A thorough preoperative examination and evaluation is the key to successful surgery. In the preoperative examination and evaluation of keratoconus, the main objectives of the surgeon are to assess the patient’s suitability for keratoconus surgery, to select the appropriate surgical procedure, and to predict the outcome of the surgery.
I. Medical history taking
When we receive a patient who is interested in undergoing keratomileusis surgery, we first take a medical history to determine whether the patient is a suitable candidate for surgery through communication with the patient.
1. Ask the patient about the purpose of refractive surgery and initially determine whether the patient is suitable for surgery. The most important thing is to understand whether the patient has unrealistic expectations and needs to exclude these patients from surgery. The surgeon needs to inform the patient that the postoperative vision is unlikely to exceed the preoperative corrected vision, and that the possibility of other postoperative ocular diseases still exists. If the patient still has unrealistic expectations, such as a guaranteed postoperative visual acuity of 1.0, then we need to carefully consider whether or not to perform refractive surgery on these patients, as these patients may have many postoperative adverse complaints.
2. Ask patients about their systemic diseases, previous surgeries and medications, and exclude contraindications to surgery. Some systemic diseases are contraindications to refractive surgery, such as uncontrolled systemic immune diseases, immunodeficiency diseases, and diabetes mellitus. Some medications may interfere with corneal healing, such as corticosteroids and immunosuppressants. Long-term use of corticosteroids can also increase the occurrence of cataracts and lead to vision loss, and patients need to be fully informed.
3. Ask the patient about eye diseases, eye trauma and surgery. Focus on dry eye, blepharitis, corneal epithelial erosion, retinal fissure or detachment. For patients with a history of previous ocular disease, trauma and surgery, a thorough preoperative examination is required to determine suitability for surgery.
4. Ask the patient about his or her occupation and social status to understand the patient’s vision needs and to help choose the appropriate procedure. Different occupational groups have different vision needs, for example, soldiers and police officers need the best distance vision, and teachers need the longest distance reading. For example, police officers and athletes have a relatively high chance of eye trauma, so it is safer to choose superficial cutting surgery.
5. Ask the patient whether the refractive status is stable in the past two years. For patients with unstable refractive status (myopia increased more than 0.75D within two years), patients need to be informed that surgery cannot achieve myopia control and that myopia continues to deepen after surgery leading to vision loss. If the patient has unrealistic expectations, refractive surgery is not recommended.
6. Ask the patient about the previous refractive correction method. Previous refractive correction methods are related to postoperative satisfaction. Find out about corneal contact lens wear, including lens type, wearing time and stop wearing time. Long-term wear of corneal contact lenses leads to changes in corneal refractive power and corneal edema, requiring a certain amount of time off to allow the cornea to recover before refractive surgery can be performed. Typically, soft contact lenses are discontinued for 1 week and RGPs are discontinued for 2 weeks. However, since there are many factors affecting corneal recovery, a combination of slit lamp microscopy and corneal topography is needed to determine if the time off wear is adequate.
Pre-operative examination
A thorough and accurate preoperative examination is a guarantee of successful surgery. Through the preoperative examination, the surgeon ultimately determines whether the patient can undergo keratoconus surgery, which surgical procedure is most appropriate, and whether the surgical outcome meets the patient’s expectations. To ensure the accuracy of the examination, we should regularly commission or update the examination equipment. It is important to keep the special examination technician and the optometrist relatively constant to ensure the accuracy of the examination results.
Preoperative examinations include: examinations related to surgical design (visual acuity and optometry, corneal topography, corneal thickness, wavefront aberration, pupil), routine eye examinations (slit lamp microscopy, intraocular pressure, triple-lens fundus examination), and visual quality assessment (contrast sensitivity).
1. Visual acuity and optometry
For refractive surgery, refractive error determines the amount of laser cut and is critical. Before optometry, bare eye distance visual acuity, the original refractive correction prescription visual acuity, and prescription prescription must be understood.
First, computerized optometry in the natural state versus subjective optometry. The accuracy of computerized optometry is very important and requires regular model eye calibration. Subjective optometry is still the gold standard of optometry, and the key to subjective optometry is the control of accommodation. It is recommended to use a comprehensive optometrist for primary optometry. In primary optometry, the end point of the spherical lens is judged by the best corrected visual acuity of the smallest negative lens or the largest positive lens, combined with the red and green optic balance to assist in judging the end point; the determination of the column lens usually uses crossed column lens + foveal optic mark, but pay attention to the need to combine computerized optometry, aberration and corneal topography and other objective examination results. During the column lens adjustment, if the column lens prescription or axial direction is found to be significantly different from the objective examination, it must be readjusted for confirmation. For people over 40 years of age, near additional (ADD), positive and negative relative adjustment (PRA/NRA) and distance use dominant eye need to be checked after the primary optometry. For patients who already have presbyopia, we can use a monocular optic design scheme, where the dominant eye is nearsighted enough to be corrected while the non-dominant eye is nearsighted enough to be undercorrected, to achieve a balance of near and distance vision. The monocular vision design may have some effect on binocular visual function, and the interference of blurred image on clear image may lead to discomfort, so a trial frame simulation must be performed before surgery to select an appropriate monocular vision design plan with the patient’s subjective comfort and clarity as the primary criteria.
Then, ciliary muscle paralysis (dilated pupil) optometry is performed. Usually, 0.5% tropicamide drops are used every 5 minutes for 3 consecutive dots, and computerized optometry is performed after waiting 15-20 minutes. For myopic patients, the dilation computerized optometry is checked against the natural state primary optometry to determine accommodation control. If the dilated computerized optometry is 0.5 D or more lower than the natural state primary optometry, then a dilated primary optometry is required and the natural state primary optometry is performed again before surgery to confirm the myopia. In myopic patients, over-adjustment is usually due to long-term wear of over-corrected glasses. If the adjustment cannot be relaxed in the natural state, we recommend that the patient be treated for adjustment spasm before refractive surgery. In farsighted patients, the adjustment is often tense in the natural state, so the pupil dilation computerized optometry must be followed by a subjective optometry, and the natural state subjective optometry must be performed again before surgery. The hyperopia detected in the natural state is considered dominant hyperopia, while occult hyperopia must be detected after pupil dilation. For hyperopic patients with particularly tight adjustment, most often seen in those who have never worn corrective glasses, we can recommend that the patient wear corrective glasses for a certain period of time to relax the adjustment before undergoing refractive surgery. It is important to note that the astigmatism and axis position are based on the natural state, not the dilated state.
2. Corneal topography
Corneal topography before refractive surgery is not only to understand the curvature of the cornea, but more importantly, to detect cone corneas, especially subclinical cone corneas, and exclude these patients from refractive surgery. It is important to note that cone keratoconus is a primary corneal disease and is not directly related to keratoconus surgery. Corneal refractive surgery is not appropriate for cone keratoconus because it may accelerate the progression of cone keratoconus due to the inevitable weakening of corneal biomechanics.
Corneal topography is the most important test for subclinical cone corneal screening. Continued improvements in corneal topography technology have also led to increasing sensitivity in the diagnosis of subclinical cone corneas. Traditional corneal topography uses the Placido ring, the main shortcoming of which is that only the anterior surface of the cornea can be examined, but not the posterior surface, and some cone corneas are undetectable in early stages with only posterior surface abnormalities. Since then, Obscan, a corneal topography using slit light scanning mode, has been introduced, and its progress is that it can examine the anterior and posterior corneal surfaces at the same time, but the accuracy of the examination of the posterior corneal surface is poor. Recently, the Pentacam Oculus, a diagnostic instrument for the anterior and posterior corneal surfaces, was developed using the rotating Scheimpflug technique, which allows simultaneous examination of the anterior and posterior corneal surfaces and measurement of corneal thickness at various points. Studies have shown that the Pentacam has a high sensitivity for the diagnosis of subclinical posterior corneal cones.
The corneal topography of subclinical cone corneas includes abnormal increase in lower corneal curvature with corneal thickness thinning, asymmetry in upper and lower corneal curvature, abnormal increase in posterior corneal surface height, and asymmetry in both eyes. For clear subclinical conical cornea or cone cornea, keratoconus is contraindicated. For patients with suspected subclinical cone corneas, we recommend suspending surgery and following up corneal topography and corneal thickness regularly for six months to one year, and considering superficial cutting surgery if the cornea is stable. For patients with predominantly intraocular astigmatism, keratoconus correction is not the best option.
Corneal topography is also one of the main tests to determine the effect of long-term contact lens wear on the cornea. Corneal topography in long-term contact lens wear shows corneal sphericity, corneal irregularity, and corneal edema thickening. The corneal topography can help the doctor determine whether the contact lens has been stopped for an adequate period of time.
Corneal topography combined with optometry can determine the source of astigmatism. The best indication for corneal refractive surgery is for patients with predominantly astigmatism in the cornea.
3.Corneal thickness
Corneal thickness is not only an important indicator to determine the feasibility of keratoconus surgery, but also determines the surgical procedure and the expected efficacy of the surgery together with refraction.
The traditional method of corneal thickness measurement is the A ultrasound. This method has many drawbacks, including contact, the influence of the examiner’s experience and patient cooperation, and the inability to measure corneal thickness in all directions simultaneously, so it is gradually being phased out by newer instruments. Currently, we use the Pentacam to simultaneously obtain corneal topography and thickness maps of all corneal points. The thinnest corneal thickness is taken as a parameter in the surgical design. After a controlled study, we found that the difference in central corneal thickness measurement between Pentacam and A-ultrasonic instrument is very small, while Pentacam is easier to operate and obtain multi-point corneal thickness in one measurement.
Corneal thickness is an important indicator of the feasibility of corneal refractive surgery. When the corneal thickness is too thin (less than 450 μm), keratoconus surgery is not recommended, even if the corneal topography is normal, because the thinness increases the risk of postoperative corneal bulging.Pentcam’s corneal thickness map is also an important reference to aid in the diagnosis of subclinical cone corneas.
Corneal thickness combined with refractive error determines which procedure is more appropriate and the expected surgical outcome. The most important consideration in the choice of surgical procedure is corneal biomechanics; the corneal flap in LASIK contributes little to corneal biomechanics and is therefore suitable for patients with normal or thick corneal thickness. To minimize the impact on corneal biomechanics, it is safer to use a thin corneal flap whenever possible. Studies have shown that a safe corneal thickness for LASIK is a stromal bed thickness >250 μm and a residual stromal bed thickness greater than 1/2 of the original corneal thickness. superficial cutting surgery has less impact on corneal biomechanics compared to LASIK and can be used for patients with high myopia and relatively thin corneas. However, it should be noted that corneal bulging can also occur after superficial cutting surgery, so it is also necessary to ensure a safe stromal thickness >360μm.
4.Wavefront aberration
Wavefront aberration examination provides objective refraction and is usually performed before the primary optometry. The most important role of wavefront aberration examination is the diagnosis of complications related to higher order aberrations with wavefront aberration-guided laser cutting. The aberration-guided or aberration-optimized procedure can reduce the increase in spherical aberration of surgical origin and improve the quality of postoperative vision. The optical causes of the patient’s visual complaints, such as the halo sign due to spherical aberration and the trailing sign due to comet aberration, can be found by aberration examination. The use of the appropriate aberration-guided remedial surgery can effectively improve the quality of vision. Wavefront aberration is also an important indicator to assess the efficacy of the procedure.
Currently, the most commonly used aberrometer is the Hartmann-Shark aberrometer. The examination can be performed in the natural state, or under ciliary muscle paralysis to reduce the effect of adjustment on spherical aberration.
5.Pupil examination
Because pupil diameter is directly related to the design of the laser cutting optical zone size and postoperative visual quality, the darkroom pupil diameter must be checked in all patients prior to dilated optometry. We use the pupil photography that comes with the computerized optometry to measure pupil diameter under a dark room. For patients with a large darkroom pupil (>7mm), if they also have a high degree of myopic astigmatism, the chance and degree of postoperative halos will increase and may even affect night driving, and we must fully communicate with the patient.
6.Slit lamp microscopy
Slit lamp microscopy examines the anterior segment of the eye with the posterior pole. Severe blepharitis and dry eye need to be treated and improved before performing keratoconus surgery. For patients with hypersecretion of tears, immune diseases such as dry syndrome need to be excluded. Conjunctival scars that are in close proximity to the cornea may interfere with negative pressure attraction, and superficial excision may be considered to avoid negative pressure attraction problems. For corneal epithelial detachment, especially in long-term corneal contact lens wearers, discontinue contact lens wear until the cornea recovers. Corneal clouding should be determined before deciding whether laser correction is feasible. Corneal granular dystrophy can recur after laser excision, so caution is needed in younger patients. Iris pigmented nevi that are large and close to the pupil may interfere with surgical follow-up, which can be simulated by having the patient under the laser or reduced by adding lateral illumination. Lens disease, such as cataracts and crystal hemianopia, are not indications for keratoconus surgery. Cup-to-disc ratios greater than 0.5 or bilateral asymmetry require exclusion of glaucoma. Patients with corrected visual acuity less than 1.0 should be concerned about the macula, and optical coherence tomography should be performed if necessary to rule out macular degeneration, clefting, and other diseases.
Macular disease is not a contraindication to keratoconus surgery, but refractive surgery must be considered only when the patient fully understands the condition and has a correct understanding of refractive surgery.
7.IOP
Measurement of intraocular pressure has a dual significance for keratoconus surgery. The first is to help rule out glaucoma. For patients with IOP >21mmHg or with suspicion of glaucoma on fundus examination further visual field and OCT examinations are needed to rule out glaucoma. Second, hormonal eye drops must be used after keratoconus surgery, and preoperative basal IOP is our main reference indicator for determining hormonal hypertension. We usually use a non-contact IOP meter to measure three times to take the average value. If the non-contact IOP is higher, it is recommended to review it again with pressure leveling IOP.
8.Triangulation fundus examination
After the dilated eye exam, a triple-lens fundus examination is performed. Special attention should be paid to the optic disc (cup-to-disc ratio), macula (degeneration) and peripheral retina (lacunae and detachment). Fundus disease is not an absolute contraindication to keratomileusis and must be treated before keratomileusis can be performed. Our team routinely performs argon laser closure of peripheral retinal fissures and waits 2 weeks before performing keratomileusis; regular follow-up is recommended for peripheral retinal degeneration without treatment. Patients with fundus disease are safer with superficial cutting surgery because the interference of negative pressure attraction to the fundus can be avoided.
9.Contrast sensitivity
Contrast sensitivity testing is not used for surgical design, but to evaluate visual quality. The contrast sensitivity curve reflects the ability to discriminate between different spatial frequencies of contrast. The increase in higher order aberrations after keratoconus surgery often does not cause a decrease in visual acuity, but rather shows a decrease in contrast sensitivity. Therefore. Contrast sensitivity is a more comprehensive reflection of the patient’s actual visual experience after keratomileusis than visual acuity or refraction. Contrast sensitivity examination can be performed with the internationally used CSV-1000E contrast sensitivity meter.
Pre-operative talk and signature
Although most of the contraindications have been eliminated by the perfect preoperative examination, and the corrective efficacy of the surgery can be predicted more accurately, due to the limitations of the sensitivity of the examination equipment, individual differences in laser energy response, individual differences in postoperative corneal healing and collagen proliferation and other uncertainties, the surgeon must have a detailed conversation with the patient before surgery to inform the expected surgical efficacy and possible The surgeon must have a detailed conversation with the patient before surgery, inform him/her of the expected outcome and possible uncertainties, and sign an informed consent form after obtaining the patient’s full understanding.
In the case of certain risk factors, such as the risk of undercorrection and regression due to high refractive error, the risk of nocturnal visual impairment due to high refractive error with large pupils, and the risk of refractive instability and presbyopia due to ageing of the lens over 40 years of age, the surgeon needs to emphasize and obtain the full understanding of the patient.