best possible corrected
  vision.
• Restricts light entering the eye and prevents
  scatter, light enters along the visual axis
  focusing the image on the retina reducing
  blurring and increasing sharpness.
• Can improve vision by up to 4 dioptres.
• Vision may not be improved with pinhole if
  cataract or any other eye pathology.

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Optimum Conditions for Testing Visual Acuity

• Comfort maximizes patient compliance.
• Unhurried explanation, privacy, illuminated chart
  at the correct level with no obstructions to
  detract from the chart.
• In these conditions patients are relaxed and a
  more accurate result can be obtained and recorded.

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Placement of Visual Acuity Chart

• Charts must be placed at correct height and
  distance to ensure accuracy and consistency.
• Reduces unnecessary referrals for further
  investigations or investigations not being
  carried out when required.

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Notation of Visual Acuity Results

• Logmar
• Results are recorded as a decimal but there is no
  indication of the distance between the patient
  and chart.
• The better the vision the smaller the decimal eg
  0.2.
• In poorer vision the decimal figure would be
  higher eg 0.7

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Notation of Visual Acuity Results

• Snellen
• The results are recorded as a fraction eg 6/12.
• The top figure being the distance between the
  patient and the chart in metres.
• Therefore in the above example the patient would
  be 6 metres from the chart. If using a 3 metre
  chart the result is recorded as 3/12.
• The bottom figure indicates how far away in
  metres a patient with normal vision could see
  that line of letters.

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Occluders

• To ensure accuracy.
• Using a hand as an occluder may cause pressure
  on the eye resulting in blurred vision.
• The patient may have the opportunity to Peek
  between their fingers possibly producing better
  vision than they have.

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Pupil Dilatation in the Screening Examination

• Mydriatic drops dilates small pupils to allow
  more light to enter the eye producing clearer,
  better images.
• Lessens the number of images inadequate for
  grading.
• Reducing the number of patients to be recalled or
  referred for slit lamp examination.

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Action of Mydriatic Drops

• Affects the sympathetic and parasympathetic
  nerves of the iris which controls accommodation.
• Tropicamide 1 is most commonly used. It is a
  rapid acting drug affecting the parasympathetic
  nerves causing the pupil to dilate and the
  ciliary muscles to paralyse.
• Phenylephrine 2.5 is a much stronger drug, which
  works by stimulating the sympathetic nerve
  endings.

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Contraindications for Use of Mydriatic Drops

• The only definitive reason would be a known
  allergy.
• Other considerations would be driving status,
  recent eye surgery, eye infections, pregnancy
  and if iris clip anterior lens was in place.

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Safe Storage of Mydriatic Drops

• Store below 25 C but refrigeration not required.
• Store in original undamaged container.
• Protect from direct sunlight.
• Before use check drug concentration, expiry date
  and batch number on box and vials.
• This information must be recorded on patients
  records. Local protocols must be adhered to.

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Mydriasis and the possibility of a critical
incident!

• Who is at risk?
• Patients with undiagnosed angle closure glaucoma
  and patients with shallow anterior chamber.
• Who is not at risk?
• Patients with glaucoma who are treated either
  topically with drugs to reduce intraoccular
  pressure or surgically e.g. with trabeculectomy
• Reaction is more likely with Phenylephrine than
  with Tropicamide

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Action in the event of a Critical Incident

• Onset can be quite rapid.
• Patients can develop, red and swollen eye,
  nausea, vomiting and headaches.
• If this occurs within clinic setting refer as an
  ophthalmic emergency to hospital eye service if
  not possible to local AE.
• Information leaflet should be given to all
  patients who have had mydriasis.

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Procedures for staff following an adverse reaction

• Record all details of incident on patient
  records.
• Inform GP
• Advise patient to make staff aware of this
  problem on subsequent visits
• Complete Datix form and inform line manager.

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THANK YOU FOR LISTENING

• GOOD LUCK WITH CITY AND GUILDS UNITS 4 AND 5
• JEAN AND JENNY