Teaching Case

A 42-year-old recreational diver presented with progressive pain at the right shoulder two hours following diving. History of recent trauma was denied.

Plain radiographs revealed no abnormalities (Fig. 1). Magnetic resonance (MR) arthrography of the right shoulder 4 weeks after the onset of pain was requested in order to exclude rotator cuff disease and bone marrow disease. MR revealed an irregular aspect of the articular side of the supraspinatus tendon (Fig. 2A). MRI also showed an extensive area of abnormal bone marrow within the proximal diaphysis of the humerus, with extension into the humeral head (Fig. 2B, Fig. 3A and B). Coronal T1-weighted images (T1-WI) of the right humerus (Fig. 3) showed a peripheral rim of low signal intensity (SI) surrounding a central zone similar to fat. On coronal fat suppressed T2-WI (Fig. 4A and B), the peripheral borders were strongly hyperintense. Coronal T2-WI (Fig. 5) showed a double-line sign consisting of a hyperintense rim centrally surrounded by a hypointense rim peripherally. There was no soft tissue involvement.

Based on the imaging findings and clinical history of the patient, diagnosis of dysbaric osteonecrosis (DON) was made.
DON is a manifestation of decompression illness (DCI). Other commonly used terms are Caisson disease, the bends, and diver’s disease. DON results from nitrogen gas bubble formation following an excessive drop in atmospheric pressure, in divers during rapid ascent, or in pilots after urgent increase of altitude [1].
DON usually affects fatty-marrow containing long bones, particularly the proximal third of the humerus and tibia and distal third of the femur [2]. This can be explained by the fact that nitrogen is more soluble in fat [3]. Obstruction of intramedullary sinusoids by gas bubbles results in blood stasis, ischemia and bone necrosis causing joint pain (acute type I of DCI). Furthermore intramedullary pressure changes may aggravate venous stasis [3]. In type II of DCI, neurologic, pulmonary and circulatory symptoms predominate.
Clinical information is crucial to make a final diagnosis as imaging of DON is not different from other causes of bone infarcts.
Plain radiography lacks sensitivity for early detection of DON. Typical radiographic findings include increased bone density, subarticular crescent sign and geographically delineated bone infarcts with sclerotic borders. These findings are only visible after weeks or months and represent late, irreversible lesions.
MRI is the preferred technique for early diagnosis. Devascularized areas tend to form well-demarcated lesions with a characteristic serpiginous geographic pattern [4]. Typically, a peripheral double-line sign is seen on T2-WI, consisting of an inner rim of high SI surrounded by an outer rim of low SI. It was initially explained by an inflammatory response around the central ischemic core resulting in a hyperintense inner zone of hyperemic granulation tissue and a hypointense outer rim of reactive sclerosis [5]. Alternatively, the double line sign may be caused by a chemical shift artefact, explaining reversal of these alternating hypointense and hyperintense rims. The central part of osteonecrotic bone marrow may have a variable SI. Most often, due to mummification of dead fatty marrow, the SI resembles fat on all pulse sequences, although edematous or fibrotic changes may occur [4].
Scintigraphy is sensitive to detect early ischemic bone marrow changes, its low specificity hampers its usefulness in diagnosing DON.
The prognosis depends upon early diagnosis and prompt treatment, consisting of hyperbaric oxygen therapy in a recompression chamber. This has been applied twice in our patient with complete resolution of pain within 1 week following treatment.