2. Contact Lens Prescribing Protocol
A systematic procedure is essential to ensure
thorough and efficient management of the contact lens
patient. Such a procedure involves the following steps:
• Patient screening. Screening is usually the first contact
that the patient will have with a practitioner. If a practitioner
is to obtain the necessary information from a prospective
contact lens patient, steps should be taken to establish good
rapport. It is during this time that an initial determination of
the suitability of the patient for contact lenses is made. It is
also an opportunity to discuss the benefits of contact
3. • Preliminary examination and measurements. Following a decision to
try contact lenses, an examination is conducted, and relevant
measurements made, preparatory to trialing
suitable contact lenses.
• Trial lens fitting. Trial lens parameters as close to the final lens order as
possible should be chosen. The trial fitting routine is aimed at determining the
final lens specifications. These specifications will be central to patient
satisfaction with the lenses.
• Lens dispensing. The contact lenses are dispensed after appropriate
verification. Once the lenses are confirmed as being acceptable, the patient is
instructed about contact lenses generally, the modes of lens wear being
recommended and proper lens care.
• After-care. After-care visits are necessary to monitor the patient’s adaptation
to the contact lenses. The visits are scheduled at regular intervals
afterdispensing. After-care visits are usually the practitioner’s only opportunity
to assess the eye’s response to contact lenses. Of special interest are objective
signs which are not reflected in the patient’s subjective response to lens wear,
e.g. superficial corneal staining, corneal oedema, etc.
4. Patient Screening
• Patients who indicate a desire to be fitted with contact lenses are
usually well motivated. The
reasons for wanting contact lenses range from recommendations from friends or
relatives to specific work needs, sport or recreation needs.
• After inquiring about the patient’s interest in contact lenses, investigate their
criteria for success relative to the types of lenses available, e.g. some contact lenses
are made in a limited range of lens parameters. A local quick reference guide is
useful for reviewing the range of lenses available (see Soft Contact Lens Quick
Reference Guide at the end of this lecture). More comprehensive lists are published
internationally, e.g. Tyler’s Quarterly Soft Contact Lens Parameter Guide, Contact
Lens Data Book (see reference list at back of
this unit) and supplements to contact lens journals.
• At the preliminary examination baseline information on ocular variables should be
collected. Such data is useful for future reference and for comparison with post-
• Screening may indicate that the patient would be better served by spectacles
rather than contact lenses. The options (if any) should be presented to the patient.
5. Patient Screening
• Anatomical and physiological. Examination of the structure,
shape and clarity of the anterior segment may reveal whether
The eye is ‘normal’. The measured features of the eye will suggest
the type and design of contact lens to be trialed.
• Psychological. Motivation, intelligence and personality influence the
likely success rate of contact lens wear. A patient, such as the one shown
in slide 5, who expresses extreme sensitivity, may be suggestive of a
potentially unsuccessful lens wearer. A comprehensive explanation of
the advantages that can be derived from contact lens wear may dispel
some of the fallacies of contact lens wear. Monitoring the compliance of
such patients to the prescribed care regimen is necessary.
Pathological. A thorough history and subsequent eye examination may
provide indications or contra-indications for contact lens wear. The
important aspects of history taking are:− general health , ocular health ,
medication, ocular history including vision corrections, special
occupational, recreational and environmental factors.
6. Ocular abnormalities such as senile ectropion ,
Sjögren’s Syndrome, etc. may contra-indicate contact
lens wear or help identify the type and/or design
which may be suitable.
• Personal and occupational needs.
Consideration of age, gender, cosmetic,occupational,
recreational, environmental and other factors may help the
choice of type and design of contact lenses to be prescribed.
• Refractive. Previous and current records of the patient’s
refractive status should always be
referred to, especially where binocular functions need to be
considered. Referral letters and record cards are good sources
of such information.
8. Trial Fitting: Lens Selection
Trial lens selection includes a tentative determination of
parameters/features. These might include:
• Back optic zone radius (BOZR). Corneal curvature measurements (in
millimetres) can be used as a starting point for choosing the initial BOZR.
• Total diameter (TD). The trial lens TD is generally found by measuring
the Horizontal Visible Iris Diameter (HVID) and adding or subtracting 2
mm for soft and rigid lenses, respectively.
Centre thickness. This is a function of the BVP of the contact lens.
Generally, plus lenses have a greater centre thickness than most minus
• Water content (for soft contact lenses). The lens BVP and the wearing
modality desired will influence how the eye’s oxygen requirements will
be met or approximated. Water content is an essential factor in such
9. Lens design (lenticular, multicurve, surface shape, etc.). The desired lens fit
can be better achieved by trialing various lens designs, e.g. using
lenticulation to improve lens centration in a highly myopic patient or using
a back surface multicurve design to improve the lens-cornea
• Lens type (spherical, toric, bifocal, etc.). This choice is dependent on the refractive
and curvature measurements of the eye. Providing the best spherical refraction may
not always give satisfactory visual acuity and toric lenses may be needed.
Lens material. Oxygen transmissibility, Dk/t, is a most important material property of the
lenses to be prescribed. Ideally, trial lenses should be of the same material as the lenses
to be ordered
because material choice can significantly affect their fitting behaviour.
• Back vertex powers (BVP) - near and distance. Select the trial lens that is closest to the
patient’s refractive error, particularly for high prescriptions. Over time, and the
failure, a practitioner will build a collection of lenses, including special lenses such as
and toric lenses, to complement their standard trial sets.
10. Trial Fitting: Procedure
• Initial lens selection. After baseline measurements have
been recorded, the first
trial lenses are selected and inserted.
• Trial lens fitting and vision assessment. After the lenses have settled,
their fit and resultant vision are assessed. Assessment should be both
quantitative and qualitative (see
record forms in the practicals of Units 3.2 and 3.3 for guidance). Once a
satisfactory fit is achieved, the prescription can be finalized and the
• Subsequent trial lens selection. If the initial lenses prove to be
unsatisfactory, further trials
are required. When the trial lenses required are not available, the
practitioner may have to resort to empirical prescribing. Empirical
prescribing involves no trial lenses. Rather, the final lens order is based
on the results of the preliminary examination and the ocular Rx as
calculated from the spectacle Rx.
11. Trial Fitting: Desired RGP Lens Fit
• Central alignment.
The example opposite shows no fluorescein
under the central portion of the lens. This
suggests that the BOZR of the lens closely
matches the central corneal curvature.
• Moderate edge clearance.
This manifests itself as a bright band of
fluorescein at the periphery of the lens.
• Good centration.
Lens positioned so that it is concentric with the
limbus in the rest position and its BOZD covers
the pupil adequately. While lens movement is
essential, rapid recentration after blink is highly
12. •Adequate size.
The lens diameter in this example should allow
the possibility of top lid attachment in normal
gaze without excessive lens movement.
Viewed dynamically, it would probably be a
With an accurate over-refraction, the vision
mount of toricity generated is based on
• Lens centration.
Good SCL centration is characterized by a
uniform amount of lens overlap of the sclera.
Blink-induced lens movement should be
followed by rapid recentration.
13. • Complete corneal coverage. Although some lens decentration
unavoidable, the practitioner has to ensure the lens covers the
cornea under all reasonable circumstances.
• Adequate movement. While some minimum movement is required
all lenses, the actual amount depends on the lens type. Generally the
amount will be between 0.2 and 1 mm. The prime reason for lens
movement being required is the dispersal of metabolic waste from
the post-lens space. Tear exchange has been demonstrated to be
minimal under soft lenses largely because of the thinness of the post-
lens tear film.
• Comfort. Soft lenses are usually very comfortable initially. Any
fitting factor that can compromise ocular health may cause some
wearer discomfort some time after insertion (usually 20 - 120 minutes
• Good and stable vision. With an accurate over-refraction, the result
should be clear and stable vision.