Contact Lenses Competency 7.7

Competency 7.7: An understanding of and the ability to fit soft contact lenses to patients with astigmatism

This competency requires the trainee to have a good understanding of how toric contact lenses work and to recognise the limitations of the different lens types.  The ability to fit these lenses requires the trainee to recognise when toric lenses are a better option over spherical lenses, and to understand what level of astigmatism can be corrected before selecting an appropriate trial lens for the patient.  Fitting of the lens includes assessing the suitability of the toric lens on the cornea and how modifications to prescriptions are to be made.  The trainee should be able to justify their fitting decisions.

Best Form Evidence

This competency could be assessed by Direct Observation during the third quarterly visit.  However, in most cases, the Assessor will expect to see your Log Book and a selection of Patient Records instead.  You need to be able to demonstrate at least one contact lens record with astigmatism over 1.50DC, and as always, three would be preferable.   You may also be asked to talk through some fitting Case Scenarios that the Assessor will set out.

What the Assessor expects to see:

  • Identification of when a toric lens is required
  • Ability to discuss lens options with the patient
  • Selection of suitable trial lens
  • Assessment of contact lens fit and toric fitting assessment
  • Final lens selection of appropriate parameters and prescription

Identification of when a toric lens is required

Spherical soft lens

Where there is more than 0.75DC, the visual acuity obtained with a spherical soft lens may not be adequate for the patient’s visual needs.  However, if the patient only wants the lens for social use, it may be an adequate level of vision.  Be prepared to explain this to the patient.

Spherical rigid gas permeable lens

Suitable for correcting up to 2.00 DC in most cases. The tears film between the toric cornea and spherical back surface of the lens neutralises any corneal astigmatism.  You need to be aware of any potential side effects of fitting a lens that deliberately does not provide an alignment fit.

Soft toric lens

Where non toric soft lenses provide inadequate acuity or RGP’s are rejected by the patient.  Soft toric lenses effectively correct corneal astigmatism and can be tailor-made to specification if required.  Know which lenses are available and in what modality.  For example, monthly disposable torics are only available in limited cyl powers and axes.

Rigid bitoric lenses

Where corneal astigmatism is too high to achieve a stable fit with spherical RGPs, and soft torics are either unsuitable or unable to correct vision to a satisfactory level.  A toric back surface stabilises the lens on the cornea, while a front toric surface corrects astigmatism.  Be prepared to discuss how you go about ordering such lenses for trial and what measurements you need to supply to the manufacturer.

Selection of suitable trial lens

As with non-toric lenses there are several things to consider, including patient motivation and lens costs, but the over-riding factor is the degree of astigmatism to be corrected.  The higher the degree of astigmatism, the less options available for the patient.

Measurements to consider:

  • Horizontal Visible Iris Diameter
  • Pupil size
  • Keratometry readings
  • Prescription
  • Tear break-up time/ tear prism height

By considering all of the above, you should be able to select a lens of suitable material and replacement modality for the patient’s needs.

Assessment of fit

Physical fit of soft torics can be assessed as you would a normal soft lens. Bitorics align themselves with the cornea and should give an alignment fit.  Once the fit has been assessed as satisfactory, the toric markings on the lens should be noted and compared to the manufacturer’s guidelines.  If the markings have rotated, remember to compensate for the degree of rotation in the final prescription ordered.

Rule of thumb: LARS-Rotation to the left, then add degree of rotation to cyl axis. Rotation to the right, then subtract.

Make sure you record all your observations in a clear and concise manner in the patient records.

Unacceptable evidence

  • Poor selection of trial lens due to incorrect interpretation of ocular measurements
  • Inappropriate selection of trial lens for patient’s needs based upon history & symptoms
  • Inaccurate estimate of contact lens fitting measurements leading to the trainee recommending a poorly fitting lens for the patient
  • Inaccurate assessment of toric markings resulting in the incorrect lens prescription being ordered
  • Insufficient number of contact lens fits completed

At the end of the fitting, you should be able to write out a full lens specification for ordering.  Note whether or not the patient needs tuition on insertion, removal and the care of contact lenses.

Author: Michelle Hanratty

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