PO Box 1097 Newcastle 2940, KwaZulu-Natal, South Africa
E-mail: pauleyes@mweb.co.za
INTRODUCTION
Spectacle lenses (Class A) and contact lenses (Class B) are registered as medical devices with the South African Health Products Regulatory Authority (SAHPRA)1. Both spectacle lenses and contact lenses are important vision-correcting devices. In the majority of cases, the patient has a choice of the device they prefer. However, there are instances when clinically one device yields superior vision, unequivocally becoming the best option. This article delves into the clinical techniques and time-honoured pearls that guide the optometrist to decide which is a better option, spectacle lenses or contact lenses and their indications based on refractive and clinical findings2.
ROUTINE TO FOLLOW TO ARRIVE AT THE TENTATIVE LENS CHOICE
Case history, keratometric/corneal topographic data, pin-hole corrected visual acuity (VA), slit-lamp biomicroscopy and patient compliance guide the practitioner’s choice between spectacle lenses and contact lenses. After conducting a comprehensive eye examination, the presence of spherical or regular astigmatism, especially when corrected spectacle VA is close to contact lens (CL) corrected VA, are indications that the patient is quite free to use spectacles or contact lenses. However, a difficult subjective refraction is the “red flag” that should prompt the optometrist to conduct a more thorough investigation to select the most appropriate device.
SPECTACLE LENS INDICATIONS
Spectacle lenses are warranted when the patient has poor contact lens regimen compliance. Generally, presbyopic patients and young patients requiring progressive addition lenses benefit more from spectacle lenses than contact lenses when optimally clear varying visual distances are critical2,3. Whenever significant horizontal and vertical diplopias are in need of relieving prisms, they are easily incorporated in spectacle lenses. Spectacle lenses are the go-to device when photochromic, gradient, polarised and fixed tints as well as anti-reflection coatings are prescribed.
CONTACT LENS INDICATIONS
Typically, contact lenses are indicated in large irregular astigmatism and corneal ectasia.
CONDITIONS ASSOCIATED WITH IRREGULAR ASTIGMATISM
- Corneal trauma
- Keratoconus
- PMD
- Post keratoplasty (corneal graft)
- Refractive surgery(RK,PRK,Lasik)
- Post HSV
- Ocular surface disease
OCULAR SURFACE DISEASE ASSOCIATED WITH IRREGULAR ASTIGMATISM
- Steven Johnson syndrome
- Neurotrophic disease
- Ocular pemphigoid
- Graft versus host disease
- Exposure keratitis
- Persistent epithelial defect
- Severe Dry eye(Sjogrens, Filamentary K, Limbal stem cell deficiency)
CONFIRMATION OF IRREGULAR ASTIGMATISM
Once you suspect corneal irregularity, you can confirm its presence by using various techniques and tests. By observing the red reflex, either with retinoscopy or ophthalmoscopy you can confirm irregular astigmatism2.
The astigmatic fan and clock are useful tools to confirm irregular astigmatism. These tests are present on many chart and electronic projectors. A patient with spherical refractive error sees all clock hands equally clear or blur while the patient with regular astigmatism after being fogged (to make patient myopic) will see a complete line clear (2 clock hands that form a complete line). Patient with irregular astigmatism will see various random lines clearly.
Looking for distortion in keratometry mires can also be helpful. However, topography is probably the best way to identify the presence of corneal irregularity, its size, location and degree of severity. It can show the
- distorted placido disc image
- location and magnitude of astigmatism (central versus limbus-to-limbus astigmatism)
- corneal versus lenticular astigmatism
- location and magnitude of irregularity (ectasia, PMD, etc)
- Nipple versus oval versus global cone
Quite often, advanced contact lens designs are necessary in irregularly/abnormally shaped corneas. Great skill and a wealth of experience are required in managing these conditions. In-depth knowledge of corneal topography, lens dynamics, software designs and specialised fitting techniques ensure success in these visually compromised eyes. In addition, novel ideas such as aberration-controlled lenses and the use of evidence-based new cutting edge technology have contributed to dramatic improvement of the results recently both in irregular and visually compromised eyes such as ectasias, corneal grafts, poor refractive surgery outcomes etc4,5. The advent of OPD scans, corneal topographers and optical coherence tomography (OCT) scan affords the optometrist an abundance of clinical data that ensures success in providing state-of-art care.
TENTATIVE CONTACT LENS OPTION
Some of the findings that are necessary for the tentative contact lens are the astigmatism found in subjective refraction, corneal astigmatism and the internal astigmatism6. Most of the internal astigmatism is from the crystalline lens and is often referred to as lenticular astigmatism. Lenticular astigmatism is the difference between refractive astigmatism and corneal astigmatism (∆K from keratometry). RGP contact lenses provide excellent vision; however, initial comfort complaints of the patient and inexperience of optometrists in fitting these lenses are the main reasons for their limited success and popularity compared to soft lenses. Rigid lenses are invaluable for managing patients with large amounts of regular as well as irregular astigmatism. Even though currently available soft toric lenses4 may be prescribed for moderate refractive astigmatism, misalignment or rotational instability may result in unacceptable vision and thus RGPs are still used because of the excellent visual acuity they offer. Despite the contemporary hype with scleral lenses, do not forget that RGP lenses have stood the test of time for decades and should form a major part of your armamentarium of vision-correction devices. In addition, the tentative lens is often chosen based on some of the following assumptions:
- Total ocular astigmatism = corneal astigmatism + lenticular astigmatism
- Most corneal astigmatism is transferred through a soft lens to its anterior surface.
- Spherical RGP lenses work well for patients with 0.75D or less of lenticular astigmatism and less than 2.50D of corneal toricity.
- Significant lenticular astigmatism (≥1.00DC) necessitates front surface toric, which is applicable to both hard and soft lenses.
- Irregular corneas and poor visual acuity through soft CL warrant RGP assessment.
- Where there is an equal choice between fitting either a toric hard or toric soft contact lens, a soft contact lens is preferred because of the comfort.
Below are some tables of the major situations encountered and the tentative contact lens choices.
Table 1. CONTACT LENS RANKING WHEN REFRACTIVE ASTIGMATISM ≤0.75. P.Ramkissoon, 2024
| REFRACTIVE SITUATION | CONTACT LENS OF CHOICE |
| Corneal astigmatism = Refractive astigmatism | Option 1:Spherical soft contact lens
Option 2: Spherical RGP |
| Corneal astigmatism ≠ Refractive astigmatism | Option 1:Spherical soft contact lens
Option 2: Spherical RGP |
Table 2. CONTACT LENS RANKING WHEN REFRACTIVE ASTIGMATISM ≠ CORNEAL ASTIGMATISM. P.Ramkissoon, 2024
| REFRACTIVE CONDITION | CONTACT LENS OF CHOICE |
| Refractive astigmatism > Corneal astigmatism (Corneal cyl < 2D) | Option 1: Toric soft Contact lens
Option 2: Front surface toric RGP |
| High corneal astigmatism | Option 1: Bitoric “CPE” RGP
Option 2: Toric Soft contact lens |
Table 3. SPHERICAL CORNEA WITH SPHERICAL REFRACTIVE ERROR. P.Ramkissoon, 2024
| REFRACTIVE CONDITION | CONTACT LENS OF CHOICE |
| Refractive astigmatism = corneal astigmatism
= 0 (Lenticular astigmatism = 0) |
Option 1: Soft contact lens
Option 2: Spherical RGP |
Table 4. SPHERICAL CORNEA WITH ASTIGMATIC REFRACTIVE ERROR. P.Ramkissoon, 2024
| REFRACTIVE CONDITION | CONTACT LENS CHOICE |
| Lenticular astigmatism = refractive astigmatism because corneal astigmatism is 0 | Option 1: Toric soft contact lens
Option 2: Front surface toric RGP |
Table 5. TORIC CORNEA WITH SPHERICAL REFRACTIVE ERROR. P.Ramkissoon, 2024
| REFRACTIVE CONDITION | CONTACT LENS CHOICE |
| Low corneal astigmatism (< 2.00DC) | Option 1: Toric soft contact lens
Option 2: Front surface toric RGP |
| High corneal astigmatism (>2.00DC) | Option 1: Bitoric “CPE” RGP
Option 2: Toric soft contact lens |
Scleral contact lens fitting philosophy dramatically differs from that of RGP contact lens fitting and design. As a result of this, new scleral lens fitters who apply RGP fundamentals to fitting and troubleshooting scleral lens fits, regrettably often do not succeed. Topography will help determine corneal shape such as eccentricity and whether the cornea is prolate or oblate. This information will help you to decide whether or not to start with a regular or a reverse geometry diagnostic lens design. Detailed understanding of tangential and elevation maps are paramount to enhanced fitting strategies of complex corneal shapes.
OCT technology is readily available to allow you to take ultra-fine measurements of a scleral lens fit, enabling more precise design and accurate adjustments. A scleral lens is indicated in instances of extreme sagittal depth and immense, inferior decentered corneal apex when the RGP fitting fails owing to inadequate pupil coverage, inferior peripheral CL edge standoff and increased CL mobility. However, your first diagnostic lens acts as a rudimentary, but effective, topographer and sagittal depth gauge.
A good scleral lens fit should allow an optimal fluid reservoir that is able to mask regular and irregular astigmatism. This fluid reservoir is a liquid bandage that therapeutically helps patients who have severe OSD by bathing the anterior ocular surface constantly. A quadrant-specific design takes into account the different e-values of the abnormal cornea6,7. Unfortunately, the major drawback is that few laboratories can manufacture this design competently.
An important advantage of scleral lenses is that their limbus-to-limbus clearance allows the lens to vault over corneal elevations, and their fluid dynamics allows the post-lens fluid to “correct” corneal irregularities. We need approximately 300 to 400 µm of apical clearance for an acceptable fit. Note that the bulbar conjunctiva has goblet cells that are compressible. As a result, a scleral lens that has 250 microns of apical clearance at dispensing has 150 µm of apical clearance after 8 hours. There are two useful ways to measure the vault of a scleral lens. One involves using a slit beam from a slit-lamp biomicroscope to compare the fluorescein-filled reservoir between the scleral lens and the cornea with the known thickness of the scleral lens. The other method is to measure the vault using OCT imaging. The scleral lens that is ordered should have a reasonable concave radius at the edge for good profile and reduce blanching6,7. Also, good scleral fitting is imperative to avoid a common problem of midday fogging7.
CONCLUSION
As excellent as the university optometry curriculum and lecturers might be, there are many aspects of patient management that you can only get from being in private practice. This is often learned the hard way, after having to deal with unhappy patients and to cover the cost of expensive lens remakes. Ondervinding is die beste leermeester – Experience is the best teacher!
REFERENCES
- South African Health Products Regulatory Authority Classification of Medical Devices and IVDs November 2019.
- Baldwin WJ. Borish’s Clinical Refraction. 1st ed. Philadelphia: WB Saunders & Co. 1998.
- Edrington TB, Barr JT. Toric RGP Indications.Contact Lens Spectrum. November 2001.
- Young G, Hickson-Curran S. Reassessing Toric Soft Lens Fitting
Contact Lens Spectrum. January 2005. - Hom MM. Manual of Contact Lens Prescribing and Fitting. Boston: Butterworth-Heinemann 1997.
- Ramkissoon P. Tentative Contact Lens Options. Vision 2010.
- Ramkissoon P. Advanced Contact Lens Designs and Management of Contact Lens Complications. 2012