Alteration of the pattern of regenerative corneal subbasal nerves after laser in-situ keratomileusis surgery.

PURPOSE: Corneal nerves exhibit high plasticity, which allows successful reinnervation after nerve damage caused by laser in-situ keratomileusis (LASIK) surgery. This study aimed to examine corneal subbasal nerve orientation during regeneration after LASIK. METHODS: This study involved 20 healthy, myopic subjects who had undergone bilateral Femto-LASIK 12-16 months prior with a superior hinge position. The corneal subbasal nerve plexus at the central, mid-temporal and mid-superior cornea on the right eye were imaged using in vivo confocal microscopy. Global nerve fibre orientation (indicating the overall pattern) and variation of nerve fibre orientation (indicating the consistency of the orientation) was determined using customised MATLAB™ software (www.mathworks.com/products/matlab.html). Differences in nerve orientation variables between groups were examined using the Mann-Whitney U test. Linear mixed models with Bonferroni adjustment were conducted to examine differences between corneal regions, and over time, after LASIK. RESULTS: There were no differences between post-LASIK and control groups in global nerve orientation at any of the examined corneal regions. The post-LASIK subjects had a greater variation of nerve orientation at the central (p = 0.007) and temporal (p = 0.049) cornea than the controls. There was a difference in global nerve fibre orientation between corneal regions (p < 0.001) in the controls but not in the post-LASIK group. The variation of nerve fibre orientation was higher at the central, compared to the temporal and superior cornea after LASIK (p < 0.001), although there were no differences between corneal regions in controls. CONCLUSIONS: Our results demonstrate that there was an increased variability in the corneal subbasal innervation patterns following LASIK when compared to controls.

Corneal Sensitivity After Ocular Surgery.

The cornea is densely innervated with free nerve endings to provide a high level of sensitivity to foreign bodies or noxious substances. They also provide trophic support to the tissues of the cornea and facilitate their repair and replacement. Any reduction in the function of the nerve endings through disease, contact lens wear, or surgery may lead to corneal disease, damage, or reduced healing. Assessment of the corneal nerve function can be made by the use of specialized instruments (aesthesiometers) that stimulate the corneal nerves using different modalities-mechanical, chemical, and thermal. Each modality assesses the function of a different cohort of corneal nerve type. Ocular surgery, particularly corneal surgery, can produce significant damage to the corneal innervation. However, for the majority of surgical procedures, corneal sensation eventually returns to preoperative levels, given enough time. The principal exceptions to this are penetrating keratoplasty, epikeratophakia, and cryo-keratomileusis, where sensation rarely returns to normal. For all types of surgery, the pattern of corneal sensation loss and recovery depends on the type, depth, and extent of incision because these influence the number of nerve fibers severed, and on the healing response of the patient.

Effects of ambient humidity on the Cochet-Bonnet aesthesiometer.

PURPOSE: The Cochet-Bonnet (COBO) aesthesiometer is the current standard in corneal sensitivity assessment. This study investigates the influence of ambient room humidity levels on the stimulus force exerted by the instrument. METHODS: A COBO instrument (Luneau Opthalmologie) with 0.12 mm nominal nylon filament diameter was placed in an environment chamber (Electro-tech systems Inc. PA, USA) at 25 °C and relative humidity (%RH) set to either 20-80%, in 10% steps. After 12 h in the chamber at a chosen %RH level, the instrument was removed and exerted force measured by pressing the nylon filament onto the plate of an analytical microbalance (Mettler-Toledo AB265; precision ±0.0001 g) at a perpendicular angle, by a predetermined amount. Exerted force onto the microbalance was recorded in grams for a specified filament length. Procedure was repeated for filament lengths 10-60 mm, in 5 mm steps. The instrument was returned to the chamber and procedure repeated 5 times, before repeating at the next %RH setting (random order). Measurements at each filament lengths were compared using one-way ANOVA and post-hoc Tukey's range test. A p-value < 0.05 denoted statistical significance. RESULTS: Significant differences in exerted force were observed with alteration in %RH levels for each filament length (all p < 0.001). Exerted force decreased significantly with an increase in %RH for all filament lengths, with the average force decreasing by 15% with each 10% rise in %RH. CONCLUSIONS: This study confirms previous suggestions that the rigidity of the COBO nylon filament is affected by ambient room humidity levels, with implications on the stimulus force delivered by the instrument. A conversion table is provided for converting filament lengths to pressure for a range of relative humidity levels.

Corneal Nerve Morphology and Tear Film Substance P in Diabetes.

PURPOSE: This work aims to characterize the relationship between tear film neuropeptide substance P and the structural integrity of the sub-basal nerve plexus in diabetes. METHODS: Seventeen healthy control participants and nine participants with diabetes were recruited in this cross-sectional study. Total protein content and substance P concentrations were determined in the flush tears of participants. Corneal nerve morphology was assessed by capturing the corneal sub-basal nerve plexus using the Heidelberg Retinal Tomograph II with the Rostock Corneal Module (Heidelberg Engineering GmbH, Heidelberg, Germany) in the central cornea. Corneal nerve fiber density (CNFD) was measured using ACCMetrics (M.A. Dabbah, Imaging Science and Biomedical Engineering, Manchester, UK) on eight captured images. Comparisons between groups were made using independent samples t-tests. Correlations between parameters were analyzed using Pearson's correlations. RESULTS: Substance P concentrations were significantly higher in the tears of the control group compared to participants with diabetes (4150 ± 4752 and 1473 ± 1671 pg/mL, respectively, P = .047). There was no significant difference in total protein content between the groups (3.4 ± 1.8 and 2.6 ± 1.7 mg/mL in the control and diabetes groups, respectively, P = .262). CNFD was significantly lower in the participants with diabetes compared to the control group (16.1 ± 5.7 and 21.5 ± 7.0 mm/mm, respectively, P = .041). There was a moderate correlation between substance P and CNFD (r = 0.48, P = .01). CONCLUSIONS: Substance P is expressed at a significantly lower level in the tears of people with diabetes compared with healthy controls. The positive correlation between substance P and corneal nerve density indicates that substance P may be a potential biomarker for corneal nerve health.

Changes in corneal subbasal nerve morphology and sensitivity during orthokeratology: Recovery of change.

PURPOSE: To investigate the recovery of orthokeratology (OK)-induced changes in corneal nerve morphology and sensitivity following lens wear discontinuation, over a 3-month period. METHODS: Sixteen myopic subjects who wore OK lenses during sleep for 3 months discontinued lens wear for 3 months. Corneal nerve morphology and sensitivity were assessed on the right eye only 3 h after waking at the pre-lens wear baseline and after lens wear discontinuation. Corneal nerve fiber density (NFD) and global nerve fiber orientation (GNFO) were assessed by sampling a 1 mm2 area of the subbasal nerve plexus at the corneal apex and temporal mid-periphery using in vivo confocal microscopy. Corneal sensitivity was measured using the Cochet-Bonnet aesthesiometer at similar corneal locations. RESULTS: Significant changes in corneal sensitivity, NFD, and GNFO were observed at various corneal locations during the lens wear discontinuation period. In the central cornea, NFD and corneal sensitivity increased during the 3-month non-lens wear period (p < 0.01); no significant changes in NFD or corneal sensitivity were noted in the mid-peripheral cornea. Central corneal sensitivity recovered to pre-lens wear levels after 1 month of non-lens wear. Central NFD appeared to remain slightly decreased after 3 months of non-lens wear compared to pre-lens wear levels, but this difference did not reach statistical significance (p = 0.09). GNFO did not change significantly at the central cornea during the 3-month non-lens wear period, but altered in a counter-clockwise rotational direction (reversal of OK-induced change) at the mid-peripheral cornea (p < 0.05), although it did not appear to fully recover to pre-lens wear orientation (p = 0.05). CONCLUSIONS: OK lens-induced reduction in corneal sensitivity recovers with cessation of lens wear, returning to pre-lens wear levels shortly after lens wear discontinuation. Changes to nerve morphology induced by OK lens wear, however, appear to recover more slowly towards pre-lens wear levels. This requires confirmation in a longer non-lens wear period.

Changes in Corneal Subbasal Nerve Morphology and Sensitivity During Orthokeratology: Onset of Change.

PURPOSE: To investigate changes in corneal subbasal nerve fiber density and orientation during a 3-month orthokeratology (OK) lens wear period and their relationship with concurrent changes in corneal sensitivity. METHODS: Sixteen subjects wore overnight OK lenses for 90 days and were assessed at baseline, Day 30, and Day 90. Nerve images at the corneal apex and temporal mid-periphery were captured from the right eye only using in vivo confocal microscopy and analyzed to calculate nerve fiber density (NFD) and global nerve fiber orientation (GNFO). Corneal sensitivity was measured using the Cochet-Bonnet aesthesiometer at similar corneal locations. Control groups of non-lens (NL) wearing and conventional rigid gas-permeable (GP) lens-wearing subjects were also examined. RESULTS: Significant changes in NFD, GNFO and corneal sensitivity were observed during OK lens wear over the study period. In the central cornea, both NFD and corneal sensitivity decreased by Day 30, with further reductions at Day 90. Reduced NFD was associated with reduced corneal sensitivity. In the mid-peripheral cornea only, GNFO was rotated in a clockwise direction at Day 30, with further rotation at Day 90. Corneal sensitivity reduction plateaued by Day 30. In the GP lens-wearing subjects, only corneal sensitivity decreased by Day 30 at both corneal locations. No changes were observed in the NL control subjects. CONCLUSIONS: Alterations in corneal nerve morphology occur rapidly with commencement of OK lens wear and appear to underpin functional sensitivity loss. Nerve fiber orientation can provide a useful index for changes in corneal nerve morphology.

Reduced Corneal Sensitivity and Sub-Basal Nerve Density in Long-Term Orthokeratology Lens Wear.

OBJECTIVES: To investigate changes in corneal sensitivity and nerve morphology in orthokeratology (OK) contact lens wear. METHODS: In a cross-sectional study, 54 subjects (aged 18-45 years) were grouped into three categories: nonlens (NL), soft lens (SCL), and OK lens wearers. Corneal sensitivity was measured at the corneal apex and 2.5 mm temporal to the apex using the Cochet-Bonnet aesthesiometer. Corneal nerve morphology was assessed by sampling a 1 mm area of the corneal sub-basal nerve plexus using the Heidelberg Retinal Tomograph with Rostock Corneal Module at the corneal apex and 2.5 mm temporal to the apex. Nerve fiber density (NFD) was calculated by measuring the total length of nerve fibers per square millimeter using Image-Pro Analyser. Comparisons between groups were made using mixed analysis of variance and post hoc paired t tests with Bonferroni correction or the Kruskal-Wallis test and post hoc Mann-Whitney U tests as appropriate. RESULTS: There was a significant difference in corneal sensitivity between the three groups (P=0.027). Central threshold was significantly higher in the OK than NL group (0.69±0.42 g/mm vs. 0.45±0.12 g/mm; P=0.048). Mid-peripheral threshold was not different between the three groups (P>0.05). There was a significant difference in NFD between the three groups (P<0.001). Central NFD was significantly less in the OK than NL and SCL groups (OK: 17.89±4.42 mm/mm, NL: 25.87±5.00 mm/mm; SCL: 24.52±4.93 mm/mm; P<0.001). Mid-peripheral NFD was not different between the three groups (P>0.05). CONCLUSIONS: Long-term OK lens wear is associated with a decrease in central corneal sensitivity and NFD. The mechanism underlying refractive change during OK treatment seems to impact both corneal sensitivity and nerve morphology.

Treatment Zone Decentration During Orthokeratology on Eyes with Corneal Toricity.

PURPOSE: To compare the magnitude of treatment zone decentration between eyes with minimally toric corneas (≤1.50 DC, LoTor group) and eyes with moderately toric corneas (1.50 to 3.50 DC, HiTor group) after a single overnight wear of spherical orthokeratology lenses. METHODS: In the LoTor group, 21 participants (9 M, 12 F, 20-40 years) were fitted using a conventional fitting approach based on the flat corneal meridian. In the HiTor group, 12 participants (5 M, 7 F, 19-45 years) were fitted using the conventional fitting method in one eye (HiTor group I) and the other eye was fitted with a lens with slightly deeper sagittal height (HiTor group II). In all groups, BE spherical orthokeratology trial contact lenses (Boston XO) were used and corneal topography data (Medmont E300) were obtained at baseline and after a single overnight wear. The magnitude of treatment zone decentration relative to vertex normal was determined from corneal topography refractive power difference maps. Treatment zone parameters including magnitude and direction of decentration were analyzed and related to baseline corneal parameters. RESULTS: After a single overnight wear, the mean magnitude of treatment zone decentration was 0.48 ± 0.20 mm in the LoTor group, 1.06 ± 0.57 mm in HiTor group I, and 0.95 ± 0.44 mm in HiTor group II. Treatment zone decentration in the LoTor group was significantly different from HiTor group I (p < 0.001), both fitted using a conventional fitting method. Treatment zone decentration was not significantly different between HiTor group I and II (p = 0.606). The magnitude of treatment zone decentration was positively correlated with the amount of baseline corneal toricity (LoTor and HiTor group I combined, p = 0.048). CONCLUSIONS: Eyes with higher amounts of corneal toricity give rise to increased amounts of treatment zone decentration in overnight orthokeratology.

Myopia control during orthokeratology lens wear in children using a novel study design.

PURPOSE: To investigate the effect of overnight orthokeratology (OK) contact lens wear on axial length growth in East Asian children with progressive myopia. DESIGN: A prospective, randomized, contralateral-eye crossover study conducted over a 1-year period. PARTICIPANTS: We enrolled 26 myopic children (age range, 10.8-17.0 years) of East Asian ethnicity. METHODS: Subjects were fitted with overnight OK in 1 eye, chosen at random, and conventional rigid gas-permeable (GP) lenses for daytime wear in the contralateral eye. Lenses were worn for 6 months. After a 2-week recovery period without lens wear, lens-eye combinations were reversed and lens wear was continued for a further 6 months, followed by another 2-week recovery period without lens wear. Axial eye length was monitored at baseline and every 3 months using an IOLMaster biometer. Corneal topography (Medmont E300) and objective refraction (Shin-Nippon NVision-K 5001 autorefractor) were also measured to confirm that OK lens wear was efficacious in correcting myopia. MAIN OUTCOME MEASUREMENTS: Axial length elongation and myopia progression with OK were compared with conventional daytime rigid contact lens wear. RESULTS: After 6 months of lens wear, axial length had increased by 0.04±0.06 mm (mean±standard deviation) in the GP eye (P=0.011) but showed no change (-0.02±0.05 mm) in the OK eye (P=0.888). During the second 6-month phase of lens wear, in the OK eye there was no change from baseline in axial length at 12 months (-0.04±0.08 mm; P=0.218). However, in the GP eye, the 12-month increase in axial length was significant (0.09±0.09 mm; P<0.001). The GP lens-wearing eye showed progressive axial length growth throughout the study. CONCLUSIONS: These results provide evidence that, at least in the initial months of lens wear, overnight OK inhibits axial eye growth and myopia progression compared with conventional GP lenses. Apparent shortening of axial length early in OK lens wear may reflect the contribution of OK-induced central corneal thinning, combined with choroidal thickening or recovery due to a reduction or neutralization of the myopiogenic stimulus to eye growth in these myopic children.

Central and paracentral corneal curvature changes during orthokeratology.

PURPOSE: To investigate regional changes in corneal curvature and power induced by overnight orthokeratology (OK) contact lens wear over a period of 2 weeks. METHODS: Corneal topography data (Medmont E300) from 21 myopes (12 M, 9F, 20 to 40 years), who had worn BE OK lenses manufactured in Boston XO material for 14 nights, were analyzed retrospectively. Enrollment criteria were myopia up to 4.50 D and corneal toricity up to 1.50 D. Custom MATLAB programs were used to determine sectorial tangential curvature and refractive power, and to investigate changes from baseline after 1 and 14 nights, and between 1 and 14 nights of lens wear in the central circular zone (CCZ) and surrounding paracentral annular zone (PCZ), with each zone subdivided into nasal, superior, temporal, and inferior sectors. RESULTS: After OK, significant asymmetry was found in tangential curvature across sectors. In the CCZ, by day 14 there was greater flattening in the temporal (-1.27 ± 0.62 D, p < 0.001) than nasal sector (0.05 ± 0.62 D, p = 0.893). In the PCZ, by day 14 there was greater steepening in the temporal (2.37 ± 1.09 D, p < 0.001) than nasal sector (0.30 ± 1.36 D, p = 0.326). In both zones, vertical sectors did not show any asymmetry. The variation in corneal curvature across sectors and the mirror asymmetry was also reflected in variations in the corneal refractive power. CONCLUSIONS: OK induces non-uniform corneal changes to the central and paracentral regions. This non-uniformity may influence peripheral refraction profiles reported with OK that have been suggested to be influential in myopia control.

Corneal sensitivity with contact lenses of different mechanical properties.

PURPOSE: To examine the changes in corneal sensitivity after overnight wear of contact lenses with different mechanical properties. METHODS: Twenty young-adult subjects wore a silicone hydrogel, rigid gas-permeable, or orthokeratology (OK) contact lens in randomized order for a single night of wear in the right eye only. All lenses were matched in Dk/t (∼46 ISO Fatt). Changes in corneal apical radius r(o), asphericity Q, and corneal refractive power (Medmont E300) were measured. Changes in central corneal sensitivity were also measured by a masked investigator using two instruments: Cochet-Bonnet (COBO) aesthesiometer and Non-Contact Corneal Aesthesiometer (NCCA). RESULTS: There were significant differences in corneal topographic change from baseline between the lens types for r(o), Q, and corneal refractive power. There were also significant differences in the change from baseline (mean ± SD) in corneal sensitivity between lens types using the COBO (silicone hydrogel, 0.02 ± 0.17 g/mm(2); rigid gas-permeable, 0.03 ± 0.20 g/mm; OK, 0.22 ± 0.33 g/mm(2)). A significant increase in threshold from baseline was only seen in the OK lenses (p = 0.006). There was no change in sensitivity thresholds from baseline for any lens type using the NCCA (p > 0.05). CONCLUSIONS: Central corneal sensitivity is reduced after a single overnight wear of OK lenses, as measured using the COBO aesthesiometer. This suggests that the mechanical force exerted by contact lenses may influence corneal sensitivity.

Mapping the corneal sub-basal nerve plexus in orthokeratology lens wear using in vivo laser scanning confocal microscopy.

PURPOSE: This study was designed to map the sub-basal nerve plexus (SBNP) in the cornea of orthokeratology (OK) lens wearers. METHODS: Laser scanning confocal microscopy (LSCM) was performed in vivo on three subjects: a non-lens wearer and two OK lens wearers. Scans were performed on the right eye while the left eye fixated a moving target. A total of 575, 430, and 676 contiguous images of the SBNP were taken from the non-lens wearing and the OK lens wearing subjects, respectively, and used to construct maps of the central to mid-peripheral SBNP. RESULTS: In the non-lens wearing eye, nerves radiated towards a whorl-like complex centered nasally and inferiorly in an overall pattern consistent with previously reported studies. In the OK lens wearing eyes, this whorl pattern was absent, replaced by a tortuous network of nerve fibers centrally, and thicker curvilinear fibers mid-peripherally, particularly in the nasal, inferior, and temporal regions. CONCLUSIONS: This study maps the corneal SBNP in OK lens wearers and provides compelling evidence that OK lens wear alters the normal SBNP distribution observed in healthy, non-lens wearing eyes.

Lens Dk/t influences the clinical response in overnight orthokeratology.

PURPOSE: To investigate the influence of lens oxygen transmissibility (Dk/t) on the clinical response to overnight (ON) orthokeratology (OK) lens wear over 2 weeks. METHODS: Eleven subjects (age, 20 to 39 years) were fitted with OK lenses (BE; Capricornia Contact Lens) in both eyes. Lenses in matched design/fitting but different materials (Boston EO and XO; nominal Dk/t: 26 and 46 ISO Fatt, respectively) were worn ON only in the two eyes over a 2-week period. Changes in logarithm of the minimum angle of resolution visual acuity, subjective refraction (spherical equivalent), corneal apical radius ro and asphericity Q (Medmont E300), and central stromal thickness (Holden-Payor optical pachometer) were measured. RESULTS: There were statistically significant differences in outcomes between the two lens materials (analysis of variance, p < 0.001) throughout the study. After 2 weeks of ON OK lens wear, changes from baseline (mean ± standard deviation) with the EO lenses were significantly less than XO for visual acuity (-0.72 ± 0.37 vs. -0.83 ± 0.41; p = 0.012), refraction [+2.19 ± 0.73 diopter (D) vs. +2.74 ± 0.70 D; p = 0.004], ro (0.34 ± 0.08 mm vs. 0.46 ± 0.11 mm; p < 0.001), and Q (0.26 ± 0.08 vs. 0.36 ± 0.08; p < 0.001). After the first ON OK lens wear, change in central stromal thickness was greater for EO compared with XO (27 ± 36 µm vs. 10 ± 31 µm; p = 0.05), but ON edema was reduced after 2 weeks for both lens materials (8 ± 25 µm vs. -1 ± 33 µm; p > 0.05). CONCLUSIONS: An increase in lens Dk/t appears to increase the clinical effects of ON reverse-geometry lens wear over the medium term. This adds further support to the recommendation that high Dk materials should be used for ON OK not only to provide physiological advantages but also to optimize clinical outcomes.

Performance standards for toric soft contact lenses.

PURPOSE: To simplify the clinical assessment of toric soft contact lens (TSCL) on-eye behavior by establishing a set of standard clinical evaluation techniques. The likely performance range expected among the TSCL wearing population was determined for a series of lens designs and acceptable performance standards indicated for each variable. METHODS: Four prism-ballast, two peri-ballast and one dynamic stabilization TSCL designs were each worn by groups of 20 subjects in a nondispensing study. After 20 min of lens wear, lenses were assessed, in right eyes only, for subjective comfort (100-point scale), lens mislocation (degrees deviation from vertical) and rotational recovery after deliberate 30 degrees mislocation (degrees/10 blinks). The percentage of lenses orienting within +/-10 degrees of target orientation (zero rotation) and the variability of orientation (standard deviation of mislocation) were also calculated for each lens group. RESULTS: Based on partitioning of the data distributions for each variable, performance was designated as excellent, acceptable or poor. Corresponding performance cut-offs were determined at > or =90, 89 to 80, and <80 for subjective comfort, < or =+/-6 degrees , +/-7 degrees to 10 degrees , and >+/-10 degrees for mislocation, >10 degrees /10 blinks, 10 degrees to 6 degrees /10 blinks, and <6 degrees /10 blinks for rotational recovery. For groups of wearers the appropriate cut-offs were > or =90%, 89 to 70%, and <70% of lenses orienting within +/-10 degrees of target orientation and <+/-6 degrees , +/-6 degrees to 10 degrees , and >+/-10 degrees for variability of orientation. CONCLUSION: Techniques suitable for the evaluation of TSCL clinical performance have been described and guidelines for the assessment of such lenses established. In the process, we have identified potential performance differences that may relate to variations in TSCL design.

Fibrillary lines in overnight orthokeratology.

This case report describes the appearance of fibrillary lines in the anterior stroma of a 39-year-old Asian woman wearing overnight orthokeratology (OK) lenses. The fibrillary lines were fine, slightly curved and sub-epithelial, arranged in a band-like annulus in the corneal mid-periphery. The lines were not associated with epithelial staining, although a marked Fischer-Schweitzer corneal mosaic was noted after blinking. Fibrillary lines are a relatively common finding in normal and keratoconic corneas and have been reported previously accompanying OK lens wear. Their origin is unknown and epithelial neural remodelling, corneal biomechanical stress and abrupt corneal curvature changes have been suggested as contributing factors. The appearance of fibrillary lines in our OK patient had no adverse consequences on vision or ocular health, at least in the medium term.

Osmolality and buffering agents in soft contact lens packaging solutions.

AIM: The study determined the effects of packaging solution osmolality and buffering agent on soft contact lens parameters. METHODS: One lens type from each FDA contact lens material category was equilibrated to phosphate buffered saline (PBS) or borate buffered saline (BBS), at three osmolality concentrations: 270, 310 and 414 mOsmol/kg. Lens diameter (LD), base curve (BOZR) and back vertex power (BVP) were measured and compared to nominal packaging label values. RESULTS: Statistically significant differences were found in all three parameters across all osmolality levels for both buffering agents. Etafilcon A showed the largest amount of parameter change, followed by alphafilcon A and balafilcon A. Lotrafilcon A was the least affected. LD of alphafilcon A lenses (greater than +/- 0.2 mm) and BVP of lotrafilcon A and etafilcon A lenses (greater than +/- 0.25 D) measured outside the International Standards Organization (ISO) tolerance from their packaging labels when measured in solutions that mimicked their packaging solutions. CONCLUSION: Osmolality and buffering agents influence lens parameters. Packaging solutions can vary the parameters of some lens types from their nominal value to outside the tolerance range set by ISO.