Teaching Case

A 56-year-old female patient presented with swelling and pain in the medial aspect of the left foot for the past month. No history of trauma or fever. She suffered from rheumatoid arthritis for several years on treatment.

A small nodular lesion showing T1W hypointense and T2W heterogeneously hyperintense signal was seen in the subcutaneous soft tissue along the medial aspect of the foot inferomedial to the first tasometatarsal joint. A central small cystic area was seen on T2W images. Mild surrounding oedema was noted. An area of focal inflammation with STIR hyperintense signal was noted in the heel pad inferior to the calcaneus, extending almost up to the skin. Multiple periarticular erosions were seen along the distal aspect of the lateral malleolus, calcaneus, talus, medial and intermediate cuneiforms. A valgus deformity of the forefoot was observed. The tendons appeared normal.

Subcutaneous rheumatoid nodules are granulomatous nodules seen in 20% of patients with rheumatoid arthritis. They are frequently found on pressure points at bony prominences and extensor surfaces adjacent to joints, elbows, and fingers, as well as the forearm, metacarpophalangeal and proximal interphalangeal joints, occiput, back, heel, and other areas. Only 1% of cases occur in the foot, typically beneath the metatarsal head region. It is usually associated with seropositivity for rheumatoid factor [1].
The pathogenesis of rheumatoid nodules still remains unclear. Histologically, a rheumatoid nodule is composed of three zones: an inner zone of central necrosis, a middle cellular palisading area, and an outer granulomatous area with perivascular infiltration of chronic inflammatory cells [2].
They can also occur in patients with systemic lupus erythematosus, ankylosing spondylitis, granuloma annulare and chronic active hepatitis [1].
These are firm, nontender and moveable nodules ranging in size from 2 mm to 5 cm. Rarely they may ulcerate and can be painful [1].
The MR imaging characteristics of rheumatoid nodules are variable and non-specific. They appear most commonly as ill-defined nodular lesions isointense to muscle on T1W with heterogeneous intermediate to high signal on T2W. These nodules can appear solid showing homogeneous enhancement with contrast or cystic with peripheral rim enhancement. Sometimes a heterogeneous enhancement with central necrosis can also be seen [3, 4, 5].
The other hallmark findings of rheumatoid arthritis such as erosions and synovitis provides a clue to the diagnosis of these soft tissue nodules [6].
Ultrasound usually shows a homogenous hypoechoic lesion in the subcutaneous plane. A central discrete hypoechoic area may be seen in nodules with central necrosis [7]. USG done in our patient showed a well-defined hypoechoic lesion with a central hyperechoic area in the subcutaneous plane of the foot.
The common differentials include plantar fibromatosis, ganglion, tophus, cellulitis with abscess, proliferative fasciitis, lymphoma or sarcoma [3]. Plantar fibromatosis typically occurs along the plantar aponeurosis and shows hypo or isointense signal to skeletal muscle on both T1W and T2W images. Tophi usually show hypointense signal on T1W and intermediate signal on T2W images with other features of gouty arthritis.
Fine needle aspiration cytology done from the nodule in our patient revealed lymphocytes, neutrophils, histiocytes, few multinucleated giant cells and fibroblasts, in keeping with a chronic inflammatory pathology.