Mucor
Summary
Mucor is a genus of fast-growing, ubiquitous saprophytic molds belonging to the order Mucorales, causing mucormycosis, an angioinvasive fungal infection primarily affecting immunocompromised hosts (uncontrolled diabetics with DKA, neutropenic patients, iron overload states). Characterized histologically by broad, non-septate (pauciseptate) hyphae branching at wide (~90°) angles, distinguishing it from Aspergillus (septate, 45° branching).
Detail
Mucor and related genera (Rhizopus, Absidia, Lichtheimia) cause mucormycosis, a rapidly progressive, angioinvasive infection with a high mortality rate. Risk factors include diabetic ketoacidosis (classic association), hematologic malignancies, neutropenia, immunosuppressive therapy, corticosteroid use, deferoxamine therapy (iron overload — Mucorales can utilize free iron), and organ/stem cell transplantation.
Pathophysiology: Spores are inhaled or inoculated into tissue, germinate into hyphae that invade blood vessels, causing thrombosis, infarction, and tissue necrosis. This angioinvasion leads to the classic black, necrotic eschar seen in rhinocerebral disease.
Clinical syndromes: - Rhinocerebral mucormycosis: most common, presents with facial pain, headache, sinusitis, orbital cellulitis, cranial nerve palsies, and black necrotic eschar on palate or nasal turbinates; can extend intracranially causing cavernous sinus thrombosis. - Pulmonary mucormycosis: seen in neutropenic/leukemic patients, presents with fever, cough, hemoptysis, and rapidly progressive infiltrates. - Cutaneous, gastrointestinal, and disseminated forms also occur.
Histopathology: Broad, ribbon-like, non-septate hyphae with irregular right-angle (90°) branching, best seen on H&E, GMS, or PAS stains. This contrasts with Aspergillus, which has septate hyphae branching at acute (45°) angles.
Diagnosis: Tissue biopsy with histopathology and culture; imaging (CT/MRI) showing sinus or orbital involvement; rapid diagnosis is critical due to fulminant course.
Treatment: Requires aggressive surgical debridement of necrotic tissue combined with high-dose IV amphotericin B (liposomal formulation preferred to reduce nephrotoxicity); posaconazole or isavuconazole may be used as step-down or salvage therapy. Reversal of underlying risk factors (e.g., correcting DKA, discontinuing deferoxamine) is essential.
Prognosis is poor without early recognition and aggressive intervention, especially in immunocompromised or diabetic ketoacidosis patients.
Sources
- First Aid for the USMLE Step 1
- Harrison's Principles of Internal Medicine
- Murray's Medical Microbiology
- UpToDate: Mucormycosis
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