Herein we report the morphologic and ultrastructural features of an iridectomy specimen taken from a patient whose brown-green iris darkened while participating in the phase 3 trial of latanoprost in Sweden.
A well-documented side effect of latanoprost, an analog of prostaglandin F2α used as an antiglaucoma medication, is iridial color change.1- 4 The color change is found particularly with green-brown, gray-brown, and blue-brown irides that become progressively more brown with the use of latanoprost.1- 4 Excessive formation of melanin with stromal release of granules could cause uveitis,5 exacerbate primary or secondary glaucoma,4 or lead to precancerous changes.
The patient was aged 67 years when she entered the clinical trial.1 She suffered from ocular hypertension in her right eye with a mean diurnal intraocular pressure of 23 mm Hg. She was treated with 0.025% timolol maleate (Timoptic) for 6 months in her right eye only (Figure 1, left). Thereafter, she was treated with topical 0.005% latanoprost once daily for 13.5 months. The color change was suspected at 11 months and she was withdrawn from the trial 2.5 months later (Figure 1, right).
Left, Photograph of the right eye before the start of latanoprost treatment. Right, Photograph of the right eye after suspension of latanoprost treatment.
In the follow-up, the patient was treated with timolol for only 1 month owing to ocular irritation. Intraocular pressure–lowering medication was not used. She underwent cataract surgery 16 months after withdrawal from the trial and a sector iridectomy was performed via a tunnel incision.
The iris specimen was fixed in 2% phosphate-buffered glutaraldehyde. At the laboratory, the specimen was rolled into a cylindrical shape with the posterior pigmented epithelium to the inside. It was washed in buffer, postfixed in osmium tetroxide for 1 hour, rewashed, dehydrated in graded alcohols, cleared in propylene dioxide, embedded in an Araldite–epoxy resin mixture, and cured overnight. Semithin sections for light and differential interference contrast microscopy and thin sections for electron microscopy were cut on an Ultracut E ultramicrotome(Reichert).
The cylindrical specimen was cut in cross-sections to produce target-shaped semithin sections with the posterior part of the pigmented epithelium in the center (Figure 2, left). In this manner, all 360° could be appreciated and a complete length of the iris leaf from pupil to periphery could be studied.
Left, A light micrograph of a semithin section through the specimen (toluidine blue, original magnification ×150). Right, Higher-power light micrograph to show the region of the nevus. Arrows indicate the thickened, pigmented anterior border, and the sphincter muscle (S) is marked (toluidine blue, original magnification ×400).
The anterior layer of the specimen was hyperplastic and thickened in the pupillary area. The stroma between the anterior border and the sphincter muscle was hypercellular — these cells contained melanin granules and some were pigmented (Figure 2, right).
The fine structure of the posterior epithelial cells was unremarkable, although some nuclear membranes exhibited infolding and invagination. Among the mature melanin granules, some exhibited incomplete melaninization or were vacuolated. Spirals of rough endoplasmic reticulum were found in some cells and multilayered basement membrane material was evident in a few focal areas.
Ultrastructural investigation of the stroma showed the pigment-containing cells were melanocytes, clump cells, and some fibroblasts (Figure 3). The clump cells were most obvious near the sphincter muscle. Tissue histiocytes were present in small numbers and some had vacuolated cytoplasm. A few mast cells were found. There was no evidence of free melanin granules within the collagen matrix. Qualitative observation of the stromal melanocytes gave the impression that the melanin content was not excessively high (Figure 3).
A transmission electron micrograph of the posterior pigmented epithelium (E), dilator (D), and stroma (ST) that shows no abnormalities. Stromal clump cells can be identified (arrows) (original magnification ×3000).
The anterior border was from 3 to 6 cells wide except in the thickened area in the pupillary zone (Figure 4). The melanocytes were heavily, but not excessively, pigmented. Most of these cells had prominent osmiophilic heterochromatin in their nuclei, but there was a subset of cells with paler-staining nuclei and dark marginal heterochromatin (Figure 4). Sometimes nucleoli, invaginations, and pseudoinclusions were noted and this subset was found in the thickened zone, border layer, and stroma. Throughout the specimen, no mitotic figures were found and premelanosomes and immature melanin granules were identified in unremarkable numbers.
Anterior border region in the area of the nevus. It is hypercellular and some melanocytes have atypical features (arrows) (toluidine blue, original magnification ×3000).
A mound of cells on the anterior border toward the pupil had the characteristics of a freckle or nevus. The presence of melanocytes with atypical features (nuclear chromatin margination, prominent nucleoli, invagination of the nucleus) was of interest since these characteristics are seen in precancerous lesions.
To our knowledge, this is the first morphologic report of an iris specimen from a patient with latanoprost-induced eye color change. Theoretically, increased pigmentation could result from proliferation of melanocytes, melanogenesis, or migration of melanocytes. In this specimen there is little indication that proliferation took place.
The melanocytes with atypical features did not appear sinister as they were not in the great abundance or in the pronounced form seen in precancerous lesions. Melanocytes exhibiting the nuclear features described above are also found in normal brown irides, and before treatment with latanoprost, the sphincter area was dark brown. The visible change in eye color was in the middle and peripheral parts of the iris where no abnormal morphologic findings were made. Stromal and anterior border melanocytes had abundant melanin granules but did not appear to have excessive pigmentation. The degenerative vacuolation of posterior epithelial melanin granules has been described in the normal aging iris and in the iris of patients with glaucoma.6
There is little evidence in this specimen or in studies using cultured melanocytes that latanoprost adversely affected the tissue.7 The possibility that latanoprost-induced melanogenesis would lead to bursting of melanocytes and extrusion of free melanin into the stroma is not supported by the morphologic features of this specimen. The incidence of mature melanin granules in the stromal melanoyctes is in no way remarkable. There did seem to be an abundance of clump cells throughout the stroma, and although the origin and nature of clump cells are not well understood, one of their suspected functions is to mop up loose melanin.5 Overall, there is little in the way of tissue abnormality in the specimen.
Accepted for publication November 23, 1998.
Research into latanoprost-induced eye color change is supported by a grant from Pharmacia Upjohn, Sweden.
Reprints: Daniel M. Albert, MD, MS, Department of Ophthalmology, University of Wisconsin-Madison, 2870 University Ave, Suite 102, Madison, WI 53705.
Grierson I, Lee WR, Albert DM. The Fine Structure of an Iridectomy Specimen From a Patient With Latanoprost-Induced Eye Color Change. Arch Ophthalmol. 1999;117(3):394-396. doi:10.1001/archopht.117.3.394