Introduction

Septic physitis is a disease reported rarely in small animals. Reports in the literature include a report of vertebral septic physitis in a dog as well as cases of septic physitis secondary to portosystemic shunting in three dogs.^1,2 As a whole, osteomyelitis is uncommon in small animals, although it is most common in the metaphysis of growing dogs, likely as a result of blood supply and hematogenous spread of infection.³ Trauma, bacteremia, surgery, or foreign bodies are some of the potential predisposing factors to development of osteomyelitis.⁴ Physitis, and osteomyelitis in general, is most often a radiographic diagnosis in veterinary medicine, characterized by the widening and irregularity of physeal margins and associated soft tissue swelling at the site.⁵ MRI has been proposed as a method of earlier diagnosis of osteomyelitis or septic arthritis in large animals.⁶ It is regularly used in human medicine, where the appearance of these disease processes on MRI has been described.^4,6,7 However, to the authors’ knowledge, the MRI appearance of appendicular septic physitis has not been reported in small animals. Here we describe the MRI characteristics and clinical outcome of septic physitis in a 5 mo old male boxer dog.

Case Report

A 5 mo old intact male boxer dog initially was presented to its primary care veterinarian for evaluation of progressive left forelimb lameness that began after a fall in a car 3 days previously. On presentation, its ears were bandaged from recent ear cropping performed with no complications. Its medical and surgical history was otherwise unremarkable, although it was being fed a raw diet. At the time of presentation, it was non-weight bearing lame on the left front and was febrile. Radiographs of the left forelimb at this time revealed evidence of an osteochondrosis lesion of the humeral heads bilaterally, mild widening of the cranial aspect of the left proximal humeral physis when compared with the right, but no evidence of soft tissue swelling. A chemistry panel was unremarkable. The patient was given carprofen^a (4.2 mg/kg subcutaneously once) and discharged with carprofen (3.7 mg/kg orally q 24 hr) and tramadol^b (2.5 mg/kg orally q 8–12 hr).

The patient presented to the emergency service 24 hr later after continued progression of lameness despite treatment. On intake exam it was found to be monoparetic on the left thoracic limb with no obvious long bone or joint pain noted. Further neurologic exam revealed decreased tone and absent withdrawal reflex on the affected limb but no further neurologic abnormalities. No soft tissue swelling was noted. The patient was found to be febrile at 40°C on presentation. Differentials at this time included traumatic nerve injury, meningitis, myelitis, discospondylitis, fracture, developmental orthopedic disease such as hypertrophic osteodystrophy, or a septic orthopedic process. An MRI of the cervical spine and brachial plexus were recommended and scheduled for the following morning.

The patient was started on ampicillin/sulbactam^c (23 mg/kg IV q 8 hr) because of persistent fever of 40.5°C, after which he became afebrile at 38.5°C. Physical exam the following morning showed no neurologic change, but significant swelling centered around the left shoulder and pitting edema of the distal left thoracic limb were noted. Repeat radiographs were recommended prior to MRI exam.

Four radiographs of the left forelimb centered over the humerus and a single lateral of the right proximal humerus (for comparison) were obtained. The cranial margin of the proximal metaphysis of the left humerus was widened and ill-defined. Moderate, amorphous metaphyseal sclerosis at this site was observed. Bilateral humeral osteochondrosis remained visible. Severe extracapsular soft tissue swelling was also noted around the proximal left humerus. The contralateral limb exhibited a normal open physis appropriate for the patient’s age.

Sagittal and transverse T2-weighted images of the cervical spine and brachial plexus were obtained. No abnormalities of the spinal cord, meninges, or vertebrae were identified. There was severe soft tissue swelling of the left thoracic limb extending into the left brachial plexus region, but no overt abnormalities of the left brachial plexus such as a mass or regions of contrast enhancement within the nerves were identified.

MRI sequences of the shoulder were run as a secondary consideration following cervical MR. Sagittal and dorsal plane T1- and T2-weighted sequences were obtained of the left shoulder prior to contrast administration, and additional sagittal and dorsal T1-weighted sequences were obtained with and without fat saturation following contrast (Figures 1, 2). Effusion was noted in the scapulohumeral joint. The osteochondrosis lesion was identified on the caudal humeral head. Minimal contrast enhancement was noted within the bone at the caudal humeral head, and the remainder of the epiphyseal bone of the humerus was within normal limits. The metaphysis of the left humerus showed marked heterogeneity in the medulla and cortex, some of which were markedly heterogeneously contrast enhancing, whereas other areas did not contrast enhance at all. The cranial margin of the proximal humeral physis was widened and irregular with increased T2-weighted signal intensity. There were pockets of T2-weighted hyperintense/T1-weighted hypointense material (thought to represent fluid) adjacent to the cranial and caudal margins of the physis, communicating with the physis itself, and extending into the fascial planes surrounding the regional musculature. This was the case with all T1 sequences obtained. Fluid-attenuated inversion recovery sequences were not obtained; however, fluid was confirmed as the source rather than fat via subsequent ultrasound exam. The musculature of the left proximal thoracic limb was enlarged and diffusely heterogeneously T2-weighted hyperintense with multifocal patchy contrast enhancement on postcontrast T1-weighted sequences. There was also regional lymphadenopathy noted that was thought to be reactive.

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Because of the appearance of fluid pocketing on MRI, ultrasound was used to locate the pocket for fine-needle aspiration and cytology. On ultrasound, echogenic fluid could be visualized spontaneously flowing between a pocket cranial to the proximal humerus and the physeal space, most likely reflecting sedimentation of cells within the fluid due to gravitation effects. This fluid was aspirated using sterile technique and submitted for cytology as well as culture and sensitivity testing.

The sample was highly cellular with numerous degenerate neutrophils and low numbers of foamy mononuclear cells. Rare small multinucleate cells were observed and thought to represent osteoclasts. There were numerous cocci and coccobacilli found both intra- and extracellularly. The interpretation was marked septic suppurative inflammation. The culture grew a beta-hemolytic streptococcus that was susceptible to all antibiotics tested.

Systemic antibiotic therapy, pain control, and recumbency care were the treatments used in this case. Antibiotic therapy in hospital was started with ampicillin/sulbactam (23 mg/kg IV q 8 hr) and transitioned to amoxicillin/clavulanic acid^d (18.6 mg/kg orally q 12 hr) prior to discharge. Antibiotics were continued for 16 days after discharge.

Initial follow-up 1 wk after discharge revealed significant clinical improvement. The left forelimb lameness at this time was graded as a 1 out of 5, improved from grade 4 on first presentation. The patient was afebrile, and the swelling was markedly improved to almost resolved. No axillary pain or withdrawal deficits were noted. Additional recheck exam with radiographs of both shoulders was performed 4 wk after discharge. The patient no longer showed any clinical signs of lameness or swelling. Radiographs showed significant improvement of the left humerus proximal physeal widening, with only a small area of lucency at the cranial aspect. Sclerosis and cortical thickening of the proximal left humerus were thought to represent remodeling secondary to previous insult versus residual or persistent osteomyelitis. The osteochondrosis lesions were persistent but less radiographically evident. The right proximal humeral physis remained radiographically open, but the left showed evidence of premature closure. No limb length discrepancy or angular limb deformity could be observed on exam at this time. Further follow-up radiographs were performed 5 mo after initial discharge. The left proximal humeral physis was found to be completely closed, and there was improvement in the previously noted sclerosis and cortical thickening. Its physical exam at that time showed no residual lameness or other changes that could be associated with the physitis.

Discussion

To this date, MRI findings of appendicular septic physitis in a dog had not been described. The most striking aspects of the MRI were the marked heterogeneity of the medulla and cortex of the metaphysis, the combination of marked heterogeneous contrast enhancement, the pocketing of fluid in the area extending among fascial planes, and the widening of the physis itself. Although no direct neurologic lesions were noted in this case, the severity of swelling in the area of the brachial plexus was thought to have led to neuropraxia causing the clinical appearance of a brachial plexus injury.

There is one reported case describing MR findings of vertebral osteomyelitis in an adult dog.³ In that report, there were sufficient secondary changes noted in the imaging of the spinal cord that neoplasia or a primary neurologic disease were not able to be definitively ruled out based on imaging.³ Ultimately, surgery was required to differentiate the cause.³ In the case described here, the MR images were suggestive of a process separate from a neurologic cause, but the severe swelling in the area of the brachial plexus did make definitive diagnosis difficult. Ultimately, the patient’s rapid and significant response to antimicrobial therapy make an underlying neurologic condition unlikely. The MRI findings observed here were consistent with the findings described in septic arthritis and osteomyelitis in horses and people. Horses with septic arthritis showed a diffuse, mottled hyperintensity within the bone adjacent to the affected joint, as well as effusion.⁶ MRI in children demonstrated that the metaphysis, epiphysis, and physis may show changes on imaging, but that there are not changes seen in the joint space itself, consistent with the findings in this dog.⁴ MRI is considered the most useful imaging technique to evaluate suspected osteomyelitis in people and has been proposed as the imaging of choice for early detection of septic arthritis and osteomyelitis in foals because of its ability to identify early changes within the bone and soft tissues.^6–8

Because the MRI showed evidence of fluid pocketing around but external to the physis, limited ultrasound exam of the shoulder was performed. This confirmed the fluid pocket identified on MRI and that it was communicating with fluid in the physis. This was suspected on MRI but was easier to confirm via ultrasound because of the dynamic nature of the fluid pocket. Sampling of the fluid pocket provided the material for cytology and culture and sensitivity testing. Ultrasound has been used to identify joint effusion or subperiosteal fluid associated with septic arthritis or osteomyelitis in children, with acute osteomyelitis being recognized by elevation of the periosteum by a hypoechoic layer of purulent material.⁷ Similar to this case, it has been used in children to guide diagnostic aspiration.⁷

MRI has been studied as an imaging modality of choice for both initial diagnosis and follow-up for physeal injury in children, including septic physitis.⁴ Complications of septic physitis in children cited included chronic osteomyelitis or physeal arrest, leading to limb shortening or angular limb deformities.⁴ Although it was a small sample, some of the children had trivial trauma that seemed to incite the infection, similar to what the dog presented here experienced.⁴ These children showed evidence of changes on MRI in combinations of the metaphysis, epiphysis, and physis, but none had changes present in the associated joint, similar to what is described in the dog here.⁴

No specific source of infection was identified for this dog. However, the recent ear cropping may represent a possible source of systemic infection. The owner did not report any irregularities with the ear cropping, and there was no visible swelling or drainage associated with the area, but this still cannot be ruled out as a potential cause. The Streptococcus species cultured from the submitted fluid could be consistent with this source of infection. Sepsis in young animals is considered to occur most commonly from wounds, which could include tail docking or ear cropping, as well as respiratory, urinary, and gastrointestinal infections.⁹ In this case, it was thought that one of these was the likely source of infection and that the trauma associated with the car incident was possibly the inciting incident that caused seeding of bacteria at that location. The timeline to development of radiographic signs of infection, with a 5 day lag, is consistent with what is reported in children.⁴

Conclusion

We have described the characteristic of limited MRI sequences in one case of septic physitis. Although septic physitis is typically a radiographic diagnosis, this case shows how MRI can be used to rule out neurologic disease and instead suggest septic physitis. The characteristics of infectious osteomyelitis with physeal involvement in this dog was similar to what has been described in children.⁴ It also demonstrated the utility of MRI for identifying a fluid pocket and, together with ultrasound guidance, allowed for needle aspiration for identification of the infecting organism and hence targeted antibiotic therapy.