Pathology of Acute Febrile Neutrophilic Dermatosis (Sweet's Syndrome)

Sweet's syndrome was first described as "acute febrile neutrophilic dermatosis" by Dr. Robert Douglas Sweet in 1964.

Classical Sweet's syndrome is characterized by high fever, elevated neutrophil count, tender erythematous skin lesions (papules, nodules, and plaques), and a diffuse infiltrate consisting predominantly of mature neutrophils typically located in the upper dermis.

These are usually located on the face, extremities and rarely on the trunk.

Sweet's syndrome may be associated with malignancy. Cases have been reported with leukaemia, specially acute myeloid leukaemia and testicular tumour.

Drug-induced Sweet's syndrome was described in 1986. The lesion was caused by trimethoprim-sulfamethoxazole.

Microscopic features of Acute Febrile Neutrophilic Dermatosis (Sweet's syndrome ):

1. Diffuse infiltrate of mature neutrophils.

2. Dense polymorphonuclear cell infiltrate in the upper dermis may be acccompanied with prominent edema of the papillary dermis.

3. Infiltrate may extend throughout the dermis and into the subcutaneous tissue

4. Leukcytoclasis (fragmented neutrophil nuclei) with formation of nuclear dust.

5. Dilated small blood vessels and swollen endothelial cells.

6. In cases with hematologic malignancy-associated Sweet's syndrome, atypical leukemic cells (leukemia cutis) are noted.

7. Eosinophils have been observed within the dermal infiltrate in some cases with either the classical or the drug-induced dermatosis.

8. Lymphocytes or histiocytes may also be present in the inflammatory infiltrate.

9. In some cases , perivascular neutrophilic infiltrate is present showing pathologic changes consistent with leukocytoclastic vasculitis.

10. Subcutaneous Sweet's syndrome lesions may have pathologic changes in the adipose tissue that can be found in either the lobules, the septae, or both.

Differential diagnosis of Sweet's syndrome includes conditions microscopically characterized by either neutrophilic dermatosis or neutrophilic panniculitis.

Rule out following condition:

Neutrophilic dermatoses - Abscess or cellulitis, bowel (intestinal) bypass syndrome, Erythema Elevatum Diutinum, granuloma faciale, leukocytoclastic vasculitis, neutrophilic eccrine hidradenitis, pyoderma gangrenosum, and rheumatoid neutrophilic dermatitis.

Neutrophilic lobular and/or septal panniculitis - Alpha 1-antitrypsin deficiency, infection, leukocytoclastic vasculitis, pancreatitis (Pancreatic Panniculitis), and rheumatoid arthritis.

Reference: Sweet's syndrome--a comprehensive review of an acute febrile neutrophilic dermatosis.