Feline corneal sequestrum: laboratory analysis of ocular samples from 12 cats

Abstract

Feline corneal sequestrum is a common ocular condition typified by brown to black discoloration of the cornea. The nature of the discoloration has not been identified. The purpose of this study was to perform a laboratory investigation of ocular samples from 12 clinical cases of feline corneal sequestrum in an attempt to characterize the nature of the discoloration. The 12 cases were referred to the Ophthalmology Unit at the Animal Health Trust between April and September 2000, and were also part of a clinical review of 64 cases of feline corneal sequestrum described separately. Five laboratory techniques that are routinely performed at the Biomaterials Unit, Aston University were employed for analysis of the ocular samples. Ocular material included corneal sequestrum, tear samples, meibomian gland secretions, and bandage contact lenses from the 12 clinical cases. High-performance liquid chromatography data showed that total tear lipid in affected eyes was significantly lower than in control eyes (P = 0.016); total tear lipid in affected eyes was lower than in the unaffected, contralateral eyes of the same cat but the difference was not significant (P = 0.29). The presence of an unknown lipid class was observed in tears and meibomian secretions of affected, contralateral and control eyes. Scanning electron microscopy showed that the discoloration in affected corneas was not due to the presence of iron. Fluorescence spectroscopic analysis of sequestra, unaffected corneas and contact lenses (from affected and contralateral/unaffected eyes) showed that lipid and protein were present but did not play an important role in sequestra. Ultraviolet-visible light absorbance spectroscopy revealed a peak at 385 nm in unaffected corneas that was absent in sequestra and the difference was significant (P < 0.0001); this peak may be a characteristic feature of the normal feline cornea. The absorbance spectra displayed a peak at 280 nm in two sequestra suggesting that chromophore groups (e.g. melanin) were present. Optical microscopy performed on 10 sequestra revealed the presence of particles, which were consistent with the appearance of melanin particles, providing laboratory evidence that characterized the nature of the discoloration as melanin for the first time.

Surgical Outcome and Histopathological Findings of Feline Corneal Sequestrum

“…In contrast to our findings previous histopathological studies on feline sequestrum report coagulative necrosis of corneal stroma, and heavy infiltration of inflammatory cells [1,10]. Studies involving the tissue characteristics of feline corneal sequestrum have reported the presence of iron [11], melanin [12] and mineralization of the tissue [13]. The varied etiological factors may incite the difference in tissue changes, therefore, need further investigation.…”

“…Recurrence of sequestrum over the previous site has been recorded, but there was no significant difference in the rate of recurrence with or without the use of a graft [12]. In our study, the previous surgical site had healed well with mild scarring but interestingly the recurrence of the lesion appeared on an adjacent site.…”

Porphyrins are not present in feline ocular tissues or corneal sequestra

“…Tear film break‐up times and goblet cell numbers in the conjunctiva of normal and affected cats have been evaluated and are similar 16 . The finding of decreased total lipids in the tears of affected cats led to the proposition that this altered tear film predisposes the corneas to desiccation, but a mechanism for sequestrum formation was still not determined 17 . In the past some have believed this pigment to be iron, but both a normal cornea and three sequestra had very low levels of iron 17,18 .…”

“…The finding of decreased total lipids in the tears of affected cats led to the proposition that this altered tear film predisposes the corneas to desiccation, but a mechanism for sequestrum formation was still not determined 17 . In the past some have believed this pigment to be iron, but both a normal cornea and three sequestra had very low levels of iron 17,18 . One study used optical microscopy to examine the sequestra and concluded that the dark particles seen were consistent with melanin, but this was not supported by an electron microscopy study 15,17 …”

Multiple imaging radiography at LNLS

“…Studies of the corneal matrix structures have been done by small-angle X-ray scattering [33,34], but never with X-ray imaging. Also, the corneal sequestra has been studied by several authors in the literature [35][36][37][38]. This disease is common in felines (Persian cats are predisposed to this condition) and is typified by the brown to black discoloration of the cornea.…”