Introduction

Osteochondrodysplasia in Scottish Fold cats, also referred to as Scottish Fold osteochondrodysplasia (SFOCD),¹ is a well-known inherited disease that affects bone growth and formation of articular cartilage in cats and leads to progressive skeletal deformities in the distal extremities and tail.^1–3 Although the severity of this condition varies widely, all Scottish Fold cats with SFOCD present with varying signs of skeletal disease.^3–5 No treatment strategy for SFOCD has been determined to date, and there are relatively few clinical reports on this feline disease.^3,5–12 This report demonstrates the efficacy of a combination of surgery, radiotherapy, and medical therapy for SFOCD with skin ulceration. To the best of the authors’ knowledge, this is the first report of triple-modality therapy for SFOCD.

Case Report

A 4 yr old 3.2 kg castrated male Scottish Fold cat was referred to Azabu University Veterinary Teaching Hospital with chronic right hindlimb lameness and bleeding from the plantar aspect of the right tarsus. The cat had a history of asymptomatic enlarged tarsi. By 3 yr and 9 mo of age, the right tarsus, swollen as a result of a large tarsometatarsal exostosis, was worn by contact with the ground and the overlying skin was ulcerated. The affected site bled intermittently and there was chronic lameness in the right hindlimb. These symptoms had failed to respond to 3 mo of treatment with nonsteroidal anti-inflammatory drugs, antibacterial ointments, and bandages by the referring veterinarian.

A physical examination, complete blood count and serum biochemistry panel, and orthopedic, neurological, and radiographic imaging examinations were performed at the first visit to Azabu University Veterinary Teaching Hospital. The cat had folded ears and relatively short limbs, and both tarsometatarsal regions were deformed by gross exostoses (Figures 1A–D). A bleeding skin ulcer measuring approximately 1.5 cm in diameter was present on the plantar aspect of the right hock (Figure 1B) and worn by contact with the ground when the cat was seated. The cat was reluctant to move and could not jump up or down from the examination table. An orthopedic examination revealed muscle atrophy in the right hindlimb and weight-bearing hindlimb lameness; a decreased range of motion was found in both carpi and tarsi. The neurological examination was unremarkable, and all blood tests were within normal range. Digital radiographs of the tarsi, carpi, chest, and abdomen were obtained and showed gross exostoses on the tarsal and metatarsal bones as well as on the phalanges of both hindlimbs. Degenerative changes were also evident in the tarsal joints, and there were deformities of the carpal bones and phalanges in the forelimbs. The thoracic and abdominal radiographs were unremarkable. The spinal column, including the tail, was not obviously affected. Based on these findings, the clinical history of the cat, and its breed, the diagnosis was suspected to be SFOCD.

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The cat was initially referred to Azabu University Veterinary Teaching Hospital for radiation therapy. However, in view of previous reports, radiotherapy was considered unlikely to decrease the mass of the exostoses significantly and to be contraindicated because of the cutaneous ulceration. Therefore, a plan was devised that consisted of a combination of surgical debridement of the large plantar exostosis that was causing ulceration and bleeding of the overlying skin on the right hindlimb, dermatoplasty for the skin damage, and radiation and medical therapy to relieve chronic pain. At this stage, although the left plantar exostosis had not yet caused bleeding, its tip had stretched the overlying skin (Figure 1D); therefore, it was treated in a manner similar to that used for the right plantar exostosis.

Premedication consisted of subcutaneous injection of atropine sulfate^a 0.025 mg/kg, morphine hydrochloride^b 0.05 mg/kg, and robenacoxib^c 2 mg/kg and IV injection of carbazochrome sodium sulfonate hydrate^d 0.5 mg/kg and tranexamic acid 5%^e 5 mg/kg. Anesthesia was induced by IV injection of 1% propofol^f 6 mg/kg and maintained after endotracheal intubation by isoflurane in 100% oxygen. A balanced electrolyte solution was administered. Cefazolin^g 22 mg/kg was administered intravenously at the time of induction of anesthesia, 1.5 hr later, and three times daily for 3 days postoperatively. A skin incision was made on the plantar aspect of the right tarsus. The largest exostosis was at the tarsometatarsal joint and visible directly under the skin (Figure 1E); it was friable and easily broken into fragments using rongeur forceps (Figure 1F). The tendons on the plantar aspect were firmly attached to the exostosis, so they were not handled in order to avoid disruption. Just enough of the exostosis was removed to permit closure of the wound (Figure 1G). Subsequently, the damaged skin was resected, and the wound, subcutaneous tissues, and skin were closed in two layers (Figure 1H). Similar surgery was performed on the left hindlimb. Thereafter, modified Robert Jones bandages were applied to both tarsi for 3 days to reduce swelling. After removal, the exostosis tissues were preserved in 10% neutral buffered formalin and submitted for histopathological examination.

Three days after surgery, palliative radiotherapy was performed using a 6-MV linear accelerator^h. The cat was premedicated with atropine sulfate 0.025 mg/kg administered by subcutaneous injection. Anesthesia was induced by alfaxaloneⁱ 3 mg/kg administered intravenously and maintained using isoflurane in 100% oxygen. The protocol consisted of six fractionated doses each of 1.5 Gy on a Monday-Wednesday-Friday schedule for a total dose of 9 Gy. The medical therapy consisted of pentosan polysulfate^j 3 mg/kg, which was administered subcutaneously every wk for 4 wk starting 1 wk after surgery.

The bleeding from the right tarsus, which had been evident preoperatively, stopped on postoperative day 3. The lameness had improved. One wk after starting radiotherapy, the cat was more active and allowed its hindlimbs to be palpated. The sutures were removed 2 wk after surgery and the cat was discharged from hospital. On the day of discharge, the cat was able to jump up and down from the examination table. The scars were healing normally and there were no acute radiation-related complications. Slight lameness was evident but appeared to be markedly improved. Histopathological examination of the submitted tissues revealed a localized mass of mature cartilage with osteoid on the bone surface and that the surface was covered with connective tissue. The finding that these tissues consisted of cartilage and osteoid was consistent with our suspected diagnosis of SFOCD.

Follow-up physical, orthopedic, and radiographic examinations were performed at 20, 48, and 80 wk postoperatively. At 20 wk, the owner reported that the cat’s activity levels had improved, with further improvement reported at 48 and 80 wk. There was no recurrence of skin ulceration on either tarsus at 20, 48, or 80 wk (Figures 1I–L). Orthopedic examination revealed intermittent lameness on weight-bearing in the right hindlimb that steadily improved after surgery. Both carpi and tarsi had the same limited range of motion as before surgery.

Digital radiographs of both tarsi were obtained at 20, 48, and 80 wk postoperatively and of the carpi at 48 wk. There was no further growth of the exostoses and no obvious worsening of the degenerative changes in either tarsus (Figures 2A–J). The areas containing exostoses, which were caudal to the tarsometatarsal joint, were measured by OsiriX imaging software on mediolateral radiographs obtained at each assessment. The measurement was started from the intersection of the exostosis and metatarsal bone, moved proximally along the bone margin, and then back to the starting point along the free end of the exostosis (Supplementary Figure I). The measurements obtained are shown in Table 1. There was no change in either carpus during 48 wk of postoperative follow-up and no radiation-related complications, such as alopecia, bone necrosis, nerve damage, or tumorigenesis. The owner reported being satisfied with the efficacy of the combined therapy, particularly with the cessation of bleeding and improvement in activity.

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TABLE 1 Areas of Exostoses Caudal to the Tarsometatarsal Joint of Each Hindlimb Measured on Mediolateral Radiograph

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Discussion

The Scottish Fold cat resulted from a mating between a queen with a spontaneous mutation and local farm cats and British shorthairs.¹³ Initially, it was thought that SFOCD only developed in cats that were homozygous for the folded ear (Fd) gene, which is inherited as an autosomal dominant trait.^14,15 However, several subsequent reports described similar but less severe lesions in heterozygous (Fd/fd) cats.^1,3,4,6 Registration of the Scottish Fold was banned in the United Kingdom in 1974; however, Scottish Fold cats are still common there and in many other countries.⁴ Some authors advocate that continued breeding of the Scottish Fold should be discouraged.^3,9 However, because there is still a large population of Scottish Fold cats at present, some treatment guidelines should be established.

SFOCD is an inherited disorder that affects both growth of the bone plates and formation of articular cartilage.^2,3 Histologically, tissues from affected kittens show defective bone formation at the growth plates, with disordered proliferation of chondroblasts and irregular groups of cells arranged in a haphazard manner.³ The exostoses removed from the cat described here consisted of osteoid and cartilage, which is consistent with a clinical diagnosis of SFOCD.

There is considerable variation in the age at which clinical manifestations occur in cats with SFOCD, as well as their severity and rate of progression.⁴ The age at onset of clinical signs has been reported to be between 5 mo and 6 yr,³ as was the case in this cat. Cats with SFOCD show a variety of signs of skeletal disease, including lameness, reluctance to jump, and a stiff stilted gait.^3,5,7 In addition to the symptoms described above, the present case also had concomitant skin involvement, which is consistent with a single previous case report.⁷ The skin lesion in this cat occurred because of the stretched overlying skin that was worn by contact with the ground and persistent licking of the area. The severity of clinical signs also varies.⁴ Some cats do not show any lameness or related signs until later in life⁶; however, others are so severely affected that the owners opt to euthanize.³ Plantar exostoses have been documented in advanced cases and were obvious in the present case.³

Given that the etiology is genetic, the main aims of treatment are to ameliorate the clinical signs and accompanying pain.⁴ Several surgical approaches, including removal of tarsal exostoses or staged bilateral ostectomies and pantarsal arthrodesis, radiation therapy, or medical therapy using pentosan polysulfate or other oral drugs have been reported.^3,5–11 However, given the variability of the presenting clinical signs, there is still no universal standard of care for SFOCD.

The case described here was referred to Azabu University Veterinary Teaching Hospital for radiotherapy. However, given earlier reports indicating that radiotherapy would not have decreased the size of the exostoses, it was anticipated that the skin damage would persist.^5,9 Furthermore, medical treatments recommended by the referring veterinarian had not resolved the skin problem. Therefore, surgery was considered necessary to treat the skin damage.

Pantarsal arthrodesis has previously been shown to resolve lameness and skin problems in a cat suffering from SFOCD.⁷ However, arthrodesis is an invasive and irreversible procedure and SFOCD could progress further in the longer term.^5,9 Considering the relatively young age of the cat in this report, the decision was taken to reduce the size of the exostoses rather than perform an arthrodesis. During surgery, the exostoses were found to be adhered to and displacing the surrounding soft tissue, including the tendons. Therefore, even if a wide debridement was performed, normal joint function would not be retained. Hence, just enough of the exostosis was excised to allow closure of the wound. Salvage arthrodesis could be considered for pain relief in this cat in the future.

All four extremities were irradiated to relieve chronic pain in this cat using the radiotherapy protocol described in previous reports.^5,9 However, a protocol specifically tailored to SFOCD still needs to be determined. Although the exact mechanisms are yet to be identified, low-dose radiation therapy has an analgesic effect that is attributed to anti-inflammatory activity and has been used to treat inflammatory or degenerative joint disease in humans.¹² Radiation therapy has been reported to relieve pain in cats with SFOCD for 24–72 mo but cannot completely suppress further growth of exostoses.^5,9 Therefore, radiation therapy for SFOCD should be considered a supplemental treatment. The cat in this report showed no signs of excessive bone proliferation at 80 wk after surgery. However, regrowth of the exostoses can occur in the long term, so continuous observation is still needed.⁵

Medical therapy was also attempted because a previous report showed that administration of pentosan polysulfate reduced lameness and discomfort in cats with SFOCD.³ The authors of that report did not put forward a reason for why this drug was beneficial in SFOCD but speculated that it would be a helpful treatment for osteoarthritis accompanying SFOCD. Pentosan polysulfate is assumed to support the anabolic activity of chondrocytes and fibroblasts while attenuating the catabolic events associated with destruction of the extracellular matrix in cartilage and has been reported to be beneficial in the treatment of osteoarthritis of the human knee.¹⁶ However, the beneficial effect of the medications cannot be confirmed because they were administered at almost the same time as the surgery and radiotherapy.

The measurements obtained on mediolateral radiographs showed that the plantar exostoses on both tarsi decreased either immediately after surgery or at a later time point during follow-up (Table 1). Two previous reports indicated that radiotherapy did not decrease the volume of exostoses during the observation period.^5,9 Mathews et al., who performed pantarsal arthrodesis, reported that exostoses of the talus that were not surgically removed decreased over time during 20 wk of follow-up.⁷ However, in that report, the articular cartilage of the tibiotarsal, intertarsal, and tarsometatarsal joints, which would have some influence on new bone proliferation, was removed, unlike in the present case. It is possible that the three therapies administered had the combined effect of decreasing the size of the plantar exostoses in this cat, but the exact reason for the decrease could not be ascertained from the previous reports.

This report has some limitations, particularly because of the subjective analyses used. A more accurate assessment could have been obtained by measurement of range of motion using a goniometer, use of an objective mobility/pain scale, force plate gait analysis, or three-dimensional evaluation of the volume of exostoses. Nevertheless, given that SFOCD is a progressive disease, ongoing monitoring for bone lesions and concurrent degenerative joint disease is also required.

Conclusion

Based on the experience with this case, a combination of surgical, radiation, and medical therapies may be an effective treatment for SFOCD with skin ulceration. However, longer-term follow-up of more cases is required for a better understanding of the long-term outcomes of using this protocol.

Supplementary Figure I

Measurement area of the exostosis caudal to the tarsometatarsal joint on the mediolateral radiograph. The measurement was initiated at the intersection of the exostosis and metatarsal bone, moved proximally along the bone margin, and then to the free end of the exostosis. Measurements were obtained on both hindlimbs at each assessment.