In our patient, clues of NM included fulfillment of criteria for systemic inflammatory response syndrome at admission and a presumed source of infection; taken together, these findings should lead to a diagnosis of sepsis until otherwise proven. The patient also reported pain that was not proportional to the skin findings, which suggested an NSTI. His lack of constitutional symptoms may have been due to the effects of prednisone, which was changed to dupilumab during hospitalization.
The clinical and histological findings of NM are nonspecific. Clinical findings include skin discoloration with bullae, blisters, vesicles, or petechiae.4 Our case adds to the descriptive morphology by including marked edema with ulceration, progressive purulence, and interconnected sinuses tracking to the fascial plane. Histologic findings can include confluent necrosis extending from the epidermis to the underlying muscle with dense neutrophilic inflammation. Notably, these findings can mirror necrotizing neutrophilic dermatoses in the absence of an infectious cause. Failure to recognize simple systemic inflammatory response syndrome criteria in NM patients due to slow treatment response or incorrect treatment can can lead to loss of a limb or death.
Workup reveals overlap with necrotizing neutrophilic dermatoses including PG, which is the prototypical neutrophilic dermatosis. Morphologically, PG presents as an ulcer with a purple and undermined border, often having developed from an initial papule, vesicle, or pustule. A neutrophilic infiltrate of the ulcer edge is the major criterion required to diagnose PG6; minor criteria include a positive pathergy test, history of inflammatory arthritis or inflammatory bowel disease, and exclusion of infection.6 When compared directly to an NSTI such as NM, the most important variable that sets PG apart is the absence of bacterial growth on blood and tissue cultures.7
Imaging studies can aid in the clinical diagnosis of NM and help distinguish the disease from PG. Computed tomography and MRI may demonstrate hallmarks of extensive necrotizing infection, such as gas formation and consequent fascial swelling, thickening and edema of involved muscle, and subfascial fluid collection.3,4 Distinct from NM, imaging findings in PG are more subtle, suggesting cellulitic inflammation with edema.8 A defining radiographic feature of NM can be foci of gas within muscle or fascia, though absence of this finding does not exclude NM.1,4
In conclusion, NM is a rare intramuscular infection that can be difficult to diagnose due to its nonspecific presentation and lack of constitutional symptoms. Dermatologists should maintain a high level of suspicion for NM in the setting of rapidly progressive clinical findings; accurate diagnosis requires a multimodal approach with complete correlation of clinical, histological, and imaging findings. Computed tomography and MRI can heighten the approach, even when necrotizing neutrophilic dermatoses and NM have similar clinical and histological appearances. Once a diagnosis of NM is established, prompt surgical and medical intervention improves the prognosis.