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Quantification of long anterior lens zonules and their resulting zonule‐free zone sizes

2015; Wiley; Volume: 43; Issue: 8 Linguagem: Inglês

10.1111/ceo.12554

1442-9071

Daniel K. Roberts, Yongyi Yang, Christina Morettin, Trinh Doan, Tricia L. Newman, Jacob T. Wilensky,

Corneal surgery and disorders

Long anterior lens zonules (LAZ) are characterized by zonular fibres, often pigmented, that extend more central than usual on the anterior lens capsule, sometimes causing marked reduction in the zonule-free zone (ZFZ) (Fig. 1).1 One LAZ variety occurs with genetic mutation and late-onset macular degeneration.2 Another type, which may have a prevalence near 2%, has unknown etiology and association with age >50 years, female gender, hyperopia, shorter axial length, and persistent pupillary iris strands.3 LAZ have received attention due to possible association with open- and narrow-angle forms of glaucoma4 and due to concern that their cutting may cause capsular tearing following cataract surgery.1 Normal zonules versus long anterior lens zonules (LAZ) (top left). Retro-illumination images of pupil with different light positions and example lens optic section photo used to calculate ratio of pupil diameter to visible iris diameter (top right). LAZ tracing of nasal/temporal halves of intra-pupillary lens, and halves merged and overlaid onto the pupil zone for illustrational purposes. Zonule-free zone (ZFZ) borders are based on tip of most anterior extending zonule. Final ZFZ is based on smaller half circle (bottom left). Number of LAZ plotted against ZFZ size. Plot includes least squares regression line with 95% confidence limits (bottom right). To further characterize the LAZ trait, we quantified LAZ and their ZFZs using subjects from a prior study who had extensive testing and standardized documentation that included post-dilation retro-illumination photos (16×) of the anterior capsule.4 To view the entire intra-pupillary lens without obscuration, photos were taken with light beams just inside the temporal and nasal pupil borders. We then developed a custom program (MATLAB 6.1, The MathWorks, Inc., Natick, MA, USA) to merge unobstructed image halves and enhance LAZ for mapping (Fig. 1). One observer (CM), masked to subjects, reviewed randomized images and marked the anterior tip and peripheral tip of each LAZ fibre, which was often limited by the pupil border (Fig. 1). Two other observers (TN, DKR) reviewed tracings before acceptance. Fibre counts and meridians were then calculated. For all subjects who had acceptable lens optic section photos at the time of the retro-illumination photos, ZFZ diameters were estimated from "pupil to visible iris diameter" ratios in relation to different diameter assumptions, that is, 11.5, 12.0 and 12.5 mm.5 Three sizes were used to provide sensitivity analysis. Another masked observer (SF) determined the ratios using software (EyeCap SL, V3) provided with the Haag-Streit BX900 photo slit lamp (Haag-Streit AG, Koeniz, Switzerland). Our analyses found that number of LAZ varied greatly, with many subjects having relatively few LAZ (Table 1). We also found a predilection for superior quadrants (Fig. 2). Mappings highlighted a propensity for fibres to extend more centrally in some clock hours than others, and estimations indicated that 50% of eyes had ZFZs ≤3.0 mm in diameter. We did not detect any relationship between ZFZ size and number of LAZ, age or refractive error (P > 0.1). Number of LAZ also did not vary significantly with age or refraction (Fig. 2). Although we did not find the highest number of LAZ in the oldest subjects, it is interesting that we have rarely detected LAZ in people <50 years of age.4 The reason for this remains unknown, and since LAZ are often non-pigmented, better visibility from pigmentation does not adequately explain their detection in later years. Since we did not find significant variation of ZFZ with age, this also suggests that ZFZ size might be established early after LAZ become clinically apparent. TOTAL SUBJECTS† N = 59 (54 female, 5 male) §Mean Age = 70.6 ± 9.0 years (53–91 years) Graph showing long anterior lens zonules (LAZ) predilection for superior quadrant (INF, inferior; NAS, nasal; SUP, superior; TEMP, temporal) (top left). Number of LAZ and zonule-free zone (ZFZ) size plotted against age. Plots include least squares regression lines with 95% confidence limits (top, bottom right). Histogram showing distribution of ZFZ diameters (bottom left). The tracings highlighted a peculiar feature of LAZ, that is, a "segmental" appearance in which fibres end abruptly without visible extension all the way peripherally (Fig. 1). Why this happens remains unexplained, but determining its reason might improve knowledge of lens/zonule physiology. In total, these data help further characterize the LAZ trait, which remains understudied, given its suggested prevalence, potential relationship to glaucoma and macular degeneration, and importance relative to cataract surgery. The authors thank Dr. Sarah Fong for technical assistance provided during components of this investigation.