What is the most likely diagnosis for a patient presenting with multiple painful, swollen nodules on the abdomen?

Multiple Painful Abdominal Nodules: Diagnostic Approach

The most likely diagnosis for multiple painful, swollen nodules on the abdomen is piloleiomyoma (benign smooth muscle tumors arising from arrector pili muscles), though gastrointestinal stromal tumor (GIST), nodular scleroderma, and inflammatory conditions must be systematically excluded through imaging and biopsy.

Initial Diagnostic Workup

Immediate Imaging

• Obtain ultrasound of the abdomen as the initial imaging modality to determine whether nodules are superficial (dermal/subcutaneous) or deep (intra-abdominal) 1
• If nodules appear intra-abdominal or ultrasound is inconclusive, proceed immediately to CT abdomen and pelvis with IV contrast, which is the standard for evaluating abdominal masses and can differentiate between superficial versus deep pathology 1

Critical Distinction: Superficial vs. Deep Nodules

If nodules are superficial (skin/subcutaneous):

• Piloleiomyoma presents as multiple painful nodules on trunk and extremities, characteristically tender to palpation, and represents benign smooth muscle tumors 2
• Nodular scleroderma (keloidal scleroderma) presents as indurated, exophytic nodules on chest and abdomen, often with sclerodactyly; occurs predominantly in African American females 3, 4
• Pyoderma gangrenosum initially appears as erythematous papules or pustules that rapidly progress to deep ulcerations; however, this typically ulcerates rather than remaining as intact nodules 1

If nodules are deep (intra-abdominal):

• GIST characteristically presents as large (often >10 cm), mobile abdominal masses with pain; approximately 70% show spindle cell morphology 5
• For masses ≥2 cm, multiple core needle biopsies are the standard approach (via ultrasound or CT guidance) to obtain tissue diagnosis before definitive surgery 1, 5

Definitive Diagnostic Algorithm

For Superficial Nodules:

1. Perform punch or excisional biopsy of representative nodule to obtain histopathology 1, 3, 4
2. Order complete blood count, ANA, anti-Scl-70 if scleroderma suspected (63% ANA positive in nodular scleroderma) 4
3. Histology showing smooth muscle bundles confirms piloleiomyoma; dermal sclerosis with collagen deposition confirms nodular scleroderma 2, 3

For Deep/Intra-abdominal Nodules:

1. CT abdomen/pelvis with IV contrast to characterize size, location, and relationship to bowel 1
2. Ultrasound-guided or CT-guided core needle biopsy (minimum 1.0 mL tissue) for masses requiring potential multivisceral resection 1, 5
3. Immunohistochemistry is mandatory: CD117 (positive in 95% of GIST), DOG1 (if CD117 negative), CD34 (positive in 70%) 1, 5
4. Mutational analysis for KIT and PDGFRA genes strongly recommended for all confirmed GIST, as this has predictive value for imatinib sensitivity 1

Management Based on Diagnosis

If Piloleiomyoma (Superficial):

• Surgical excision of symptomatic nodules is the definitive treatment 2
• Calcium channel blockers or alpha-blockers may reduce pain in multiple lesions not amenable to complete excision 2

If GIST (Deep):

• For masses >2 cm: surgical resection with negative margins is standard treatment 5
• If easily resectable, proceed directly to complete surgical resection; if requiring multivisceral resection, consider neoadjuvant imatinib to downstage (requires confirmation of imatinib-sensitive mutations first) 5
• Critical: avoid tumor rupture during surgery, as intra-abdominal spillage dramatically increases recurrence risk 5
• Surgery should be performed by surgeons trained in sarcoma management 5

If Nodular Scleroderma:

• Methotrexate is a reasonable initial treatment option for progressive disease, resulting in reduced firmness and prevention of new lesions 3
• Systemic corticosteroids may be considered for inflammatory component 3, 4

Critical Pitfalls to Avoid

• Never assume superficial nodules are benign without biopsy confirmation, as nodular scleroderma can have systemic involvement including pulmonary fibrosis (28% of cases) 4
• Do not perform surgical debridement if pyoderma gangrenosum is suspected, as pathergy phenomenon can worsen lesions 1
• For suspected GIST, avoid Bouin fixation of biopsy specimens, as this prevents molecular analysis required for treatment planning 1
• Do not delay imaging in patients with palpable abdominal masses, as larger GISTs (>10 cm) have higher malignant potential and require prompt surgical evaluation 5
• Avoid FNA if core biopsy is feasible for deep masses, as core biopsy provides superior tissue architecture for definitive diagnosis 1, 5

Specific Features Favoring Each Diagnosis

Piloleiomyoma: Multiple small (typically <2 cm) tender nodules, superficial location, pain triggered by cold or pressure 2

GIST: Single large (>10 cm) mobile mass, deep location, spindle cell morphology on biopsy, CD117/DOG1 positivity 5

Nodular scleroderma: Indurated keloidal nodules, sclerodactyly present, predominantly African American females, ANA positive 3, 4