Fig. 13.1
Measurement of eye axial length. (a) Measurement of axial length with contact A-mode ultrasound; (b) measurement of axial length with IOL Master; (c) measurement of axial length with Lenstar LS 900
As there is direct or indirect contact with the cornea during both the contact and the immersion measurements, it is hard for young children (especially those under the age of 3 years) to cooperate. Therefore, these measurements need to be performed under anesthesia. Because the measurement accuracy will be affected by the possible occurrence of Bell’s phenomenon among children under light anesthesia, the measurement should be performed when the child’s anesthesia score reaches 1 (being unresponsive to patting or shaking) and the Bell’s phenomenon disappears to improve the accuracy. Besides, as the pediatric eye is soft and easily compressed, the measurement error of the axial length may be even higher especially in cases of poor cooperation. Therefore, an average value should be obtained with multiple repeated measurements so as to reduce the error [1, 17].
Optical Measurement
Nowadays, IOL Master and Lenstar LS 900 are the most frequently used equipments for optical measurement of eye axial length.
IOL Master
This device utilizes the principle of partial coherence interferometry. It divides the laser emitted from the laser diode into two independent rays of axial light, which reach the cornea and the retinal pigment epithelium along the axis. The reflected light passes through the light splitter and is captured by the image detector. Then the axial length is calculated [18, 19]. IOL Master is a technique of noncontact biometry. Besides axial length, it can also simultaneously measure corneal curvature, anterior chamber depth, and horizontal corneal diameter and provide formulas for IOL power calculation. IOL Master has the advantages of being multifunctional, noncontact, accurate, efficient, safe, and simple to operate [19, 20].
The traditional ultrasound method only measures the distance between the anterior corneal surface and the internal limiting membrane of the retina, whereas IOL Master measures the distance between the anterior corneal surface and the retinal pigment epithelium, which is the axial length in real sense and is about 0.2 mm longer than that measured with ultrasound [21]. Meanwhile, IOL Master is optimized for special conditions like silicone oil-filled eyes and pseudophakic eyes with corresponding measurement pattern, which guarantees a convenient and accurate axial length measurement [22–24].
Measurement procedures and techniques: The child is placed in a sitting position, with the chin on the chin rest and the eyes fixating on the marker in the machine (Fig. 13.1b). After the examiner enters the information of the child and clicks to log into the axial length measurement procedure, white spot and green cross-shaped markers appear on the screen. When examining cataractous eyes, larger spots are suggested (with a size close to the green circle regardless of the measurement distance). When examining pediatric eyes with nuclear cataracts, smaller spot and slight vertical deviation from the optical axis are suggested. If the examined eye has a refractive error over 5D, it should be measured with spectacles to enhance fixation and improve the accuracy of the results. If the child wears contact lenses, measurement error may occur.
IOL Master examination needs to be carried out in cooperative children with stable fixation. It is rather challenging to get measurements in young children with poor compliance, in the eyes with inadequate fixation and in the eyes where the light fails to be effectively transmitted due to dense cataracts [24, 25].
Lenstar LS 900
Lenstar LS 900 (Fig. 13.1c) is based on the principle of optical low-coherence reflectometry (OLCR). The single beam of 820-nm laser emitted from the laser diode reaches the surface of each ocular structure and is reflected backward and received by the detector. The target data is obtained after analysis by the embedded software. Like IOL Master, Lenstar LS 900 is also a noncontact measurement device, and in addition to axial length, it delivers nine parameters, including corneal curvature, central corneal thickness, corneal diameter, anterior chamber depth, lens thickness, pupil diameter, angle kappa, and retinal thickness. IOL Master measures eye axial length using the partial coherence interferometry principle, while Lenstar LS 900 measures all the nine parameters based on the principle of OLCR. Though the axial length values obtained by Lenstar LS 900 are greater than those obtained by IOL Master (about 0.01–0.026 mm) [26], the two devices show a high consistency and correlation in various situations including the normal eyes, eyes with various types of cataracts, pseudophakic eyes, aphakic eyes, and silicone oil-filled eyes [22, 26, 27].
Similar to IOL Master, Lenstar LS 900 is suited to cataractous children who are cooperative, have stable fixation without dense cataracts.
13.2.1.2 Measurement of Corneal Curvature
The precision of corneal curvature measurement is another important factor that affects the accuracy of IOL power. An error of 1D in corneal curvature measurement may lead to an error of 0.8–1.3D in IOL power [7, 28]. Corneal curvature can be measured with a manual keratometer, an autorefractor, corneal topography, an aberrometer, and IOL biometry devices based on polarization optics [4]. Common measurement techniques for children include the following types.
Manual Keratometer
Manual keratometer includes the JS (Javal–Schiotz) model and BL (Bausch–Lomb) model manual keratometers (Fig. 13.2a). The keratometer obtains the results by utilizing the principle of Purkinje imaging and measures corneal curvature at the central 3-mm diameter zone. It is suited to cooperative older children and has advantages of being simple, quick, and accurate. But it is not applicable for the eyes with flat (<40D) or steep (>50D) corneal curvature or the eyes with irregular corneal astigmatism.
Fig. 13.2
Measurement of corneal curvature. (a) Measurement of corneal curvature with a manual keratometer; (b) measurement of corneal curvature with a handheld automated keratometer
Automated Keratometer
Automated keratometer includes a desktop model (usually attached to an automated optometer) and a handheld model (Fig. 13.2b). The desktop automated keratometer is suitable for cooperative children with stable fixation, while the handheld keratometer is suitable for supine-positioned anesthetized children. With a mean measurement error of approximately ±0.25D [29], the desktop automated keratometer is more accurate and reproducible compared to the handheld model [9, 13, 30]. The handheld keratometer can be used in children under sedation and anesthesia, but the measurement error may be as high as 6.0D due to the lack of fixation [31]. Some studies have demonstrated that when the eyelid is opened with a speculum, the lacrimal film maintained with lubricating eye drop and fixation is kept with scleral depressor; the corneal curvature measurement has no significant difference compared with that obtained under the state of natural fixation [13].
IOL Master
IOL Master can measure both axial length and corneal curvature simultaneously. The camera with the charge-coupled device (CCD) in this machine captures and measures the distances among the six reflected light spots to calculate corneal curvature. When all six spots in the green circle are clear, the results can be obtained after pressing the button. There is strong homogeneity between the results obtained with IOL Master and that obtained with manual or automated keratometer [30].
Lenstar LS 900
Lenstar LS 900 can also measure eye axial length and corneal curvature on the same machine. When measuring corneal curvature, the stability and reliability of measurement are guaranteed by its multiple measuring spots. Meanwhile, it monitors the patient’s blink and fixation loss, and only the measurements that strictly conform to the standard can be analyzed. Different from IOL Master, the corneal curvature readings of Lenstar LS 900 are the data from multiple measuring spots, which can better demonstrate the information of corneal curvature and morphology, and reduce the measurement error resulting from misalignment of the measuring direction and the axis of reference points [21]. A strong agreement is found between the measurement results of Lenstar LS 900 and that of IOL Master [25, 27].
All these four measurement techniques for corneal curvature are accurate, objective, and reproducible. However, it is still possible for measurement error to occur with any technique. The measurement errors of axial length and corneal curvature are important causes for refractive surprises [28, 32]. To enhance the accuracy of IOL power calculation, remeasurement should be conducted if the readings of axial length and corneal curvature are not within the average range, the IOL power calculated exceeds the predicted limits, or the binocular results are apparently asymmetric.
13.2.2 Formulas for IOL Power Calculation
At present, there is still no formula specially designed for the calculation of pediatric IOL power [33], and calculation is presently conducted with the adult formulas. Both the regression and the theoretical formulas are established on the basis of adult data. But for children with developing eyes, their short axial length and large corneal curvature will give rise to errors with the usage of adult formulas [34–36]. Moreover, the shorter the axial length and the larger the corneal curvature are, the higher the resulting error will be [10]. Besides, effective lens position (ELP) is considered in some IOL power calculation formulas, but the postoperative ELP in children is different from that in adults, which can also lead to certain errors when these adult formulas are used in IOL power calculation for children [37].
Nihalani [10], in a retrospective study, reported that the mean prediction error of IOL power was over 0.5D in 57 % of the pediatric eyes after surgery for 4–8 weeks, and the error was more significant in children under the age of 2 years with axial length shorter than 22 mm and corneal curvature larger than 43.5D. Up till now, lots of ophthalmologists have compared the predictive accuracy of different formulas for IOL power calculation. Andreo LK and colleagues [38] compared the predictive accuracy of four formulas: the SRK-II, SRK-T, Holladay, and Hoffer Q for pediatric IOL power calculation. They found that no significant difference existed among the four formulas at 2 months after surgery. But when the axial length was shorter than 22 mm, the Hoffer Q formula was more accurate, while the SRK-II formula was slightly less accurate. However, the difference between them was not statistically significant. Trivedi [39], in a study of 16 eyes with axial length shorter than 20 mm, also confirmed that the prediction errors of the Holladay II formula and the Hoffer Q formula were similar, ranging from −2.56D to 2.54D and −2.63D to 2.92D, respectively. However, the prediction errors of the Holladay I formula and the SRK/T formula were relatively higher, ranging from −2.94D to 1.86D and −3.24D to 1.63D, respectively. Nihalani [10] proposed that Hoffer Q formula could give a better prediction compared with the SRK-II, SRK/T, and Holladay I formulas for younger children with shorter axial length. In addition, some ophthalmologists demonstrated that in the eyes with extremely short axial length (<19 mm), the Haigis formula had the least refractive error (+0.51 +/− 0.12D), followed by the Hoffer Q formula (−0.70 +/− 0.14 D) and the Holladay I formula (−1.11 +/− 0.13D), and the SRK/T formula had the greatest refractive error (−1.45 +/− 0.14D) [40]. Therefore, for pediatric eyes with short or extremely short axial length, the Haigis and Hoffer formulas are recommended. In view of the unique anatomic features of pediatric eyes, further large-scale randomized controlled clinical trials are needed to determine which formula is more appropriate for younger children (especially for those under the age of 2 years).
13.3 Selection of IOL Power for Children
With the development of ophthalmic microsurgery, IOL implantation is more and more common in pediatric cataract cases. However, there is still no consensus for the selection of pediatric IOL power. Currently, it is commonly believed that when selecting IOL power for children, ophthalmologists should take into account the pediatric patient’s age at IOL implantation, the target refraction, as well as the refractive status of the contralateral eye.
13.3.1 The Age at Surgery of IOL Implantation
As pediatric eyes are still developing, myopic shift can occur after IOL implantation, and the extent varies with age [41–43]. Therefore, it is hard to predict myopic shift precisely, especially in young children [41, 42]. A clinical study demonstrated that the children receiving IOL implantation at the age of 2–3 years had a mean myopic shift of 4.6D, ranging from 0.5 to 10.75D; those at the age of 6–7 years had a mean myopic shift of 2.68D, ranging from 0.5 to 6.60D; those at the age of 8–9 years had a mean myopic shift of 1.25D, ranging from 0.75 to 2.60D; and those at the age of 10–15 years had a mean myopic shift of only 0.61D, ranging from 0 to 1.9D [41]. Another study showed comparable results which also confirmed that the younger at IOL implantation, the greater the myopic shift would be [43]. The mean myopic shift was 5.96D for children who were 1–3 years old at IOL implantation, 3.66D for children at 3–4 years old and 3.40D for children at 5–6 years old. The myopic shift decreased linearly after the age of 3 years [43]. Due to myopic shift and individual differences, the selection of IOL power for children is further complicated.
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