New insight into non-healing corneal ulcers: iatrogenic crystals

Abstract

Aims: To characterise and correlate crystalline precipitations implicated in non-healing corneal ulceration in two patients with a previous history of acanthamoeba keratitis.

Materials and methods: Persistence of acanthamoeba and secondary bacterial infection was excluded with negative corneal scrapes. Confocal microscopy identified crystal-like deposits within the corneal stroma. To investigate possible precipitating combinations, all concurrent treatments at the time of presentation were mixed in wells, with observation of precipitate formation. Precipitates were observed with phase-contrast microscopy, and subsequently characterised via crystallography techniques and electrospray ionisation mass spectrometry.

Results: Combinations of dexamethasone 0.1% minims and chlorhexidine gluconate 0.2% formed an amorphous material characterised by electrospray ionisation mass spectrometry as an insoluble chlorhexidine salt. Combinations of chloramphenicol drops and timolol 0.5% formed a crystal identified via X-ray crystallography as santite (K(B₅O₆(OH)₄).(H₂O)₂). This is a borate mineral identified in nature, arising from thermal springs, but never reported in biological tissues. Clinical improvement was observed following the cessation of the implicated precipitating combinations.

Conclusion: Our observations suggest iatrogenic precipitate formation, with a potential deleterious effect upon healing. The substrates for these precipitates include several frequently prescribed topical ophthalmic treatments. These findings shed new light on the aetiopathogenesis of non-healing corneal ulceration, and have broad implications on topical prescribing for this challenging condition.

Figure 1

(a) Case 1 slit-lamp photograph showing predominantly superior corneal neovascularisation, diffuse corneal oedema and localised infiltrate. Reflective bodies were evident within the anterior stroma, concentrated around the edges of the epithelial defect. (b) Case 1 slit-lamp photographs with fluorescein staining, evidencing well-demarcated epithelial defect. (c) Case 1, confocal microscopy evidencing highly refractive crystal-like entities, concentrated within the anterior stroma. (d and e) Case 1 appearances 2 weeks after cessation of gutte chloramphenicol/gutte timolol combination. Despite continued corneal oedema and persistent infiltrate, there is near complete closure of the epithelial defect. (f) Case 2 slit-lamp photograph showing diffuse infiltrate around a persistent epithelial defect. Tiny reflective bodies were evident in the anterior stroma. (g) Case 2 slit-lamp photograph with fluorescein staining, evidencing large epithelial defect. (h) Case 2 confocal microscopy, evidencing highly refractive entities, concentrated within the anterior stroma within the constraints of the epithelial defect.

Figure 2

(a) Photograph of a crystal of santite from the timolol/chloramphenicol eye drop mixture mounted on a glass fibre for single-crystal X-ray diffraction. The photograph shows the formed crystal (circled), 8 μm in diameter. (b) The crystal structure as viewed down the [100] direction.

Figure 3

Electrospray ionisation mass spectrometry results for chlorhexidine/dexamethasone 0.1% minims (a), and chlorhexidine alone (b). The two highest peaks in both spectra reflect the molecular ions for chlorhexidine (CHX). The molecular mass of CHX is 504. Using ESI as an ion source, the compound encompasses a proton, increasing the molecular mass by +1. The two peaks at 505 and 507 reflects the two Cl atoms present in the structure which have isotopes at 35 (75% total abundance) and 37 (25% total abundance). Peaks with lower masses represent fragments of the molecule as it breaks apart in the ion source, and are compound characteristic. It can be seen that the top panel (precipitate) exactly matches that of chlorhexidine. The few extra small peaks reflect contamination.