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By Laird Harrison
Posterior capsule rupture rates are surprisingly low in UK cataract patients with very short or very long axial length, researchers say.
The rate of posterior capsule rupture or vitreous loss in a series of 180 114 cataract procedures in eyes with axial lengths under 20.0 mm was less than a third of the rate in some previous studies, write researchers from the Royal College of Ophthalmologists and 3 other UK institutions.
The researchers published their findings online in the journal Eye 23 October 2015.
The study is one in a series of analyses being performed on the Royal College of Ophthalmologists’ National Ophthalmology Database. The college established this database to provide national audit and research data, and to help ophthalmologists anonymously compare their surgical outcomes with those of their peers.
For this analysis, the researchers used anonymised data on 180 114 eyes from 127 685 patients undergoing cataract surgery between August 2006 and November 2010 collected prospectively from 28 National Health Service sites.
They restricted the analysis to cataract operations performed on patients aged 18 years or older using phacoemulsification where the primary intention was cataract surgery not combined with any other procedure. They excluded eyes with missing baseline axial length or a baseline axial length measurement of less than 15 mm.
Of the 127 653 patients, 40.6% were male and 59.1% were female. (The gender was not specified for 0.3%.) At the time of the first eye cataract surgery, the median age of the patients was 77.1.
Increasing axial length has been associated with more advanced levels of nuclear cataract and lower age at time of cataract surgery in previous studies, but in this study there was no association between axial length and age at cataract surgery.
The researchers found that the mean baseline axial length measurement was 23.43 with a standard deviation of 1.38 mm. The baseline axial length was less than 20.00 mm for 0.2%, less than 21.00 mm for 1.6%, less than 22.00 mm for 10.2% and greater than 28.00 mm for 1.0% of the eyes.
The investigators combined posterior capsular rupture and vitreous loss into one statistic, abbreviated as PCR. They defined it as as unintentional communication with the posterior segment from various types of intraoperative complication.
They documented an overall PCR rate of 1.95%, with 3.6% in those with an axial length of less than 20.0 mm and 1.95% in those with an axial length of greater than 20.0 mm.
The PCR rate was lower than in previous trials. For example, one study found a rate of 11.7% in an axial length of less than 20.5 mm, and 12.5% with less than 20 mm.
In this study there was no apparent increase in PCR rates as axial length increased above 26.0 mm as would be expected based on previous studies. One possible explanation is that consultant surgeons performed more cases at the extremes of axial length. In a previous National Ophthalmology Database study, intraoperative complication rates were lower for consultant surgeons than other surgeon grades.
At the extremities of the axial length scale, the presence of brunescent or white cataracts correlated with an increased rate of PCR as follows:
The researchers speculate that this association between brunescent or white cataract and extremes of axial length suggests surgeons were more reluctant to operate on these cases until there was more advanced cataract, perhaps due to expected poorer visual outcomes as a result of higher ocular co-pathology rates or anticipated higher PCR rates in eyes at axial length extremes.
During the study period 41.0% of patients had cataract surgery to both eyes where the median time between the first and second operations was 0.3 years and the median difference in axial length measurements between the eyes was 0.01 mm.
Preoperative visual acuity was defined as the better of uncorrected or corrected distance visual acuity. Measurements were recorded for 82.2% of the eyes, of which 67.9% were corrected distance visual acuity (CDVA) measurements, 29.4% were uncorrected distance visual acuity (UDVA) measurements and 2.7% were eyes for which the CDVA equalled the UDVA.
The median and mean logMAR visual acuity were 0.50 and 0.63 respectively, (Snellen approximations 6/19 and 6/25).
The investigators found ocular copathology (excluding high myopia and unspecified ‘other’) in 31.6% of the eyes. The most frequently recorded ocular co-pathologies were: age-related macular degeneration (AMD) in 10.0%, glaucoma in 8.0%, and diabetic retinopathy in 4.7%.
The researchers found more ocular copathology at the extremes of the axial length scale. Between 22 and 23 mm of axial length, the proportion of eyes with AMD and diabetic retinopathy peaked. It then decreased as the axial length increased.
Several case-control studies have associated hyperopia with AMD, though in population-based studies, this association has been less consistent. The axial length determinant of refraction may be the underlying factor for this association as well, the researchers write.
The proportion of eyes with other macular pathology was constant at 0.9% until 28 mm and increased with axial length thereafter.
The proportion of eyes with either glaucoma or uveitis/synaechiae was 1.0%. Of these co-pathologies, the highest was at the lower end of the axial length scale and rapidly decreased with increasing axial length measurements.
The association of glaucoma with both shorter and longer axial lengths have been reported by other researchers. Such studies have found an increased risk of primary open angle glaucoma (POAG) in eyes with myopia. But models investigating predictors of POAG found the association with moderate or high myopia to be no longer significant when also adjusting for axial length. This suggests that globe axial length is the main biometric risk factor for POAG.
The researchers found brunescent or white cataract in 2.9% of the eyes, and amblyopia in 1.5%. The proportions of eyes with either of these ocular copathologies were highest at the extremes of the axial length scale.
In 1.6% of the eyes, previous vitrectomy surgery had been conducted. This proportion also increased as the axial length increased.
The researchers found the following proportions of other pathologies:
If confirmed, these findings may provide insight into underlying pathological mechanisms they concluded.