OCULUS Pentacam, Руководство по толкованию

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18.2 Step 1: Evaluation of corneal irregular astigmatism
Although there is no inherent problem in performing cataract surgery in patients with mild
pterygium, subclinical keratoconus, or mild corneal scar, it is possible for irregular astigmatism
associated with these corneal diseases to affect the quality of vision of the eye after surgery
[40]
. In
times when cataract surgeries were only performed in patients with advanced visual loss surgeons
did not need to pay as much attention to conditions of mild irregular astigmatism because this
seemed negligible given the remarkable improvement in visual acuity achieved. Today however, in
patients with relatively mild cataract or premium IOL recipients, mild irregular astigmatism can be a
later cause of dissatisfaction when postoperative visual acuity or contrast sensitivity did not improve
as expected.
Preoperative evaluation of corneal irregular astigmatism and obtaining the patient’s informed
consent regarding the effects of corneal irregular astigmatism on quality of vision can help to avoid
claims after surgery also when a conventional IOL is being considered. Even mildly elevated HOAs
can be the cause of suboptimal results with multifocal IOLs, as has become clear from the improved
outcomes achieved with aspherical multifocal IOLs. Currently we have our cut-off value for total
HOAs at 4 mm diameter at 0.3 μm for mild irregular astigmatism and at 0.5 μm for moderate
irregular astigmatism.
18.3 Step 2: Detection of abnormal corneal shape
After many years Laser in Situ Keratomileusis (LASIK) has become a popular and well-established
method of correcting refractive errors. A current topic of discussion however is how postoperative
refractive errors can be avoided following cataract surgery in post-LASIK patients. As is well known,
patients for whom IOL power was calculated by conventional methods often experience a hyperopic
shift in postoperative refraction, disappointing their hopes for good uncorrected visual acuity
following cataract surgery.
To avoid postoperative errors in these patients it is important to review the tomographic map so as
not to overlook any abnormality of corneal shape. IOL power should be calculated using suitably
modified methods
[41]
, while in the no-history methods one can use total corneal refractive power.
18.4 Step 3: Evaluation of corneal spherical aberration
Aspherical IOLs are widely applied for correcting the average corneal SA [42] . However, corneal SA
varies widely even in the normal population. In addition, there are reports of myopic LASIK patients
having higher positive SA
[43]
and hyperopic LASIK and keratoconus patients having lower negative
SA. Measuring corneal SA in candidates for an aspherical or spherical IOL is in any case a reasonable
approach. At present we use a cut-off value of 0.1 μm or higher for aspherical IOLs.
18.5 Step 4: Evaluation of corneal cylinder
Toric IOLs are effective in obtaining good uncorrected visual acuity in patients with regular corneal
astigmatism. However, severe irregular corneal astigmatism is considered as a contraindication
to toric IOL implantation. Therefore, it is critical to evaluate not only corneal regular astigmatism
with a manual keratometer but also corneal total HOA with a corneal tomographer. Comparisons
between manual keratometry and wavefront analysis results on the magnitude and axis of regular
astigmatism value may also be helpful in confirming data reproducibility.
18 Corneal tomographic analysis is essential before
cataract surgery - 4 steps in screening candidates
for premium IOLs