Madura Foot: MRI

Case Report  - A 45 year non-diabetic male pt. presents with pain &  swelling in right foot  of about 2-3 months duration for MRI foot  with clinical suspicion of actinomycetoma.

Radiological Findings

1.    4^TH Metatarsal shows cortical thickening and sclerosis in shaft & mild expansile lytic lesions in the base with ill defined T2 hypointense soft tissue around the base. Multiple lytic lesions also seen in talus (neck and anterior body), anterior calcaneum, inferior cuboid, intermediate & lateral cuneiforms.

2.    Ill defined T2 hypointense soft tissue seen in the subcutaneous plane of mid foot sole & in the dorsum of foot  over the head of  1^st & 2^nd metatarsal showing small rounded areas with faint iso to hyperintense signal with hypointense rim & central tiny dot like hypointense focus – suggesting dot-in-circle sign.

3.    Ill defined T2 hypointense soft tissue also seen in the superficial & deep muscle planes of the plantar aspect of mid & hind foot with similar signal pattern.

Findings are suggestive of chronic osteomyelitis of foot due to granulomatous etiology most likely due to actinomycetoma, consistent with history. Differential – TB ( less likely).

Teaching Points by Dr MGK Murthy, Dr GA Prasad

1.    Mycetoma or Madura foot is a chronic granulomatous infection of the dermis and epidermis caused by the bacteria Actinomyces (Actinomycetoma) or by true fungi (eumycetoma). It was first described in the Indian district of Madura in 1846, hence the eponym Madura foot, affecting mainly the feet, which are more prone to trauma, and hence more likely to get infected,  other sites - lower legs, hands, head, neck, chest, shoulders and arms, common in males between the ages of 20 and 50 years.

2.    The infecting organism is presumed to be directly inoculated after penetration of the skin with a sharp object  e.g., a thorn later forming painless subcutaneous nodules and fistulae, from which a purulent exudate may be discharged. The process is usually indolent but with a potential for abscess formation, draining sinus tracts, osteomyelitis, and fistula formation, with severe deformity and disability ensuing if treatment is not provided.

3.    The “dot-in-circle” sign has recently been proposed as a highly specific magnetic resonance imaging (MRI) and ultrasonography (USG) sign of mycetoma, which may allow a noninvasive as well as early diagnosis.

4.    MRI shows lesions with low signal on T1W and T2W images, possibly due to susceptibility from the metabolic products of the “grains”. The “dot-in-circle” sign, seen as tiny hypointense foci within the hyperintense spherical lesions on T2W, STIR, and T1W fat-saturated gadolinium-enhanced images. The high-signal areas seen on MRI represents inflammatory granulomata, the low-intensity tissue seen surrounding these lesions represents the fibrous matrix, and the small central hypointense foci within the granulomata represents the fungal balls or grains.  Differential for the “dots” is rice bodies – hypointense foci seen in the synovial fluid of patients with articular or tendon tuberculosis.

5.    Biopsy  (with demonstration of the characteristic features) or staining and microbiological culture of the discharge from the lesion usually gives the definitive diagnosis, both are time-consuming procedures and diagnosis may be difficult to achieve, especially with fastidious organisms.

6.    Antifungal  medication is successful in almost 90% of cases, lesions not arising in the foot or due to fungus tend to have a worse prognosis and require surgery.