Scientific publication about the use of accelerated cross-linking for the treatment of melting keratitis in cats.

1. Veterinary Ophthalmology (2013) 1–10
DOI:10.1111/vop.12112
Evaluation of accelerated collagen cross-linking for the treatment
of melting keratitis in ten cats
Frank Famose
DVM, Cert. Veterinary Ophthalmology, Clinique Vétérinaire des Acacias, 42 avenue Lucien-Servanty, 31700 Blagnac, France
Address communications to:
F. Famose
Tel.: +33 5 61 71 24 02
Fax: +33 5 61 71 65 52
e-mail: frankfamose@gmail.com
Abstract
Objectives Melting keratitis is a serious condition presenting a high risk of permanent
blindness and is caused by infectious or noninfectious factors. In humans, the clinical
efficacy of collagen cross-linking (CXL) has been described in the treatment of refractory infectious keratitis by arresting keratomalacia. The aim of this study was to evaluate the efficacy of accelerated CXL for the treatment of melting keratitis in cats.
Animals studied Ten cats were treated for unilateral melting keratitis by accelerated CXL.
Procedure Corneas were irradiated by UVA (370 nm) at 30 mW/cm² irradiance for
3 min after soaking with 0.1% riboflavin in 20% dextran for 30 min (D1). Follow-up
was conducted 3, 7, 14, and 30 days after treatment.
Results Pain improvement was noted for all cases at D4 examination. Epithelial
healing was observed at D8 for 9 of 10 cases and at D15 for 1 of 10 cases. Resolution
of cellular infiltration was observed for all cases at D8 examination. The corneal
vascularization was reduced for 9 of 10 cats by D31. At D31, all cases presented a
variable degree of corneal fibrosis, but all eyes had visual function. No recurrent
infection was observed.
Conclusion Accelerated CXL appears to be a valuable option for the treatment of
melting keratitis in cats. All the cases have reached a satisfactory outcome despite the
individual differences in the conditions prior to the CXL treatment and the variable
presence of infectious agents.
Key Words: accelerated cross-linking, cat, corneal melting, cross-linking, keratitis,
optical coherence tomography
INTRODUCTION
In cats, melting keratopathies are serious conditions presenting a high risk of permanent blindness.1 In melting
keratitis, stromal damage is initiated by various mechanisms including bacterial proliferation, toxin secretion,
and microbial or corneal protease activation. An imbalance
between the endogenous and exogenous matrix metalloproteinases (MMP) and the proteinases present in the cornea and the precorneal tear film leads to the destruction
of corneal collagen.2–4 Microbial infection is usually suspected to be responsible for corneal melting, but cannot
always be demonstrated.1 There are few corneal pathogens
that are associated with primary corneal infections, and
infectious corneal melting is typically due to secondary
bacterial infections.1,5 Melting can also occur in the
absence of infection and is thought to be secondary to an
imbalance between proteolytic enzymes and protease
inhibitors produced by resident and inflammatory cells.2–4
© 2013 American College of Veterinary Ophthalmologists
Medical treatment is based on the administration of topical antibiotics and protease inhibitors either commercially
available preparations or fortified compounded preparations.6,7 Despite treatment, vision loss can occur due to
the progression of keratomalacia leading to corneal perforation. Perforations are managed by tectonic surgeries
such as conjunctival grafts, biomaterial grafts, or amniotic
membrane transplantation.8–11 Collagen cross-linking
(CXL) is a technique that creates intrafibrillar covalent
bonds in the collagen fibers of the corneal stroma via the
photo-activation of riboflavin by ultraviolet-A (UVA) light
and has been used since 1998 in humans for the treatment
of progressive keratoconus, pellucid marginal degeneration, and ectatic complications of refractive surgeries.12–15
For these indications, safety and efficacy of this procedure
have been widely established.12–21 The antimicrobial activity of CXL against numerous bacteria and fungi22 has
been demonstrated under experimental conditions. In
addition, an increased collagen resistance against

2. 2 famose
enzymatic digestion23 has been demonstrated under experimental conditions in vitro. Recently, the clinical efficacy
of CXL has been described in the treatment of presumed
infectious keratitis and corneal melting in humans with
promising results.22,24–31 Spiess et al. have recently
described the application of CXL in three dogs and three
cats in a pilot study,32 and Hellander-Edman et al. have
described its use for the treatment of ulcerative keratitis in
nine horses.33 Both studies used an adaptation of the
treatment protocol used for human keratoconus described
by Wollensack12 with the use of a compounded riboflavin
solution. Accelerated cross-linking is a recent adaptation
of this traditional technique based on the use of a higher
irradiance UV light source and a shorter irradiation time.
Efficacy and safety of this technique for the treatment of
melting keratitis have been recently evaluated in eight
dogs.34 The aim of the present study was to evaluate the
efficacy of accelerated CXL for the treatment of melting
keratitis in cats.
MATERIALS AND METHODS
Inclusion criteria
This prospective, nonrandomized clinical study included
cases referred for the evaluation after progression of clinical signs despite initial medical therapy. To be included in
the study, cases had to have a clinical diagnosis of melting
keratitis characterized by epithelial and anterior stromal
loss, anterior stromal dissolution, cellular infiltration, and
corneal vascularization. Cases with impending of confirmed corneal perforation were excluded from the study.
Owners’ consent was obtained prior to the inclusion of
the animals into the study. All procedures were performed
in accordance with the French guidelines for animal care
and followed the ARVO guidelines for animal use.
Ophthalmologic examination
Keratitis evaluation was made under the following criteria:
•
•
•
The specific clinical signs of keratitis were evaluated by
slit-lamp examination (Hawkeye TM, Dioptrix, Toulouse, France). A clinical score modified from Tajima
et al.35 (0–3; 0 = absent, 1 = mild, 2 = moderate, and
3 = severe) was used to grade the severity of mucopurulent discharge, corneal edema, corneal vascularization,
conjunctivitis, blepharitis, and uveitis. The highest
possible total score was 18.
A pain score modified from the Melbourne University
scoring36 (0–1; 0 = absent and 1 = present) was used to
grade pain signs that included prostration, aggressive
behavior, blepharospasm, enophthalmos, photophobia,
ocular pruritus, and defense reaction to examination.
The highest possible total score was seven.
Cellular infiltration had generally a round or elliptic
shape. The length of the major and minor axes (a and
b) was measured with a manual caliper, and the area
•
was calculated as p*a/2*b/2 (in mm²) as described by
Price et al.30
Ulceration size was measured with a manual caliper,
and the length of the major and minor axes of the
ulceration was recorded. The area was calculated as
p*a/2*b/2 (in mm²). Fluorescein staining was not used
for initial measurements of epithelial defect, because it
can interfere with UVA absorption.37
Tear production was evaluated by Schirmer test I in all
cats (Test de Schirmer, Virbac, Carros, France). A corneal
sample was collected from each cat with a Kimura spatula
(Moria-Surgical, Antony, France) and submitted for bacterial culture and antimicrobial sensitivity (Laboratoire Meynaud, Toulouse, France). A PCR test for FHV-1 was
performed for all cats (Scanelis, Colomiers, France).
Measurements of the corneal thickness
The corneal thickness was evaluated by optical coherence
tomography (OCT). The cats were anesthetized with IM
300 lg/m² medetomidine (DomitorTM, Pfizer, NY, USA)
and 5 mg/kg ketamine (Imalgene 1000TM, Merial, Lyon,
France). Cats were placed in dorsal recumbency, and the
pachymetry was performed with an optical coherence
tomography (OCT) device (iVue TM, Optovue, Fremont,
CA, USA).38 Measurements were taken with the focus at
the center of the corneal lesion. Minimal and maximal
corneal thicknesses were evaluated with the OCT caliper
tool (Fig. 1). For safety reasons, only the cats with a minimal corneal thickness >300 lm were included in the
study.
Treatment by accelerated cross-linking
The eyelids were kept open with the use of a lid
speculum. After the instillation of a topical anesthetic
(Oxybuprocaine 0.4%, Cebesine 0. 4%TM, Bausch &
Lomb-Chauvin, Montpellier, France), the ulcer margins
were cleaned, and the debris was removed from the corneal surface with a microsurgery sponge as described by
Rosetta et al.26 A solution of isotonic riboflavin (riboflavin
0.1%, dextran 20%, VibexTM, Avedro, Waltham, MA,
USA) was instilled on the corneal surface for 30 min (one
drop every 2 min). The corneal surface was rinsed with
BSS at the end of riboflavin instillation. Penetration of
riboflavin through the cornea was confirmed by visualizing
the fluorescence of the riboflavin in the anterior chamber
with slit-lamp biomicroscopy using the cobalt blue light.
The corneas were irradiated with UVA (wave
length = 370 nm) for a total dose of 5.4 J/cm², as
described in human studies,22,24–29 delivered by the
KXLTM system (Avedro) for 3 min with an irradiance of
30 mW/cm². The beam of 9 mm in diameter was centered
and focused on the center of the corneal lesion. Care was
taken not to irradiate the corneal limbus. One drop
of riboflavin was instilled after 2 min of irradiation. All
animals received a single CXL treatment at D1.
© 2013 American College of Veterinary Ophthalmologists, Veterinary Ophthalmology, 1–10

3. corneal melting in cats 3
(a)
antibiotic treatment (Tobrex 0.3%TM, 1 drop q12 h) was
continued until the next examination.
Photographs were taken for each case at each follow-up
appointment.
RESULTS
(b)
Figure 1. Case 6. Central epithelial and stromal loss (green arrows),
infiltration (red arrow), and vascularization (black arrow) are present
(a). On OCT picture (b), the residual stromal thickness can be
measured at the thinner part of the cornea (green arrows) at 447 lm.
Cellular infiltration appears as a heterogeneous hyper-reflective zone
(red arrow).
Postoperative treatment
Each eye was treated twice daily with one drop of a tobramycin solution (Tobrex 0.3%TM, Alcon, Rueil-Malmaison,
France) after the CXL treatment until complete epithelial
healing. All anticollagenase and other previous treatments
were discontinued.
Follow-up
To evaluate corneal healing and the symptom reduction
over a 30-day follow-up period, all eyes were examined 3,
7, 14, and 30 days (D4, D8, D15, and D31, respectively)
after treatment using the same pretreatment protocol
minus the Schirmer test. Epithelial integrity was evaluated
after instillation of a drop of fluorescein solution. Fluorescein dye staining of the cornea was interpreted as a positive result. In cases of epithelial healing, topical treatment
was discontinued. In cases of epithelial defect, topical
Pre-operative features
Ten cats were treated between April 2012 and February 2013
by the same clinician (Frank Famose). The pre-operative
findings of each cat were recorded (Table 1). Five cats were
Persians, four were Domestic Short-haired, and one was a
Singapura.
Keratitis duration before CXL treatment ranged from
7 days to 2 months. None of the cases had a bacteriologic
analysis before initiation of the treatment by the referring
veterinarian. Six of the 10 cats had received a surgical
therapy prior to CXL (one epithelial debridement, three
nictitans membrane flaps, one conjunctival pedicle graft,
and one superficial keratectomy). All cases had received
topical antibiotics, and 6 of 10 had received a topical anticollagenase prior to referral.
Bacterial analysis was performed in all cases after referral, prior to CXL therapy. Four cultures were positive
(three Pseudomonas aeruginosa and one Staphylococcus chromogenes). The bacteriological data and antimicrobial sensitivity are summarized in Table 2.
Two cats were positive for FHV1 (case nos four and
six).
Maximal corneal thickness ranged from 650 to
1200 lm, and minimal corneal thickness ranged from 305
to 567 lm. The relative depth of the ulceration ranged
from 16 to 75% of the corneal thickness. The size of epithelial defect ranged from 2 to 8 mm in maximal diameter. The surface of cellular infiltration ranged from 3.14
to 56.55 mm².
Postoperative features
The individual scores for clinical scores, pain scores, and
the areas of epithelial defects and cellular infiltrates are
summarized in Table 3.
Epithelial healing was observed at D8 for 9 of 10 cases
and at D15 for one case (Fig. 2).
The mean clinical and pain scores showed a marked
decrease at D4 and D8 (Fig. 2). Cellular infiltration of the
cornea was present for all cases at the time of CXL and
had resolved by D8 (Fig. 3).
Corneal vascularization was present in all cases at D8,
even in the cases in which it was not observed at D1. At
30 days post-treatment, it had resolved in 2 of 10 cases
and was mild in 7 of 10 cases (Fig. 4) and moderate for 1
of 10 cases (Fig. 5). At 1 month, all cases had a variable
degree of corneal fibrosis (Fig. 6), but all eyes were visual.
No clinical sign of corneal infection (corneal ulceration,
ocular discharge) was observed during the follow-up
period.
© 2013 American College of Veterinary Ophthalmologists, Veterinary Ophthalmology, 1–10

4. 2 years
OS
1 month
Gentamicin
(14 mg/ml)*
NAC
Framycetin
Ciprofloxacin
NAC
Epithelial
scraping
17
Sterile
Negative
567
1020
54%
392
Conjunctival
graft
15
Sterile
Negative
493
1170
58%
695
292
696
Pseudomonas
aeruginosa
Positive
543
650
16%
12
15
Staphylococcus
chromogenes
Negative
380
660
42%
3rd eyelid flap
Chloramphenicol
Ciprofloxacin
2 months
OS
1 month
Singapura
4
None
Ciprofloxacin
Domestic
Short-haired
8 years
OD
1 month
3
*Gentamicin was used at fortified concentration of 14 mg/ml.
†
CT, corneal thickness.
Previous surgical
treatment
Schirmer test
(mm/min)
Presurgical
bacteriology
PCR for FHV1
Min. CT† (lm)
Max. CT (lm)
Ulcer depth (% of
corneal thickness)
Ulcer size (mm)
Age
Affected eye
Duration prior to
CXL (from first
symptoms)
Previous topical
medical treatment
Persian
Domestic
Short-haired
1 year
OD
3 weeks
Breed
2
1
Case number
Table 1. Individual cases data
897
Negative
305
1200
75%
Sterile
17
None
Gentamicin
(14 mg/ml)*
NAC
7 years
OS
10 days
Persian
5
898
Positive
447
1200
63%
Sterile
18
3rd eyelid flap
Chloramphenicol
NAC
3 years
OD
2 months
Persian
6
896
Pseudomonas
aeruginosa
Negative
320
680
53%
14
3rd eyelid flap
Framycetin
NAC
Domestic
Short-haired
1 year
OD
1 month
7
292
Negative
420
780
46%
Sterile
13
None
Framycetin
NAC
8 years
OS
15 days
Persian
8
392
Negative
480
950
49%
Sterile
16
Ciprofloxacin
Gentamicin
(14 mg/ml) *
NAC
None
Domestic
Short-haired
6 years
OD
15 days
9
494
Pseudomonas
aeruginosa
Negative
420
720
42%
Superficial
keratectomy
15
Chloramphenicol
Ciprofloxacin
4 years
OD
7 days
Persian
10
4 famose
© 2013 American College of Veterinary Ophthalmologists, Veterinary Ophthalmology, 1–10

5. corneal melting in cats 5
Table 2. Pre-operative bacterial culture results
Sensitivity
Case
Bacterial species
Aminoglycosides
Quinolones
Tetracyclines
Chloramphenicol
3
4
7
10
Staphylococcus chromogenes
Pseudomonas aeruginosa
Pseudomonas aeruginosa
Pseudomonas aeruginosa
R
S
R
R
R
R
S
S
R
S
S
S
S
NT
NT
NT
S, sensitive; R, resistant; NT, not tested.
DISCUSSION
Table 3. Postoperative scores from D1 to D31
Case
number
Case 1
Case 2
Case 3
Case 4
Case 5
Case 6
Case 7
Case 8
Case 9
Case 10
Score
D1
D4
D8
D15 D31
Clinical score
Pain score
Ulcer surface
Infiltration surface
Clinical score
Pain score
Ulcer surface
Infiltration surface
Clinical score
Pain score
Ulcer surface
Infiltration surface
Clinical score
Pain score
Ulcer surface
Infiltration surface
Clinical score
Pain score
Ulcer surface
Infiltration surface
Clinical score
Pain score
Ulcer surface
Infiltration surface
Clinical score
Pain score
Ulcer surface
Infiltration surface
Clinical score
Pain score
Ulcer surface
Infiltration surface
Clinical score
Pain score
Ulcer surface
Infiltration surface
Clinical score
Pain score
Ulcer surface
Infiltration surface
13
5
23.56
32.99
13
6
4.71
9.42
13
4
3.14
7.07
10
5
28.27
28.27
12
5
43.98
56.55
10
7
50.27
50.27
10
4
37.7
49.48
11
4
3.14
3.14
12
6
4.71
9.42
10
5
12.57
15.71
11.4
9
2
4.71
9.42
8
3
1.57
4.71
10
2
0.79
3.14
6
3
3.14
3.14
8
2
11.78
18.85
7
4
9.42
15.71
7
4
11.78
18.85
4
1
0.79
0
7
4
0.79
3.14
4
3
3.14
7.07
7
3
0
0
0
5
0
0
0
5
0
0
0
3
0
0
0
6
0
0.79
0
4
0
0
0
5
1
0
0
4
0
0
0
2
0
0
0
2
0
0
0
3.9
2
0
0
0
1
0
0
0
1
0
0
0
1
0
0
0
2
0
0
0
2
0
0
0
3
0
0
0
2
0
0
0
1
0
0
0
2
0
0
0
1.7
1
0
0
0
0
0
0
0
1
0
0
0
1
0
0
0
1
0
0
0
2
0
0
0
1
0
0
0
1
0
0
0
0
0
0
0
1
0
0
0
0.9
5.1
2.8
0.1
0
0
21.21
4.79 0.08 0
0
26.23
8.40 0
0
Average
clinical score
Average
pain score
Average
(mm²)
ulcer surface
Average
(mm²)
infiltration
surface
0
Accelerated collagen cross-linking was used for the treatment of feline melting keratitis in 10 cases. All treated animals showed reduced pain 3 days after treatment. This
observation is similar to the results in humans in which
ocular pain improvement was achieved over the same time
duration.22,24,25
Complete epithelial healing was achieved in 7 days for
9 of 10 cats and at 14 days after treatment in one case.
The healing rate in our study was shorter than that
observed by Spiess et al.32 where epithelial healing was
achieved in 7 days for one cat and 15 and 18 days for the
two others. The difference of healing rate between these
two studies could be explained by the difference in the initial size of the ulceration and the degree of the epithelial
surface removal before riboflavin application. In the study
by Spiess et al.,32 epithelium debridement was 5 to 11 mm
in diameter, whereas in the present study, it was limited
to the ulcer margins (from 2 to 8 mm) as described by
Rosetta et al.26
Corneal melting resolved in all treated eyes within
7 days after CXL treatment. This was observed by biomicroscopic examination as a reduction in corneal thickness
and cellular infiltration with recovery of corneal transparency. These results are similar to previous reports in the
literature22,24–31,39 and can be attributable to direct effects
of CXL. Clinical observations and experimental data have
shown that CXL may have three distinct effects on cornea:
a bactericidal effect, an increase in corneal resistance to
mechanical forces and enzymatic digestion, and a reduction in corneal inflammation.
A bactericidal effect has been established in experimental conditions40 with CXL. This effect is manifested by
bacterial DNA and membrane alterations41 secondary to
the liberation of free radicals by photo-activation of riboflavin. However, an immediate reduction in the bacterial
load has not been demonstrated in the different clinical
reports.22,24–31
Collagen cross-linking is reported to increase the
corneal resistance to mechanical forces and enzymatic
digestion secondary to mechanical and biochemical modifications of corneal structure42,43 and the creation of intralamellar covalent collagen bonds.44 This mechanism is
thought to be limited to the first anterior 200 lm of the
treated cornea45 and to contribute to increased resistance
© 2013 American College of Veterinary Ophthalmologists, Veterinary Ophthalmology, 1–10

6. 6 famose
Figure 2. Progression of the average clinical score (red), average pain score (blue), average ulcer surface (gray), and average infiltration surface
(green) during the observation period.
(a)
(a)
(b)
(b)
Figure 3. Case 3. Pretreatment presentation (a) and 7 days after the
treatment (b) Cellular infiltration has disappeared, and transparency
has been recovered. Epithelial healing was complete.
Figure 4. Case 2. Reduction in vascularization from pretreatment (a)
to 1 month post-treatment (b) Residual vascularization was scored
‘mild’.
© 2013 American College of Veterinary Ophthalmologists, Veterinary Ophthalmology, 1–10

7. corneal melting in cats 7
(a)
(a)
(b)
(b)
Figure 5. Case 7. Pretreatment presentation (a) and 1 month
after treatment (b) with persistent central superficial corneal
vascularization scored as ‘moderate’.
to proteases.23 Experimentally, an increase in mechanical
rigidity and in resistance to proteolytic enzymes has been
shown for human and swine corneas.23,46–48
Collagen cross-linking reduces the corneal inflammatory
response by the induction of apoptosis of the keratocytes
located in the anterior part of the stroma.49,50 This may
contribute to modification of local immune response mediated by Langerhans and dendritic cells and may reduce
corneal melting and vascularization.39 In the current
study, a dramatic reduction in cellular infiltration and in
corneal melting was observed. In addition, corneal vascularization increased in the first 7 days and regressed over
the next 3 weeks, but was present at completion of the
study in a reduced state in 8 of 10 eyes.
Limited data have been published on the corneal effects
of CXL in cats. A recent publication32 reports a modification of the treatment protocol used for human keratoconus described by Wollensack12 with the use of a
compounded riboflavin solution. Accelerated cross-linking
(with the KXLTM) is a recent adaptation of this technique
Figure 6. Case 9. Pretreatment presentation (a) and 1 after CXL
treatment (b) with two remaining areas of marked corneal fibrosis.
and uses a higher irradiance UV light source and a shorter
time of irradiation.51,52 The energy delivered by both protocols (30 mW/cm² for 3 min for the accelerated protocol
or 3 mW/cm² for 30 min in the study by Wollensack
et al.12) is the same (5.4 J/cm²) achieving the same biological effects.51,52 However, in experimental conditions, the
biochemical stiffness of the cornea seems to decrease in
higher irradiances due to rapid oxygen depletion, because
CXL is an oxygen-dependent process.53 The influence on
the treatment of the melting keratitis is unknown. Compared with the Wollensack et al.12 protocol, the use of
accelerated cross-linking reduces operating time and thus
the duration of anesthesia. In human patients, no additional adverse effects have been observed with irradiances
>3 mW/cm².51,52 The results observed in the present
study are similar to the previous evaluation of accelerated
cross-linking for corneal melting in dogs.34 The isotonic
riboflavin solution used in the present study (VibexTM) is
commercially available in Europe, and its concentration is
the same as the compounded solution used by Spiess
et al.32 The absorption spectra of fluorescein and
© 2013 American College of Veterinary Ophthalmologists, Veterinary Ophthalmology, 1–10

8. 8 famose
riboflavin for UV-A are very close. The presence of fluorescein in the anterior stroma limits UV-A absorption and
could explain some differences observed in healing rates in
human patients.37 Therefore, in the present study, fluorescein staining was used only in the follow-up period.
Case inclusion was based on clinical diagnosis of melting keratitis by the observation of epithelial loss, corneal
edema, cellular infiltration, and stromal dissolution. As
stated earlier, microbial infection is usually suspected to
drive the inflammatory state responsible for corneal melting by the contamination of a previous epithelial defect.
However, it cannot always be demonstrated. Although
bacterial contamination was suspected in all cases, only 4
of 10 cases had a positive culture in our study, which correlates with literature data.54 These results are similar to
those presented by Spiess et al.32 in which none of the six
cases (three dogs and three cats) had a positive bacteriologic culture. This can be either explained by the inhibition of bacteria by the previous medical treatments or
related to a sterile keratitis and the activation of the proteases by other mechanisms.1,2
In our study, all the cases have achieved the same outcome regardless of the presence of bacteria or of the duration of the condition prior to CXL treatment. Similar
observations have been reported in a series of 25 human
cases.39 Makdoumi et al. concluded that CXL should be
considered as the initial treatment for keratitis without the
use of antibiotics, arguing that this could be a way of
reducing the risk of antibiotic resistance.39 However, due
to the limited data evaluating the efficacy of CXL in the
treatment of melting keratitis in cats, topical antibiotic
therapy was maintained until complete epithelial healing
to prevent a secondary bacterial contamination. Tobramycin was used at twice daily for a preventive purpose
although the frequency is unlikely to decrease bacterial
growth.
Although justified on a scientific basis to compare CXL
with a traditional therapy for melting keratitis, no control
group was included in our series. Animals were presented
after a previous medical treatment (topical antibiotics and,
in some cases, antiproteases) with no improvement in or
worsening of the clinical signs at the time of referral.
Treatment with antiproteases may have yielded similar
results as described in this study. Because no control
group was included, a comparison between the two treatment effects was not possible. However, all antiproteases
treatments were stopped at the time of CXL; thus, the
arrest of corneal melting could be attributed to the CXL
procedure.
In human patients, adverse effects of CXL have been
described: postoperative infection, herpes virus exacerbation, and corneal endothelial lesions. With CXL treatment
of human keratoconus, cases of postoperative infections
have been described in the days or weeks following the
procedure.55–57 In all cases, stromal infection was present
3–5 days after the procedure and was attributed to the
surgical removal of corneal epithelium. In the present
study, no postoperative infection was observed during
the follow-up period, as described in the human studies.19–21,26 Herpetic keratitis has been described in a
human patient 5 days after his treatment for keratoconus
by CXL.58 In our series, two cats tested positive for
FHV-1. However, there was no evidence of active disease
in the immediate postoperative period. Therefore, no
treatment against FHV-1 was prescribed. The risk of viral
activation by CXL treatment in feline patients is unknown
and should be investigated further because UV light has
been used for reactivation of herpes simplex virus in animal models.59–63 However, the UV-B used in some experimental procedures61–63 had wave lengths (280–315 nm)
significantly different from those used for collagen
cross-linking (370 nm). Endothelial lesions may be
directly attributable to the CXL treatment by the cytolytic
effect of riboflavin photo-activation on endothelial
cells.64,65 Experimentally, the maximal absorption depth
for UVA in a riboflavin-saturated cornea is thought to be
approximately 300 lm. UVA absorption does not stop at
300 lm, but absorption levels drop below the toxic
threshold in the deeper parts of the cornea and eye as a
whole. Stiffening effects of CXL seem to be limited to the
more superficial 200 lm of the stroma,43,45 and apoptotic
effects of the procedure appear to be rare in the deeper
parts of the cornea (beyond 300 lm).45 This concept,
often referred to as ‘riboflavin shielding’, correlates
directly with the minimal corneal thickness necessary for
safe CXL treatment. In this study, corneal thickness was
>300 lm in all cases. No endothelial effects were observed
in all treated cases. Before treatment, many eyes in this
study presented with a thick cellular infiltration, which
can interfere with UV penetration and produces a heterogeneous photo-activation of riboflavin. No difference in
the results was noted between these different cases regardless of the corneal thickness or the severity of cellular
infiltration. In human patients, infected corneas with a
thickness less than 300 lm have been successfully treated
by CXL after soaking with hypotonic riboflavin.26 Very
thin corneas or corneas with impending perforation were
not included in this study. However, the prospect of treating such corneas is very promising as perforation might be
prevented by the stiffening effects of CXL. Extension of
our study to a larger group should allow us to optimize
treatment parameters according to the size and the depth
of the corneal loss.
In the study by Spiess et al.,32 a case of corneal sequestrum was observed within 15 days post-CXL. Keratocytes
apoptosis has been hypothesized as a cause of sequestrum
formation,66 and because CXL induces anterior apoptosis,
corneal sequestrum formation could be a potential
adverse effect. In the present study, no sign of sequestrum development was observed. In the study by Spiess
et al.,32 it is not clear whether the keratitis or the CXL
treatment or both were factors in the development of the
© 2013 American College of Veterinary Ophthalmologists, Veterinary Ophthalmology, 1–10

9. corneal melting in cats 9
sequestrum. In the present study, variable corneal fibrosis
was observed in all cases. Corneal haze has been
described in human patients with keratoconus after CXL
treatment.14–18 In this study, it is not clear whether the
corneal fibrosis can be attributed to the initial keratitis,
the treatment procedure, or both. Follow-up for a longer
period could be useful to evaluate the long-term effects
of the treatment.
All the cats treated in this study completely healed
regardless of the presence of bacterial agents, the extension
of the initial corneal lesions, the duration of the disease
before treatment, and the previous treatments. The results
achieved in this small series suggest that CXL could be a
valuable therapeutic option for melting keratitis in cats.
The CXL procedure requires a precise focusing of the UV
beam and thus requires general anesthesia. Because the
duration of the anesthesia is reduced in comparison with
the traditional CXL protocol, accelerated cross-linking
could present a practical advantage while providing the
same biological effects. However, accelerated CXL is performed with a commercially available riboflavin (VibexTM)
with a price significantly higher than that of compounded
riboflavin. These disadvantages (anesthesia, price) have to
be taken into account in the treatment decision. CXL
could also be considered as primary treatment for keratitis
with or without the use of antibiotics.
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