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Effect of axial length and keratometry measurement error on intraocular lens implant power prediction formulas in pediatric patients
Volume 12 Number 4 / August 2008
manifest deviation is apparent, we agree with this method. The cover test introduces dissociation that may cause decompensation prior to the true level of control; however, it is occasionally necessary to ascertain the alignment of the eyes when the deviation is small, but dissociation should be kept to a minimum. When referring to BVA as acuity during BSV, it has classically been taught in the United Kingdom that the “maximum level of binocular visual acuity is equivalent to the weakest monocular visual acuity.”6 We dispute this and think that the more useful measure is of the maximum level of acuity that can be read when the deviation is controlled, whether or not this exceeds the acuity of the weaker eye. In summary, we suggest using the term controlled binocular acuity to represent the level of visual acuity achieved in the presence of BSV, which is not governed by the level of acuity achieved monocularly and which is tested during an increasing requirement for the correct amount of accommodation and fusional vergence. The end point of the test should be determined by observation of the eyes and a cover test used only where doubt concerning control exists. Alison Y. Firth, MSc, DBO(T) Helen Davis, DBO(T) Academic Unit of Ophthalmology and Orthoptics University of Sheffield Sheffield, United Kingdom References 1. Campbell FW, Green DG. Monocular versus binocular visual acuity. Nature 1965;208:191-92. 2. Wallace DK, Chandler DL, Reck RW, Arnold RW, Bacal DA, Birch EE, et al. Treatment of bilateral refractive amblyopia in children less than 10 years of age. Am J Ophthalmol 2007;144:487-96. 3. Laird PW, Hatt SR, Leske DA, Holmes JM. Distance stereoacuity in prism-induced convergence stress. J AAPOS 2008; Epub March 31, 2008, doi:10.1016/j.jaapos. 2008.01.013. 4. Walsh LA, Laroche GR, Tremblay F. The use of binocular visual acuity in the assessment of intermittent exotropia. J AAPOS 2000;4: 154-57. 5. Ansons AM, Davis H. Diagnosis and Management Of Ocular Motility Disorders (ed 3). Oxford: Blackwell Science Ltd.; 2001. p. 128-9. 6. Rowe F. Clinical orthoptics (ed 2). Oxford: Blackwell Science Ltd.; 2004. p. 69. doi:10.1016/j.jaapos.2008.05.003 J AAPOS 2008;12:424-425. Copyright © 2008 by the American Association for Pediatric Ophthalmology and Strabismus. 1091-8531/2008/$35.00 ⫹ 0
EFFECT OF AXIAL LENGTH AND KERATOMETRY MEASUREMENT ERROR ON INTRAOCULAR LENS IMPLANT POWER PREDICTION FORMULAS IN PEDIATRIC PATIENTS To the Editor: In their article on intraocular lens (IOL) implant power prediction formulas in pediatric patients,
Journal of AAPOS
Letters to the Editor
425
Eibschitz-Tsimhoni et al1 highlight the contribution of short axial length to errors in IOL power calculation formulas.1 I would like to point out that once the exact axial length is found, the choice of formula might not yield such a huge discrepancy in results. What are the sources of error in axial length measurement? Factors such as off-axis measurement and poor cooperation were mentioned in the article,1 but an equally important variable was not mentioned. Whenever we use ultrasound, we assume the same velocity for ultrasound in the aqueous and vitreous humor of 1532 m/s and a greater velocity in blood and hyphema of 1550 m/s.2 Is this actually the case? As a vitreoretinal surgeon, I can attest to the different consistencies of vitreous humor in infants, children, and adults: A child’s vitreous is denser than an adult’s—it cannot be aspirated with even a large-gauge needle. The speed of sound (including ultrasound) depends only on the properties of the medium—not on the frequency of the wave.3 Even solute in water can make sound travel faster. (Sound travels 40 m/s faster in sea water than it does in fresh water.) Because sound travels faster in semisolid materials than it does in fluids,3 the greater velocity in children’s vitreous results in a lower axial length measurement. If vitreous sound velocity were assumed to be 1530 m/s instead of 1580 m/s, vitreous cavity length will be 15.3 mm instead of 15.8 mm. Even when one uses the best formula, this incorrect measurement leads to an error of at least 1.25 D. To prevent this error, the velocity of sound in the vitreous of infants and children should be determined and used in calculations of axial length. (Optical coherence methods can also be used instead of ultrasonographic methods for more accurate axial length measurement in children.) Morteza Mehdizadeh, MD Poostchi Ophthalmology Research Center Shiraz Medical School Shiraz University of Medical Sciences Shiraz, Iran References 1. Eibschitz-Tsimhoni M, Tsimhoni O, Archer SM, Del Monte MA. Effect of axial length and keratometry measurement error on intraocular lens implant power prediction formulas in pediatric patients. J AAPOS 2008;12:173-6. 2. Green RL, Byrne SF. Diagnostic ophthalmic ultrasound. In Ryan SJ, Hinton D, Schachat A, Wilkinson P, editors. Retina. 4th ed. St. Louis: Elsevier; 2006. p. 266. 3. Halliday D, Resnick R, Krane KS. Physics. 5th ed. New York: John Wiley & Sons; 2002. p. 431. doi:10.1016/j.jaapos.2008.05.007 J AAPOS 2008;12:425. Copyright © 2008 by the American Association for Pediatric Ophthalmology and Strabismus. 1091-8531/2008/$35.00 ⫹ 0
REPLY: To the Editor: We read with interest the questions raised by Dr Mehdizadeh concerning our recent paper.1 Our paper
manifest deviation is apparent, we agree with this method. The cover test introduces dissociation that may cause decompensation prior to the true level of control; however, it is occasionally necessary to ascertain the alignment of the eyes when the deviation is small, but dissociation should be kept to a minimum. When referring to BVA as acuity during BSV, it has classically been taught in the United Kingdom that the “maximum level of binocular visual acuity is equivalent to the weakest monocular visual acuity.”6 We dispute this and think that the more useful measure is of the maximum level of acuity that can be read when the deviation is controlled, whether or not this exceeds the acuity of the weaker eye. In summary, we suggest using the term controlled binocular acuity to represent the level of visual acuity achieved in the presence of BSV, which is not governed by the level of acuity achieved monocularly and which is tested during an increasing requirement for the correct amount of accommodation and fusional vergence. The end point of the test should be determined by observation of the eyes and a cover test used only where doubt concerning control exists. Alison Y. Firth, MSc, DBO(T) Helen Davis, DBO(T) Academic Unit of Ophthalmology and Orthoptics University of Sheffield Sheffield, United Kingdom References 1. Campbell FW, Green DG. Monocular versus binocular visual acuity. Nature 1965;208:191-92. 2. Wallace DK, Chandler DL, Reck RW, Arnold RW, Bacal DA, Birch EE, et al. Treatment of bilateral refractive amblyopia in children less than 10 years of age. Am J Ophthalmol 2007;144:487-96. 3. Laird PW, Hatt SR, Leske DA, Holmes JM. Distance stereoacuity in prism-induced convergence stress. J AAPOS 2008; Epub March 31, 2008, doi:10.1016/j.jaapos. 2008.01.013. 4. Walsh LA, Laroche GR, Tremblay F. The use of binocular visual acuity in the assessment of intermittent exotropia. J AAPOS 2000;4: 154-57. 5. Ansons AM, Davis H. Diagnosis and Management Of Ocular Motility Disorders (ed 3). Oxford: Blackwell Science Ltd.; 2001. p. 128-9. 6. Rowe F. Clinical orthoptics (ed 2). Oxford: Blackwell Science Ltd.; 2004. p. 69. doi:10.1016/j.jaapos.2008.05.003 J AAPOS 2008;12:424-425. Copyright © 2008 by the American Association for Pediatric Ophthalmology and Strabismus. 1091-8531/2008/$35.00 ⫹ 0
EFFECT OF AXIAL LENGTH AND KERATOMETRY MEASUREMENT ERROR ON INTRAOCULAR LENS IMPLANT POWER PREDICTION FORMULAS IN PEDIATRIC PATIENTS To the Editor: In their article on intraocular lens (IOL) implant power prediction formulas in pediatric patients,
Journal of AAPOS
Letters to the Editor
425
Eibschitz-Tsimhoni et al1 highlight the contribution of short axial length to errors in IOL power calculation formulas.1 I would like to point out that once the exact axial length is found, the choice of formula might not yield such a huge discrepancy in results. What are the sources of error in axial length measurement? Factors such as off-axis measurement and poor cooperation were mentioned in the article,1 but an equally important variable was not mentioned. Whenever we use ultrasound, we assume the same velocity for ultrasound in the aqueous and vitreous humor of 1532 m/s and a greater velocity in blood and hyphema of 1550 m/s.2 Is this actually the case? As a vitreoretinal surgeon, I can attest to the different consistencies of vitreous humor in infants, children, and adults: A child’s vitreous is denser than an adult’s—it cannot be aspirated with even a large-gauge needle. The speed of sound (including ultrasound) depends only on the properties of the medium—not on the frequency of the wave.3 Even solute in water can make sound travel faster. (Sound travels 40 m/s faster in sea water than it does in fresh water.) Because sound travels faster in semisolid materials than it does in fluids,3 the greater velocity in children’s vitreous results in a lower axial length measurement. If vitreous sound velocity were assumed to be 1530 m/s instead of 1580 m/s, vitreous cavity length will be 15.3 mm instead of 15.8 mm. Even when one uses the best formula, this incorrect measurement leads to an error of at least 1.25 D. To prevent this error, the velocity of sound in the vitreous of infants and children should be determined and used in calculations of axial length. (Optical coherence methods can also be used instead of ultrasonographic methods for more accurate axial length measurement in children.) Morteza Mehdizadeh, MD Poostchi Ophthalmology Research Center Shiraz Medical School Shiraz University of Medical Sciences Shiraz, Iran References 1. Eibschitz-Tsimhoni M, Tsimhoni O, Archer SM, Del Monte MA. Effect of axial length and keratometry measurement error on intraocular lens implant power prediction formulas in pediatric patients. J AAPOS 2008;12:173-6. 2. Green RL, Byrne SF. Diagnostic ophthalmic ultrasound. In Ryan SJ, Hinton D, Schachat A, Wilkinson P, editors. Retina. 4th ed. St. Louis: Elsevier; 2006. p. 266. 3. Halliday D, Resnick R, Krane KS. Physics. 5th ed. New York: John Wiley & Sons; 2002. p. 431. doi:10.1016/j.jaapos.2008.05.007 J AAPOS 2008;12:425. Copyright © 2008 by the American Association for Pediatric Ophthalmology and Strabismus. 1091-8531/2008/$35.00 ⫹ 0
REPLY: To the Editor: We read with interest the questions raised by Dr Mehdizadeh concerning our recent paper.1 Our paper