The Abnormal Lens Capsule in Asteroid Hyalosis

Visualization of the posterior capsule is difficult during cataract surgery on eyes with asteroid hyalosis (AH) because of the increased translucence and decreased foldability of the capsule (Figure 1). The anterior capsulorhexis is also intrinsically different in these eyes because the anterior capsule is more brittle than usual. Although the preoperative appearance of the anterior capsule is typical in AH eyes, its postoperative appearance is qualitatively different at the slit lamp.

This article discusses ultrastructural differences we have identified in anterior capsule samples obtained from normal and AH eyes and the practical implications of these differences.

BACKGROUND

In the Beaver Dam Eye Study, AH was present in 1.2% of the population, and its incidence increased with age.¹ AH is primarily unilateral, and its prevalence is higher in men. The association of AH with diabetes is controversial. Some investigators have found a positive association, whereas others have found none.^2,3

OCT is the preferred method of retinal imaging in advanced cases where fundal visibility is reduced.⁴ The asteroid bodies, which contain calcium and phosphorus,⁵ may impede the A-scan determination of axial length.⁶

Silicone IOLs are known to opacify in AH eyes⁷ and are wtherefore contraindicated.

STUDY METHODS AND RESULTS

Methods. Anterior capsule specimens were obtained from patients with AH and healthy patients who served as controls. Examinations were carried out using electron microscopy in two separate phases because of refurbishments to the department. A total of five normal and nine AH specimens were obtained.

Results. The normal human lens capsule is formed by layers of basal lamina composed of collagen type IV secreted by the lens epithelial cells.⁸ The capsule is the only basement membrane that is formed continually throughout life⁹ Its external surface is composed of fibrils similar to those of the zonules.¹⁰

In the first phase at magnifications ranging from 500x to 1,000x, the AH specimens showed a banded phenotype appearance not observed in the normal specimens. In the second phase, there was a reversal of this banded appearance between the two groups (Figure 2). This may be explained by some difference in staining technique and the model of microscope used. At high magnification (15,000x), some difference was again observed between the two groups; the AH specimens exhibited a less directional arrangement of microfibrils (Figure 3).

Scanning electron microscopy of the anterior capsular surface revealed more pronounced features in the AH specimens (Figure 4). No significant difference in the thickness of capsule specimens was observed between the groups.

ASSOCIATIONS

Polychromasia capsulare. First described in 1969,¹¹ this rare, benign, autosomal dominant ocular condition is not associated with other ocular findings or systemic abnormalities.¹² The iridescent colors seen at the slit lamp are associated with an abnormal ultrastructural regular pattern at high magnification.¹³ This association may bear some relation to our findings in AH: Light interacts differently with the lens capsule where ultrastructural changes are found.

Other conditions associated with capsular anomalies. Pseudoexfoliation, originally thought to be limited to the lens capsule,¹⁴ is a more widespread condition both within the eye¹⁵ and systemically.¹⁶ Other conditions with recognized anomalies of the anterior lens capsule are Marfan syndrome,¹⁷ Peters plus syndrome,¹⁸ and Alport syndrome.¹⁹ Ultrastructural changes have also been noted in the capsules of eyes that develop cataracts secondary to uveitis.²⁰

CONCLUSION

Our findings suggest that AH involves the lens capsule, adding to its known ocular manifestations. AH is usually a unilateral condition and is not known to be hereditary. Nor is it known to have systemic implications.

The difficulty of cataract surgery on eyes with dense AH has been described (see Six Reasons to Take Extra Care With Cataract Surgery in Eyes With AH).²¹ The use of trypan blue dye to increase visibility of the posterior capsule in eyes with extensive AH is recommended.²² The asteroid bodies may compromise visualization of the posterior capsule during cataract surgery, but our research suggests that the capsule itself is more translucent as a consequence of ultrastructural changes in AH and less discernible as a result.

On a practical level, AH of any degree should be recognized as likely to increase the risk of complications during cataract surgery, and the condition should be incorporated into preoperative risk grading systems.²³