Metaphyseal osteopathy in a British Shorthair cat

Abstract

Metaphyseal osteopathy, otherwise known as hypertrophic osteodystrophy, is a disease that causes pyrexia and lethargy accompanied by pain in the thoracic and pelvic limbs of rapidly growing large-breed dogs. While metaphyseal osteopathy has been descibed in association with slipped capital femoral epiphysis in cats, it has not previously been reported as a cause of limb pain and pyrexia in this species. A 7-month-old British Shorthair cat presented with a 1 month history of pyrexia, lethargy and pain in all limbs. Investigation included radiographs of the limbs and chest, abdominal ultrasound, serum biochemical analysis, haematology, bone biopsy, joint fluid aspiration and cytology. Findings were consistent with a diagnosis of metaphyseal osteopathy. The cat’s clinical signs resolved following the administration of prednisolone. Symptoms recurred 1 month after the cessation of prednisolone therapy, but resolved when administration was resumed.

Case Report

A 7-month-old entire male British Shorthair cat was referred to our facility with a 1 month history of pyrexia, lameness and joint pain. The cat was fed a high quality commercial dry diet, and vaccination had been performed 4 months previously. Five months prior to presentation, the cat had been treated for dermatophytosis with griseofulvin, and the conditon had resolved.

Clinical signs were first observed 4 weeks prior to referral. Initial signs of pyrexia and inappetence were followed in 24 h by the development of a mild lameness of the right pelvic limb, which could not be localised. Investigation at the time included serum biochemistry and haematology. Haematology revealed a mild mature neutrophilia. All other parameters were within the reference ranges. Tests for feline leukaemia virus antigen, feline immunodeficiency virus antibody and heartworm (Dirofilaria immitis) antigen were performed and were negative. The cat had been treated with amoxicillin–clavulanate (Clavulox; Pfizer) 12.5 mg/kg PO q12h for 7 days, followed by clindamycin (Antirobe; Pfizer) 15 mg/kg PO q12h for 4 days. The cat’s appetite had improved 2 weeks after the onset of disease. Pyrexia persisted and the cat became reluctant to walk 3 weeks after the onset of clincal signs, prompting referral.

At referral presentation the cat was pyrexic, with a rectal temperature of 40.1ºC (104.2º F). Its heart rate was 190 beats per minute, and its respiratory rate was 25 breaths per minute. Pain was present to differing degrees bilaterally on palpation of the carpi, stifles and hips. Pain was considered to be most severe in the right carpus.

Radiographic abnormalities were present bilaterally, and were most marked in the distal radius and ulna (Figures 1 and 2). Abnormalities were also present bilaterally in the proximal and distal tibiae and fibulae, and the distal femora (Figure 3). These included diffuse areas of radiolucency within the metaphyses. Cuffs of mineralisation were present on the distal radius and ulna, and soft tissue swelling over the affected metaphyses was seen. Abdominal ultrasound was unremarkable.

Figure 1

Dorsopalmar radiograph of the right carpus. Transversely oriented lucent zones are present within the distal radial and ulna metaphyses which run parallel and adjacent to the physes (arrow). A cuff of mineralisation with soft tissue swelling is present over the distal radius and ulna

Figure 2

Mediolateral radiograph of the right carpus. Transversely oriented lucent zones are present within the distal radial and ulna metaphyses which run parallel and adjacent to the physes (arrow). A cuff of mineralisation with soft tissue swelling is present over the distal radius and ulna

Figure 3

Craniocaudal radiograph of the left crus. Subtle transversely oriented lucent zones run parallel and adjacent to the metaphyses of the proximal and distal tibia and fibula, and the distal femur (arrows)

Cytology was performed on smears made from joint fluid aspirates. Fluid from the right carpus comprised neutrophils (60%) and synovial cells (40%). The left carpal aspirates comprised synovial cells (96%) and small lymphocytes (4%). Fluid from the left stifle showed copious red blood cells with neutrophils (70%), small-to-medium lymphoid cells (4%) and synovial cells (26%; probable blood contamination of the joint fluid). The total cell counts of each sample were not elevated based on an examination of joint fluid smears. As no bacteria were seen on cytology, cultures of joint fluid aspirates were not performed. Bone biopsies were performed, using a Jamshidi needle, on the right distal radius, proximal to the growth plate. To facilitate the procedure the cat was premedicated with methadone (Methadone; Ilium) at 0.1 mg/kg IV. Anaesthesia was induced with propofol IV to effect (Repose; Norbrook Laboratories) and maintained with gaseous isoflurane (Delvet Isoflurane; Ceva).

Histopathological examination was performed on the bone biopsy samples (Figures 4–6) and revealed trabeculae of bone with increased cellularity. Spaces between the trabeculae were partly or completely filled with well differentiated osteoblasts. Thickened periosteum was also present in the samples. The differential diagnoses for this histological appearance include osteomyelitis, osteogenic sarcoma and metaphyseal osteopathy. However, no inflammatory focus or infectious agents suggestive of osteomyelitis were visualised, and the proliferating cells more closely resembled myofibroblasts rather than the fibroblasts seen with fibrosis. No mitotic figures or evidence of malignancy were found. Some histological features of metaphyseal osteopathy in dogs were absent, such as prominant osteoclastic activity, suppurative inflammation of marrow contents and periosteal aggregates of neutrophils. This is likely to be owing to the biopsy site chosen. However, the histological appearance was considered to be more consistent with a diagnosis of metaphyseal osteopathy than any other potential diagnosis. These histological findings combined with the radiographic appearance of the lesions and the cytological findings of inflammatory synovial fluid in the right carpus supported a diagnosis of metaphyseal osteopathy.

Figure 4

Photomicrograph of bone biopsy sample obtained at x 40 magnification. Bone trabeculae with increased cellularity, with the intervening spaces partly or completely filled by proliferating spindle cells, including well differentiated osteoblasts, can be seen

Figure 5

Photomicrograph of the bone biopsy sample at x 200 magnification

Figure 6

Photomicrograph of the bone biopsy sample at x 400 magnification. Bone trabeculae with increased cellularity, with the intervening spaces partly or completely filled by proliferating spindle cells, including well differentiated osteoblasts, can be seen

The cat was given prednisolone (Microlone; Mavlab) 1 mg/kg PO q12h and buprenorphine (Temgesic; Reckitt Benckiser) 0.01 mg/kg sublingually q8h for 4 days. Clinical signs improved significantly over the next week and the prednisolone dose was tapered to stop after 7 days of treatment. Symptoms recurred 1 month later, but responded well to another course of prednisolone. The dose was tapered to stop over 12 weeks. The cat’s symptoms did not recur.

Metaphyseal osteopathy is a rare disease mainly recognised in large breed dogs. The aetiology is unknown,^{1 –3} with proposed causes including nutritional imbalance, vaccination, canine distemper virus and heritability in some dog breeds.^1–7 Age at onset of symptoms is usually between 2 and 8 months,^1–4 and relapse of symptoms is common until skeletal maturity.^1,2,4 Typical clinical signs include pyrexia and lethargy with metaphyseal pain and swelling.^1,2,4 Lesions are usually apparent in the distal radius, ulna and tibia, ⁴ with radiography revealing transversely oriented lucent zones within the metaphysis, which run parallel and adjacent to the physes (sometimes called a ‘double physis’ sign or ‘scorbutic lines’). These zones are considered pathognomonic for metaphyseal osteopathy in dogs, ² and diagnosis is usually made on the basis of characteristic radiographic changes and clinical signs. ⁴ Arthrocentesis will often reveal an increased volume of transparent, straw-coloured fluid with normal viscosity and increased neutrophil numbers. ² Histopathological features include a zone of osteoblastic activity,^8,9 and an infracted area of suppurative and fibrinous inflammation where necrosis of the metaphyseal marrow and bone leads to trabecular collapse and fracture without external distortion of the bone. ⁹ In this case, biopsy samples were intentionally taken from the proximal area of osteoblastic activity and not the infracted area of metaphysis owing to concerns of iatrogenic fracture through the already damaged and more delicate feline radius. Fracture of bones affected by metaphyseal osteopathy is reported in the dog. ¹⁰

Treatment of metaphyseal osteopathy is supportive, and based on pain relief and anti-inflammatory therapy, with rehydration, nutritional support and nursing care as required.^1,2,4–6 Some authors suggest avoiding corticosteroids in large-breed dogs to avoid physeal growth disturbances, ⁵ but there is evidence that corticosteroids produce a faster remission and may be a more effective therapy. ⁶ Corticosteroids and pain relief were effective in this case, and although the cat had a relapse of symptoms, this is common in canine cases.^1,2,4 This is the first published report of metaphyseal osteopathy in the cat. Similar diseases such as slipped capital femoral epiphysis or proximal femoral metaphyseal osteopathy,^11–13 and osteodystrophy of mature cats fed a meat- or liver-rich diet are reported, but these diseases differ significantly from metaphyseal osteopathy in the location of lesions and clinical signs.^{12 –14}

Conclusions

This is the first published case report of probable metaphyseal osteopathy in the cat. This case has many similarities to reported canine cases of metaphyseal osteopathy, including clinical signs, radiographic findings, joint aspirate findings, histological findings, and response to treatment with pain relief and corticosteroids.