Teaching Case

This is a case of a two-month-old baby, with a previously drained left forearm abscess who was referred for a 2-phase bone scan in our facility for suspected multifocal osteomyelitis due to persistent fever despite negative cultures.

A 2-phase whole body bone scan was done and showed multiple sites of increased vascularity and osteoblastic activity respectively (Fig. 1). However, due to the limitations of planar imaging a single photon emission computed tomography (SPECT/CT) was ordered to accurately localise the abnormal foci of uptake. SPECT/CT localised the abnormal foci of uptake to soft tissue adjacent to the bones (Fig. 2 and 3). The CT only images showed these to be possible soft tissue calcification, which was confirmed on subsequent X-ray imaging (Fig. 4). The sites of abnormal soft tissue calcification corresponded to sites of previous trauma due to intravenous line insertions, incision and drainage (Fig. 5).

Background

Heterotropic ossification (HO) is defined as the presence of bone in soft tissue, where it does not normally exist. [1] Most cases are acquired, usually following trauma, but could be congenital on rare occasions. [2] It is believed to result from transformation of primitive mesenchymal derived cells present in soft tissue into bone forming cells. [3]

Clinical perspective

The clinical diagnosis especially in the early stages of the disease could be difficult because of the non-specific symptoms, which includes pain, warmth, swelling and fever. [4] It is therefore difficult to distinguish it clinically from conditions like osteomyelitis that share similar symptoms. [5-8] This is where anatomical and functional imaging becomes important, as they can differentiate between the two conditions like in our case. The more commonly acquired form of HO may occur in virtually any form of musculoskeletal trauma. [1] In our case, the child experienced multiple sites of musculoskeletal trauma from previous drip insertions and the incision and drainage of the left forearm (Fig. 5).

Imaging perspective

A multiphase bone scan is used to aid in the diagnosis of HO and other conditions like osteomyelitis. [1] However, the limitations of planar imaging which include poor anatomic and spatial data has given rise to the use of SPECT/CT which has a more useful anatomic and spatial data, thereby improving localisation. [9] In our case, the SPECT/CT clearly showed that the abnormal foci was extraosseous and not in keeping with osteomyelitis. The soft tissue calcifications seen and confirmed on X-ray were in keeping with multiple sites of HO. Even though this patient is just 2 months old, literature has shown that a bone scan can be positive 2.5 weeks after injury and precede a positive radiography by 1-4 weeks. [10]

Outcome

Our findings were able to prevent further use of intravenous antibiotics in this case, knowing that 80% of patients with HO run a benign course. [4] The patient was discharged form the hospital about a week after imaging as fever had subsided, with a plan put in place for further management of HO if needed.

Take home message

Paediatricians should be aware that HO could occur as a complication of multiple drip insertions, considered as a possible diagnosis when osteomyelitis is being suspected. Appropriate imaging can differentiate the two and prevent wastage of resources on intravenous antibiotics and prolonged hospital stay. This is the first case of paediatric HO in our department.

Written informed patient consent for publication has been obtained.