Infection vs Infarction: The Problem in Sickle Cell Anaemia
Patients with sickle cell anaemia have a high predisposition to osteomyelitis. This is due to recurrent splenic infarctions leading to autosplenectomy, as well as the presence of necrosis foci within the bones themselves. Pathogen number one is Salmonella, followed by staphylococci.
The main challenge in diagnostic imaging is to differentiate osteomyelitis from aseptic bone infarction. Both conditions cause acute pain and alter the MRI signal.
- Radiography: Uninformative in the first 10 days. Later, signs of osteolysis and periosteal reaction appear.
- MRI: The optimal method. Infection produces pronounced bone marrow oedema and post-contrast enhancement. Infarctions, however, have a characteristic tortuous, serpiginous appearance.
- Scintigraphy: Leukocyte scintigraphy and 99mTc-sulphur colloid imaging help to definitively differentiate these two conditions.
Tuberculous and Fungal Lesions
Specific bone infections frequently develop in immunosuppressed individuals or those who have visited endemic areas.
Tuberculosis: Characterised by the formation of "cold" abscesses. For example, in lumbar tuberculous discitis, purulent masses can descend along the psoas muscle into the pelvic cavity, which is well visualised on T2-weighted MRI.
Fungal infections (mycoses): Manifest as granulomas, fistulae, and multifocal osteomyelitis (mixed lytic and sclerotic lesions on radiographs).
- Aspergillus fumigatus: Haematogenous seeding from the lungs, frequently affecting the spine (discitis).
- Blastomycosis: Affects the spine and lower extremities, mimicking bone tumours.
- Candida: Lytic lesions without a pronounced periosteal reaction in immunocompromised patients.
- Mycetoma (Mycetoma / Madura foot): Chronic soft tissue and bone infection of the foot. On MRI, it produces a specific dot in a ring sign—multiple small cavities with a low-signal centre and intense peripheral contrast enhancement. Visually, it can mimic a haemangioma.
Features of Lesions in HIV
Osteoarticular infections in the setting of HIV (especially in intravenous drug users) are becoming increasingly common and carry an unfavourable prognosis. They are caused by opportunistic agents: Candida, Clostridium, Mycobacterium avium, as well as ordinary Staphylococcus and Salmonella.
Large joints—the knee and ankle—are typically affected. Radiographic findings include periarticular osteopenia, bone destruction (osteolysis), and marked surrounding soft tissue oedema. MRI reveals massive joint effusions and chondrolysis.
Differential Diagnosis and Biopsy Tactics
The MRI picture of osteomyelitis (oedema and contrast enhancement) is not strictly specific. The most important differential diagnosis is made with bone tumours, which also produce lytic or sclerotic cortical destruction with a wide zone of transition. In addition, infection can be mimicked by Charcot joint (acute diabetic osteoarthropathy), storage diseases (amyloidosis), and histiocytosis.
Principles of Management and Morphological Diagnostics:
- Blood cultures are drawn strictly before initiating antibiotic therapy.
- When infection or tumour is suspected, percutaneous biopsy is performed under ultrasound, CT, or X-ray guidance.
- The needle trajectory is coordinated with surgeons (in case of confirmed malignant tumour, to avoid tissue seeding).
- The punctate is immediately sent to the microbiological laboratory in a transport medium, as well as for cytology and histology.
- Histologically, accumulations of neutrophils and thrombosis are detected in acute processes, whereas caseous necrosis is found in tuberculosis.
Briefly: answers to topical questions
What stages of tuberculous spondylitis are distinguished during radiological examination?
Three stages of tuberculous spondylitis are distinguished during radiological examination and clinical evaluation.
- Prespondylitic phase (pre-arthritic) — radiographs reveal signs of primary osteitis, including a limited cavity in the vertebral body or destruction of bone trabeculae.
- Spondylitic phase (arthritic) — accompanied by destruction of the vertebral body and extension of the process to adjacent discs.
- Post-spondylitic phase (post-arthritic) — represents the outcome of the disease and stabilisation of the pathological process.
What radiopharmaceuticals are used for scintigraphy in the diagnosis of osteomyelitis?
Several different radiopharmaceuticals are used for scintigraphy in the diagnosis of osteomyelitis.
- Technetium-99m (99mTc) — used in bone scintigraphy to assess blood flow and osteoblastic activity.
- 99mTc-sulphur colloid (99mTc-sulphur colloid) — used in conjunction with labelled leukocytes to differentiate infection from bone infarction.
- Indium-111 (111In) — used for leukocyte labelling to specifically identify the localisation of the purulent-inflammatory process.
- Gallium-67 (Gallium-67) — used primarily for the diagnosis of spinal infections.
For MRI differentiation, remember: bone infarction has a serpiginous appearance, whereas infection produces a continuous, diffuse bone marrow oedema.
Class and exam questions
When do the first radiological signs of osteomyelitis appear?
In the early stages, radiography is often uninformative. The initial changes (osteolysis and periosteal reaction) may become noticeable only 10 days after the onset of the disease.
What is the "dot in a ring" sign and where is it found?
This is a pathognomonic MRI sign of mycetoma (e.g., Madura foot). On T2-weighted images, abscesses appear as rings with a low-signal area (dot) in the very centre.
What else to study on this topic
- Radiological signs of acute diabetic neuropathic osteoarthropathy (Charcot joint).
- Specific infectious pathogens in HIV patients: the role of Mycobacterium avium complex.
- Technique and navigation of CT- and ultrasound-guided percutaneous bone biopsies.
- Pathogenesis and imaging features of chondrolysis in septic arthritis.
- Labelled leukocyte scintigraphy in the differential diagnosis of complex osteitides.
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