Erratum:  In the 11/11 issue of Paths to Practice Perfection, we incorrectly listed the author Jay Lieberman, DPM as Director of Podiatric Medical Education, Northwest Medical Center.  Dr. Lieberman is former Residency Director of that institution, and has been succeeded by Richard Brietstein, DPM. Our apologies to both Dr. Brietstein and Lieberman for the error.

For tonight's Residency Insight, we have a guest case presentation from the Kern Residency program at Southeast Michigan Surgical Hospital. Dr. Brian L. Goodwin, and Dr. Smitha Mantha, PGY-2 present
an interesting case of a soft tissue mass in the interdigital space mimicing an interdigital neuroma.

   —Ryan Fitzgerald, DPM

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Ryan Fitzgerald, DPM PRESENT RI Associate Editor Hess Orthopedics & Sports Medicine Harrisonburg, Virginia

Case Report:
Rheumatoid Nodule Mimicking an Interdigital Neuroma

Introduction:

Rheumatoid arthritis is a systemic inflammatory disease with a widespread clinical and serological presentation, as well as varying degrees of functional impairment.  Synovial inflammation leads to joint damage that can progress from mild cartilage degeneration to severe joint erosion⁵.  Extra-articular manifestations often occur with this inflammatory disease, of which rheumatoid nodules are the most common, occurring in approximately 20-30% of patients¹.  These lesions form predominantly on pressure points, such as the olecranon process^2,4, or on the ball and heel of the foot.  We report a case of an elderly female with rheumatoid arthritis and a large rheumatoid nodule causing nerve pain.  

Case report:

An 80 y/o female with a recent diagnosis of rheumatoid arthritis presented with a one year history of a slowly growing mass located at the plantar aspect of her forefoot. She complained that in the last six weeks, she had started having increasing sharp shooting pain and burning discomfort, worse while wearing tighter shoes and ambulation, and mostly unrelieved with rest.  She was not on any disease modifying medications at the time of presentation.

click image below to enlarge
Figure 1: Pre-op plain films show the mass displacing the 3rd toe medially

Physical examination revealed intact pulses, reproducible pain with forefoot squeeze and to the third interspace, with a positive mulder’s sign. A small, firm, and freely movable palpable mass was appreciated at the plantar aspect of her foot, lateral to the third metatarsal head, with an overlying callous.  No pain was reproducible with palpation of this mass. A plain film examination revealed splayed third and fourth digits, with a medial dislocation of the third proximal phalanx (figure 1).  The patient was given a clinical dual diagnosis of a Morton’s Neuroma with a rheumatoid nodule, and was boarded for a concomitant surgical excision of both lesions.

 A generous serpiginous incision was planned at the plantar aspect of the foot between the third and fourth metatarsals, from the sulcus to the bases of the metatarsals. 

The mass was at once visualized as the incision was deepened into the subcutaneous tissues. The mass appeared to be poorly circumscribed, lobulated, entangled in neighboring vascular and soft tissue structures, and had eroded into the deep transverse intermetatarsal ligament. 

The mass itself was seen to extend distally into the third interspace, as well as medially into the second interspace, and was carefully removed with soft tissue pedicles (figure 2).  Once removed, the mass was sent in total for pathological and histological analysis (figure 3).  The actual measurement of the mass was 4.5 cm x 2.5cm x 1.8cm.   The proper digital nerve was not visualized with further examination of the surgical site.  The wound was then closed in layers, with attention towards elimination of any dead space(figure 4).

Click images below for a larger view.
Figure 2: the large, poorly circumscribed nodule is difficult to separate from surrounding soft tissues		Figure 3: The mass was removed with some soft tissue pedicles, measuring approximately 4.5 cm x 3.0 cm		Figure 4: The long serpiginous incision was closed in layers to eliminate dead space.

Post-operatively, the patient was prescribed a two week regimen of cephalexin and was kept strictly non-weightbearing with a rollabout walker.  At one week post-op, the patient reported full resolution of symptoms with sensation and vascularity intact to the third and fourth digits.  Sutures were removed at three weeks.  The patient has since progressed into ambulation in a tennis shoe.  Although there are some palpable adhesions at the surgical site, the patient continues to be pain free.

Histopathology:

Histological analysis of the submitted specimen revealed a palisaded granulomatous dermatitis with intraluminal fibrinous necrosis, most consistent with a rheumatoid nodule (figure 5).  A PAS reaction and acid-fast bacterial stain failed to reveal fungal elements or mycobacterial organisms, and no neoplastic processes were appreciated (figure 5).

Figure 5: histological analysis revealed granulomatous dermatitis with a center of fibrinous necrosis, quite common for a rheumatoid nodule

Discussion:

Rheumatoid nodules are the most common soft tissue manifestation of rheumatoid arthritis, occurring in approximately 20-30% of patients with the disease.  However, it has been reported that only 1% these lesions manifest in the foot1.  The presence of a rheumatoid nodule is thought to indicate an advanced stage of the disease^1,3,4.  Although these lesions are generally asymptomatic, a nodule can become quite painful as a space occupying lesion in the tight compartments of the foot.  In some cases, like our patient, the nodule can even exhibit the symptoms of an interdigital neuroma, as it grows large enough to compress the proper digital nerve.  Interestingly enough, even though the deep intermetatarsal ligament was infiltrated by and removed along with the nodule, a neuroma was not found in the official post-operative histopathological analysis. However, the extent of the invasion of the mass leads us to the conclusion that the proper digital nerve must have been imbedded within the nodule.    

The rheumatoid nodule is a curious specimen whose histological composition is formed in three distinct layers: an inner layer of necrosis, a middle layer of palisading cells, and an outer layer of perivascular infiltration of chronic inflammatory cells⁴.   In some patients the nodule can grow slowly for years, while in others it can spontaneously regress and disappear.  A recent study utilized immunohistology and monoclonal antibodies to determine that a rheumatoid nodule behaves similarly to a granuloma.  The composition of both a rheumatoid nodule and a granuloma is determined by the balance of recruited inflammatory cells, such as T-cells and macrophages, and the apoptosis rate of those cells³ A disturbance of that delicate balance can account for the discrepancy between growth and regression, as well as explain the characteristic accumulation of necrotic debris at the nucleus of the rheumatoid nodule.

As the symptoms of a rheumatoid nodule can be a vague and diffuse, a wide range of differential diagnoses must be considered.  This includes, but is not limited to, plantar fibromatosis, ganglion cyst, giant cell tumor, sarcoma, and an infectious abscess.  Although an MRI was not obtained prior the surgical excision of the rheumatoid nodule of our patient, ancillary imaging, such as plain films, MRI, and ulstrasound, can be used along with the clinical history to aid the clinician towards a more precise diagnosis.

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We at PRESENT love hearing from you.  I encourage all of our readers to participate in the online forum in the E-talk thread on this topic, and post your thoughts, pearls, and perspectives regarding this (or any other) interesting case.

References:

1. Sanders TG, Linares R, Su A, Rheumatoid nodule of the foot: MRI appearances mimicking an indeterminate soft tissue mass, Skeletal Radiol (1998); 27:457-460
2. Mcclelland D, Darby A, Hay S, Rheumatoid nodule as a cause of  Morton’s metatarsalgia, Foot and Ankle Surgery (2002); 8: 111-113
3. Highton J, Hessian PA, Kean A, Chin M, Cell death by apoptosis is a feature of the rheumatoid nodule, Ann Rheum Dis (2003); 62: 77-80
4. Miyasaka N, Sato K, Yamamoto K, Goto M, Nishioka K, Immunological and Immunhistochemical analysis of Rheumatoid Nodules, Ann Rheum Dis (1989); 48, 22-226
5. Smolen JS et al., The need for prognosticators in rheumatoid arthritis. Biological and clinical markers: where are we now? Arthritis Research and Therapy (2008) 10: 208

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