The preliminary examination includes the taking of a full history and initial patient assessment (see Chapter 33 ), ocular measurements, refraction, slit-lamp biomicroscopy and additional tests. The examination enables the practitioner to advise patients on their suitability for contact lens wear and of appropriate lens choices for fitting. The information obtained from the preliminary examination is vital in the selection of the most suitable lens type (soft, corneal or scleral), replacement frequency, care system and wearing time.
The preliminary examination also forms a baseline for the monitoring of the patient’s eye condition. This includes an evaluation of conditions such as myopia (see Chapter 31 ), keratoconus (see Chapter 25 ) or a corneal graft (see Chapter 29 ).
Standard infection controls apply to the preliminary examination for contact lenses, although additional specific issues may apply for COVID-19 infection control. It is known the virus may occur in patient conjunctival tissue or tear film ( ). Ocular involvement in COVID-19 infections and possible ocular transmission may occur even in patients without ocular symptoms ( ).
Some elements of the preliminary examination may be ‘contactless’, such as during the history, where the practitioner can maintain a standard social distance (1.5–2.0 m) from the patient. For the measurement of vision, the use of an occluder may be avoided by asking the patient to use their hand to cover the contralateral eye. Patients wearing contact lenses may be asked to remove the lenses themselves and the patient may be asked to retract their own eyelid when the clinician instils eyedrops and vital stains. Procedures such as slit-lamp examination, keratometry, fundus imaging and corneal topography involve patient proximity and may be aided with plastic shields for the instruments and adequate room ventilation ( ).
This chapter provides an overview of the typical full routine of the preliminary contact lens examination following the case history. The sequence of examination described is shown in Fig. 35.1 . Details of the instrumentation referred to in this chapter can be found in Chapter 34 .
Measurement of Vision
After the case history, vision is usually the first measurement in the preliminary examination. The initial measurement gives the habitual vision of the patient, unaffected by later test procedures that use lights or ocular manipulations. An additional benefit of the baseline measure of vision is for medicolegal reasons.
The computer-presented visual acuity chart has advantages over other types of vision chart for contact lens practice, but in particular computer-generated optotypes it can be randomized to prevent the contact lens patient from learning the letter sequences at successive visits.
Vision should be measured with and without the habitual distance spectacles of the patient at both distance and near. The level of vision will be useful information to relate to the history, refraction and the binocular vision examination. The unaided vision is of interest because patients who are commonly or intermittently uncorrected will compare that with the level of acuity they achieve with contact lenses.
Objective and Subjective Refraction
The first phase of the refraction is to undertake a preliminary objective assessment. Digital autorefractors are now being used more commonly; these devices are combined in a single instrument that can also perform tonometry and/or corneal curvature assessment. A fully objective digital assessment of the optical quality of the eye requires a wavefront refraction (see later in this chapter).
The long-established, skilled art of retinoscopy still has utility in contact lens practice; as well as allowing objective determination of refractive status, it also enables the quality of optics of the eye to be assessed. Possible indications for retinoscopy include:
vision with spectacle refraction of less than 6/6 (20/20)
astigmatism of higher levels, or changing magnitude or direction of astigmatism, where keratoconus may be suspected
looking for clues of corneal irregularity, such as a scissors reflex ( )
pseudomyopia, whereby less minus may be revealed.
The subjective refraction may be best performed in a trial frame, since many contact lens patients are young and the trial frame may be less likely to induce accommodation than the phoropter. Even if 6/6 (20/20) vision is achieved with the patient’s current spectacles or contact lenses, it is useful to check the subjective refraction in each eye for excess minus power that could account for symptoms unrelated to visual acuity (such as asthenopia or binocular vision problems).
It is necessary for nonpresbyopic patients to adopt a technique with the refraction so as to relax their accommodation. Measuring a blur function is one such method, whereby the addition of +0.50 or +0.75 D lenses is expected to almost completely blur the 6/6 (20/20) line.
Measurements of the dimensions of anatomical structures of the anterior eye should be routinely made for contact lens fitting. Because of the differences in fitting characteristics between soft and rigid lenses, different sets of measurements may be required.
Horizontal corneal diameter is usually between 10 and 13 mm (average about 11.5 mm) ( ). A cornea of less than 10 mm may be defined as microcornea and one greater than 13 mm as megalocornea. The cornea reaches adult dimensions by about 2 years of age.
The pupil diameter is usually between 3 and 6.5 mm under photopic conditions, and about 5–8 mm under mesopic conditions. Patients with large pupils are usually more sensitive to refractive error and even small amounts of corneal irregularity. For these patients, contact lenses with larger optical zones may be beneficial.
For rigid lens fitting, it is useful to record the upper and lower eyelid positions in relation to the cornea. Assessment of lid geometry assists in the selection of lens diameter and design for the optimization of lens comfort. Most often, the upper eyelid covers the superior limbus by 1–2 mm, and the lower eyelid is very near the inferior limbus ( ). An unusually high or lower eyelid positions should be noted.
Assessment of corneal shape is a standard part of the contact lens preliminary examination. Digital topographers that provide a detailed map of the shape of the cornea are rapidly becoming the norm in contact lens practice (see below); however, the optical keratometer can measure the curvature of the principal meridians of the central 3 mm of the cornea, known as K-readings, which are useful measures for many aspects of contact lens fitting.
For soft lens fitting, particularly disposable lenses, which might, for example, be available in only two back optic zone radii (BOZR), the assessment is often used simply to identify steeper corneas, which require the lens with the steeper (smaller) BOZR. In rigid lens fitting the K-readings are used directly to select the BOZR of the initial trial lens.
The amount of keratometric astigmatism is usually predictive of the ocular astigmatism, in a linear relation known as Javal’s rule ( ). This also identifies lenticular astigmatism, which may be the cause of residual astigmatism in rigid lens wear. If the keratometry mires appear distorted, then irregular astigmatism may be present, i.e. where the principal axes are not orthogonal.
Keratometry is a simple, rapid and noninvasive test, but it does have some limitations. It measures only a central corneal radius, assumes a spherical cornea with regular astigmatism and has a limited range of powers (36.00–53.00 D). Extreme corneal powers can be measured by interposing a −1.00 D lens (for low corneal powers, i.e. very flat corneas) or a +1.25 D lens (for high corneal powers, i.e. very steep corneas) in front of the keratometer. The keratometer reading is converted to the actual corneal power using tables ( Appendix E ).
As well as the keratometer, K-readings may also be obtained from many autorefractors, axial length biometers ( ) ( Fig. 35.2 ) as well as with corneal topography and tomography (see later in this chapter).
In the preliminary examination, biomicroscopy is used to assess the health of the anterior eye and to identify conditions or features that may be relevant to contact lens wear. It is also important to record the baseline appearance of the eyes for medicolegal reasons.
Six areas of the anterior eye should be assessed in the preliminary examination, and any signs should be reconciled with symptoms and assessment of the corneal curvature ( Fig. 35.3 ).
Characteristics of the normal anterior eye, and the severity of any abnormalities that are detected, can be recorded with the assistance of grading scales (see Chapter 38 ). have demonstrated that effective grading can be undertaken in a few seconds, conserving valuable chair time in clinical practice.
A normal technique is to use a variety of illumination methods – such as those described below ( ) – and cobalt blue light for fluorescein staining.
Diffuse Wide Beam
A diffuse wide beam is generally used to provide low-magnification views of the opaque tissues of the anterior segment, including the bulbar conjunctiva, sclera, iris, eyelid margins and the tarsal conjunctiva of the everted eyelids ( Fig. 35.4 ).
A broad, even illumination over the entire field of view may be produced in a variety of ways. Some slit lamps have a ground-glass filter which may be inserted to diffuse the focused light beam, otherwise the slit-beam intensity is reduced and opened to full width.
Typical uses for the diffuse wide beam involve assessment of pigmented lesions in the conjunctiva or eyelids, the bulbar conjunctiva vasculature, roughness or opacity of the conjunctiva and abnormal eyelash position or orientation. Such signs could be indicative of conditions such as trichiasis, bulbar injection, pterygium or papillary conjunctivitis.
In assessing the eyelid margins, consider the apposition of the lids and puncta against the globe. Also, look for clear glands near the base of the lashes, and flaking or scaling of the eyelid skin. These may indicate the presence of ectropion, blepharitis or epiphora.
Direct Focal Illumination
This describes any illumination technique where the slit beam and viewing system are focused at the same location. The illumination beam is placed at an angle of 30–50° on the side of the microscope corresponding to the section of the cornea to be viewed. This technique is most commonly used for assessing the transparent optical media of the eye (i.e. the cornea and crystalline lens) because the oblique illumination of the tissue creates a dark background for maximum contrast. Typically a beam width of 2–3 mm is chosen initially, and this may be reduced so as to bring more contrast (due to less light scatter) to an area of interest.
Several specific types of direct illumination are possible, including parallelepiped, optic section and specular reflection.
Using the set-up described above, a 0.1–1.0-mm wide illuminating beam is swept smoothly over the ocular surface ( Fig. 35.5 ), particularly to assess the ocular media (i.e. the cornea and crystalline lens). The parallelepiped is the most commonly used direct illumination technique and is employed, for example, to assess corneal scarring, infiltrates and corneal staining. The corneal profile may show signs of ectasia, thinning or asymmetry. Whilst scanning the external ocular surface, a low-to-medium magnification is initially chosen and the magnification is increased if any area of interest needs to be examined more closely.