Julian D. Stevens

Estimation of true corneal power after keratorefractive surgery in eyes requiring cataract surgery: BESSt formula

PURPOSE: To describe a new formula, BESSt, to estimate true corneal power after keratorefractive surgery in eyes requiring cataract surgery. SETTING: Moorfields Eye Hospital, London, United Kingdom. METHODS: The BESSt formula, based on the Gaussian optics formula, was developed using data from 143 eyes that had keratorefractive surgery. The formula takes into account anterior and posterior corneal radii and pachymetry (Pentacam, Oculus) and does not require pre‐keratorefractive surgery information. A software program was developed (BESSt Corneal Power Calculator), and corneal power was calculated in 13 eyes that had keratorefractive surgery and required cataract surgery. RESULTS: In the eyes having phacoemulsification, target refractions calculated with the BESSt formula were statistically significantly closer to the postoperative manifest refraction (mean deviation 0.08 diopters [D] ± 0.62 [SD]) than those calculated with other methods as follows: history technique (−0.07 ± 1.92 D; P = .05); history technique with double‐K adjustment (0.13 ± 2.39 D; P = .05); Holladay 2 with K‐values estimated with the contact lens method (−0.76 ± 1.36 D; P = .03); Holladay 2 with K‐values from Atlas topographer (Humphrey) (−0.55 ± 0.61 D; P<.01). Using the BESSt formula, 46% of eyes were within ±0.50 D of the intended refraction and 100% were within ±1.00 D. CONCLUSIONS: The BESSt formula was statistically significantly more accurate than the other techniques tested. Thus, it could significantly improve intraocular lens power calculation accuracy after keratorefractive surgery, especially when pre‐refractive surgery data are unavailable.

Fixed dilated pupil (Urrets-Zavalia syndrome) after air/gas injection after deep lamellar keratoplasty for keratoconus.

PURPOSE To describe three cases of fixed dilated pupil and presumed iris ischemia (Urrets-Zavalia syndrome) after anterior chamber air/gas injection after deep lamellar keratoplasty for keratoconus. METHODS Interventional case series. Three eyes of three patients with keratoconus underwent deep lamellar keratoplasty and intraoperative or postoperative injection of air/gas in the anterior chamber to appose the host-donor lamellar graft interface. RESULTS Urrets-Zavalia syndrome was diagnosed on clinical grounds in three cases and was associated with the Descemet membrane microperforation intraoperatively and introduction of air/gas into the anterior chamber intraoperatively or postoperatively. CONCLUSION A fixed dilated pupil is an uncommon complication of penetrating keratoplasty for keratoconus that can also develop after deep lamellar keratoplasty. Leaving an air or gas bubble in the anterior chamber of a phakic eye after deep lamellar keratoplasty is a risk factor and should therefore be avoided.

Extracapsular Cataract Extraction in Proliferative Diabetic Retinopathy

PURPOSE To establish the visual prognosis, prevalence of complications, and optimal strategy for management of proliferative diabetic retinopathy (PDR) in isolated extracapsular cataract extraction (ECCE). METHOD This is a retrospective review of 56 patients with PDR who underwent ECCE with lens implantation. Results were compared with 64 patients with background diabetic retinopathy (BDR) operated on during the same time period. RESULTS Final visual acuity (67% > or = 20/40) was better in BDR eyes compared with PDR eyes (21% > or = 20/40; P < 0.001). In eyes without maculopathy, 94% with BDR achieved a final visual acuity of at least 20/40 compared with 52% with quiescent proliferative retinopathy (P < 0.001). Final visual acuity in eyes with maculopathy was better in BDR eyes (36% > or = 20/40) than in PDR eyes (5% > or = 20/40) (P < 0.02). No patient with active proliferative or preproliferative retinopathy achieved a final visual acuity of more than 20/80. Postoperative deterioration of retinopathy occurred in 50% of patients with active proliferative retinopathy compared with 10% with quiescent proliferative retinopathy (P < 0.01) and 3% with BDR (P < 0.001). Immediate postoperative fibrinous anterior uveitis, which prevented early panretinal photocoagulation, developed in over half the patients with active proliferative retinopathy. CONCLUSIONS Final visual acuity after cataract extraction in diabetic patients with proliferative retinopathy is generally poor; however, in patients with quiescent proliferative retinopathy and no maculopathy, visual acuity may be good. Active proliferative retinopathy at the time of surgery is a poor prognostic indicator for final visual acuity and is associated with postoperative deterioration of retinopathy and fibrinous uveitis, which may preclude immediate postoperative panretinal photocoagulation.

Mathematical modelling of the elastic properties of retina: a determination of Young's modulus.

The retina can be regarded as an elastic membrane or sheet which stretches and deforms when a force is applied to it. Isolated bovine retina was taken and a graded traction force applied to determine retinal profile as a function of force. The resulting profile can be modelled mathematically and the model then used to determine a value for the elastic constant. The value of the elastic constant obtained by this method is approximately 2 N/m. This value of the elastic constant, combined with the observed retinal thickness, yields a value of Youngs modulus for retina of approximately 2 × 104 Pa, which is about 2 orders of magnitude weaker than typical rubber. This value can then be used in modelling retinal behaviour in vivo when forces are applied to detached retina.

Astigmatic Excimer Laser Treatment: Theoretical Effects of Axis Misalignment

OBJECTIVE: Theoretical assessment of the effect of axis misalignment on excimer laser astigmatic treatment using vector analysis. STUDY DESIGN: Vector analysis and spreadsheet solver function used to calculate the effects of an axis misaligned, under correcting, fully correcting and over correcting cylinder. CLINICAL SETTING: Excimer laser photoastigmatic keratectomy has been successful in reducing but often not completely eliminating astigmatism. The assessment and alignment of axis has proved to be a problem, resulting in residual cylinder and a change in astigmatic axis which may leave the patient symptomatic with asymmetric cylinder axes. Understanding the effects of cylinder axis misalignment is important in planning which parameters to use for treatment. RESULTS: A correcting cylinder of equal magnitude to the cylinder present, but axis misaligned produces a resultant cylinder and a small misalignment produces a large rotation in the axis of the resultant cylinder. At an axis misalignment of 30° there is no reduction at all in the magnitude of the original cylinder. When axis error is present the optimal correcting cylinder is not a cylinder of the same magnitude as the original cylinder, a cylinder of less magnitude can provide less resultant cylinder with a smaller amount of induced axis change. Over-correction of a cylinder results in marked axis swing if there is axis misalignment and may leave residual cylinder at an undesired oblique axis. CONCLUSIONS: The slight undercorrecting tendency of current excimer astigmatic treatment has the benefit of compensating for small angles of axis error. Increasing cylinder treatment power may result in marked axis swing of resultant cylinders.

Repair of Descemet's membrane detachment after intraocular surgery

Abstract We repaired three unselected cases of Descemet’s membrane detachment. A visually successful outcome was achieved in one case, an anatomically successful outcome in another, and no improvement in the last. We believe that large detachments should be repaired early rather than waiting for possible spontaneous reattachment, and we advocate sulfur hexafluoride fluid‐gas exchange as the procedure of choice.

Outcomes of phacoemulsification and intraocular lens implantation in microphthalmos and nanophthalmos

Purpose To evaluate the outcomes of phacoemulsification and intraocular lens (IOL) implantation in microphthalmos and nanophthalmos. Setting Moorfields Eye Hospital, London, United Kingdom. Design Retrospective case series. Methods Eyes with an axial length (AL) less than 21.0 mm had elective phacoemulsification and IOL implantation. Results One hundred three eyes (63 patients) were enrolled. The median AL was 20.65 mm (interquartile range [IQR], 20.26 to 20.86) and the median follow‐up, 6.3 months. Complications occurred in 16 cases (15.5%). Zonular dehiscence, severe uveitis, and aqueous misdirection accounted for the majority of complications. Complication rates were 6 (7.3%) of 82 cases with an AL from 20.0 to 21.00 mm and 10 (47.6%) of 21 cases with an AL less than 20.0 mm (P=.0001). Only AL (odds ratio [OR], 0.52 per mm; P≤.0005) and abnormal intraocular pressure (IOP) of 22 mm Hg or more or on topical IOP control (OR, 10.1; P=.001) were significant independent risk factors for complications. For the cohort after adjusting for abnormal IOP, an AL less than 20.5 mm was associated with a 4 times higher odds of any complication (P=.028), an AL less than 20.0 mm was associated with a 15 times higher odds of any complication (P≤.0005), and an AL less than 19.00 mm was associated with a 21 times higher odds of any complication (P≤.0005). Conclusions Phacoemulsification and IOL implantation in microphthalmos/nanophthalmos was challenging but appears safer than previously reported. A shorter AL and abnormal IOP were significant risk factors for complications. Financial Disclosure No author has a financial or proprietary interest in any material or method mentioned.

Prospective Study of Photorefractive Keratectomy for Myopia Using the VISX StarS2 Excimer Laser System

PURPOSE To evaluate the safety, efficacy, and predictability of excimer laser photorefractive keratectomy (PRK) for compound myopic astigmatism using the VISX StarS2 excimer laser system with international version 3.1 software. METHODS We report a prospective consecutive study of myopic excimer laser PRK, performed in a multi-surgeon environment with 200 eyes of 117 patients, to correct naturally occurring compound myopic astigmatism of between -0.50 to -5.90 D manifest refractive sphere and up to -3.50 D manifest refractive astigmatism. Patients were assessed prior to surgery and at 1, 3, 6, and 12 months after treatment. RESULTS One hundred and ninety-eight of 200 treatments (99%) were reviewed 1 year after surgery; 193 of 198 eyes (97%) achieved 20/40 or better uncorrected visual acuity and 163 of 198 eyes (82%) achieved 20/20 or better. One eye lost two lines of Snellen visual acuity assessed at 12 months but recovered acuity when assessed at 18 months. Mean spherical equivalent corneal plane power was reduced from -3.50 to +0.90 D 1 month after treatment and 0 D at 12 months (SD 0.67 D). Three eyes of three patients underwent further treatment, two with LASIK and one with PRK for residual refractive error. Refractive astigmatism of >1.00 D was reduced from a mean -1.70 to -0.70 D at 1 year after treatment. Vector magnitude was 79% of that intended and mean vector axis error (absolute) was 8.5 degrees. No eye had a severe haze response. Pelli-Robson contrast acuity was significantly reduced after treatment from a mean 1.72 D preoperatively to 1.63 D at 12 months (P<.01). CONCLUSIONS PRK for myopia using the VISX StarS2 excimer laser system was effective in the treatment of low myopic astigmatism, although there was a significant reduction in Pelli-Robson contrast sensitivity.

The effect of recurrent pterygium on corneal topography.

Pterygium is known to cause corneal topographic changes that can result in decreased visual acuity. We present a case of recurrent pterygium causing blurred vision and diplopia, and document the corneal topographic changes, showing astigmatism of 15 diopters induced by lateral gaze, prior to surgical excision.

Comparison of dynamic digital pupillometry and static measurements of pupil size in determining scotopic pupil size before refractive surgery

Purpose: To compare 2 digital infrared pupillometers. Setting: Anterior Segment Service, Moorfields Eye Hospital, London, England. Methods: Eighty‐eight eyes of 44 healthy subjects were recruited into the study. The scotopic pupil size was measured with 2 instruments, a digital pupillometer (Procyon) and a Hartman‐Shack wavefront aberrometer (Visx WaveScan). Agreement between the instruments was assessed. Results: There were 21 men and 23 women with a mean age 38.1 years ± 9.2 (SD) (range 23 to 62 years). The mean scotopic pupil diameter was significantly larger with the WaveScan (6.61 ± 0.92 mm) than with the Procyon pupillometer (6.40 ± 0.90 mm), P<.001. Conclusions: There was good agreement in measurement of scotopic pupil size between the two instruments. The larger pupil diameter found with the Visx WaveScan than with the Procyon digital pupillometer probably represents the different illumination level used by each instrument, the effects of accommodation, and target fogging.