Axial length estimation error caused by hidden double-peak on partial coherence interferometry in an eye with epiretinal membrane: a case report
A 52-year-old female presented with ERM in her left eye (). Her visual acuity at the first visit was 20/25, with mild cataract on slit-lamp examination. The spherical equivalent was −1.5 D. ERM was identified on fundus examination and spectral domain optical coherence tomography (Spectralis®; Heidelberg Engineering, Dossenheim, Germany); this clearly showed a strained line with high intensity above the retina corresponding to the ERM. After 6 months, her retinal thickening became severe due to proliferating ERM, and her visual acuity dropped to 20/100 (). Combined cataract surgery with pars plana vitrectomy was planned.\nOn preoperative examination, the IOLMaster® (version 5.4; Carl Zeiss AG, Oberkochen, Germany) was used for measurement of axial length by PCI. The measurement was repeated 20 times, and a signal curve synthesized from individual curves using an algorithm built into the IOLMaster was displayed as a summary.\nThe highest peak in the summary display was used for measurement of axial length because the signal curve appeared to be, at a glance, a single peak (). Keratometry readings were obtained by the automated keratometer incorporated in the IOLMaster. The SRK/T formula was used for intraocular lens power calculation.\nAfter giving informed consent, the patient underwent combined cataract surgery with pars plana vitrectomy. Phacoemulsification was performed with a 2.2 mm superior self-sealing clear corneal incision, and a foldable acrylic intraocular lens (AcrySof® IQ Model SN60WF; Alcon, Fort Worth, TX, USA) was injected in the capsular bag with full covered continuous curvilinear capsulorhexis. Transconjunctival pars plana vitrectomy was performed using the Alcon 25 gauge vitrectomy system (Alcon). The internal limiting membrane was peeled using indocyanine green staining after removing the ERM.\nOne month postoperatively, her best-corrected visual acuity was improved to 20/20 (). The spherical equivalent was −2.62 D despite aiming for −1.04 D, ie, the postoperative refractive error was −1.58 D.\nRetrospectively viewing all the individual signal curves in the IOLMaster, eight of 20 curves showed more than two peaks. When the amplitude of the second highest peak was higher than 30% of the highest peak and the amplitude of the third highest peak was lower than 50% of the second highest peak, the curve was defined as a double peak, and when the third highest peak was higher than 50% of the second highest peak, the curve was defined as a multi-peak. According to this rule, six eyes showed a double peak and two eyes showed a multi-peak. In cases with a double peak, five curves had a higher anterior peak, whereas one curve had a higher posterior peak. The distance from the anterior peak to the posterior peak of these curves was approximately 0.6 mm. If the posterior peak of the double peak is applied to obtain the axial measurement, the postoperative refractive error is reduced to −0.37 D.\nThe other 14 curves showing single peaks might correspond to the retinal surface because the distance was almost equal to that of the anterior peak of the double peak ().