The Postmortem Features of Mucormycosis

Abstract

Mucormycosis is a rare and severe invasive fungal infection caused by ubiquitous fungi of the order Mucorales. Infection often occurs in immunocompromised hosts and includes cutaneous, pulmonary, gastrointestinal, rhinocerebral, and disseminated forms of disease. Although the clinical characteristics of mucormycosis are well established, infection can be difficult to diagnose antemortem, resulting in frequent postmortem diagnoses. Despite this, the gross appearance of mucormycosis at autopsy has not been well described. In the present report we illustrate the gross and histologic findings in four autopsy cases of mucormycosis, including one case of pulmonary disease and three cases of disseminated mucormycosis with cerebral, pulmonary, hepatic, renal, and gastrointestinal involvement. In all cases autopsy examination demonstrated characteristic hemorrhagic infarcts with a targetoid appearance in the affected organs. These findings are secondary to fungal angioinvasion with subsequent thrombosis and tissue necrosis. Mucormycosis should be suspected at autopsy when these characteristic infarcts are identified within the proper clinical context, and a high suspicion for atypical infections should be maintained postmortem in immunosuppressed patients.

Introduction

Mucormycosis, formerly known as zygomycosis, is a rare life-threatening infection caused by the ubiquitous and angioinvasive fungi of the class Zygomycetes, order Mucorales. Mucorales includes the genera Rhizopus, Mucor, Apophysomyces, and Lichtheimia, among others, but does not include the Entomophthorales. Infection results from inhalation of spores or inoculation via disrupted skin or mucosa, and most commonly occurs in immunocompromised hosts, such as those with a history of hematologic or solid organ transplantation, hematologic malignancy, or neutropenia. Additional risk factors include poorly controlled diabetes mellitus, iron overload syndromes, and iron chelation therapy with deferoxamine, which is no longer commonly used (1 -3). In the United States health care system the mean incidence is 0.3 cases per 100,000 patients per year (4). Mucormycosis includes cutaneous, pulmonary, gastrointestinal, rhinocerebral, and disseminated subtypes of disease, in addition to rare forms such as peritonitis. Disseminated mucormycosis is associated with the highest mortality rate (96% to 100%), followed by gastrointestinal (85%) and pulmonary disease (76%) (3).

There are frequent descriptions and illustrations of the clinical appearance of mucormycosis in the literature. In contrast, the gross appearance of mucormycosis at autopsy is not well characterized, and there are relatively few published articles depicting the lesions postmortem (1,5 -8). We present four autopsy cases of mucormycosis (three disseminated, one pulmonary) that demonstrated characteristic hemorrhagic infarcts with a targetoid appearance in organs infected by Mucorales.

Case 1

A 32-year-old man with acute myeloid leukemia (AML) status-post allogeneic hematopoietic stem cell transplant, complicated by gastrointestinal graft-versus-host disease (GVHD), was admitted to our institution for relapsed AML. Upon admission, the patient described nonspecific symptoms including fever, chills, and vomiting. Empiric antibiotic therapy was initiated and voriconazole was continued for antifungal prophylaxis. The patient subsequently developed progressive abdominal pain and diarrhea. Computed tomography (CT) of the abdomen showed a small bowel obstruction and changes consistent with neutropenic enterocolitis. Several hypodensities concerning for microabscesses were noted in the liver. The patient later developed acute right upper quadrant pain and transaminitis. Magnetic resonance imaging (MRI) of the abdomen identified diffuse infiltrative hypointense liver lesions. The patient was transitioned to caspofungin and developed shock and multi-organ failure. He was transferred to the intensive care unit and received one dose of intravenous (IV) amphotericin B but died shortly after. An abdomen-only autopsy was requested.

External examination revealed jaundice and distension of the abdomen. Internal examination of the abdomen revealed 2300 mL of serous fluid and significant dilation of the small intestine. There was patchy petechial hemorrhage and dark red discoloration of the parietal and visceral peritoneum (Figure 1A and B). The omentum and mesentery appeared congested. The liver was soft with vaguely geographic, mottled discoloration (Figure 2B). Upon opening the colon, numerous dark red, nodular, targetoid mucosal lesions, measuring up to 3 cm in diameter, were discovered (Figure 2A and C). The cecum was filled with blood and the wall was markedly thickened and hemorrhagic (Figure 2D).

Figure 1:

Case 1. Examination of the abdominal cavity revealed patchy petechial hemorrhage and dark red discoloration of the parietal and visceral peritoneum (A and B).

Figure 2:

Case 1. Numerous well-circumscribed, dark red, targetoid mucosal lesions were present in the colon (A and C). There was also geographic mottled discoloration and softening of the liver (B). The cecum was markedly thickened and hemorrhagic (D).

Microscopic examination revealed hemorrhagic tissue necrosis and innumerable large, nonpigmented, ribbon-like, pauciseptate hyphae, morphologically consistent with a mucormycete (Figure 3A-D). Tissue- and angioinvasive hyphae were identified in the liver, gallbladder, right kidney, ileum, cecum, colon, and intra-abdominal soft tissues. The colonic lesions were determined to represent nodular infarcts. Given the limited nature of the autopsy, the extent of disease beyond the abdomen was not known, but due to the extensive abdominal involvement, disseminated disease was favored. The cause of death was disseminated mucormycosis complicating recurrent AML. Mucormycosis had not been suspected clinically.

Figure 3:

Case 1. Histologic examination of the organs revealed innumerable angioinvasive hyphae, with involvement of medium to small caliber renal arteries (A). The hyphae were large, ribbon-like, and pauciseptate, consistent with a mucormycete (B-D). (Hematoxylin-eosin, magnifications ×50 [A], ×200 [B], and ×400 [C and D]).

Case 2

A 67-year-old man with focal segmental glomerulosclerosis status-post kidney transplant, on chronic immunosuppression, was admitted to a local hospital with right middle lobe pneumonia, diabetic ketoacidosis, and acute kidney injury. He was treated and discharged but returned a few days later with new-onset anuria. Blood and urine cultures were positive for fluconazole-resistant Candida albicans. He subsequently had a complicated clinical course that included persistent pulmonary symptoms. A tissue culture of the right lower lobe of the lung demonstrated growth of a mucormycete in addition to Candida. Candida overgrowth of the specimen limited definitive fungal identification. A biopsy of the right lower lobe confirmed the presence of fungal hyphae morphologically consistent with a mucormycete. Antifungal therapy with amphotericin B was initiated but the patient’s condition progressively worsened. He ultimately developed large volume hemoptysis and experienced a cardiac arrest. A complete autopsy was requested.

Autopsy examination revealed 1400 mL of serosanguinous fluid in the right pleural cavity with evidence of hemoaspiration in the airways and lungs. There were large, hemorrhagic, and targetoid infarcts of the right lower and left upper lobes of the lungs (Figure 4). Microscopic examination confirmed the presence of innumerable angioinvasive fungal forms morphologically consistent with a mucormycete. The cause of death was pulmonary mucormycosis complicating kidney transplant for focal segmental glomerulosclerosis.

Figure 4:

Case 2. Large, hemorrhagic, targetoid infarcts were identified in the lungs.

Case 3

A 62-year-old woman had a history of myelodysplastic syndrome (refractory anemia with excess blasts-1) status-post allogeneic hematopoietic stem cell transplant. Her post-transplant course had been complicated by acute gastrointestinal GVHD, cytomegalovirus viremia, and eventual relapse. She was admitted to a local hospital with neutropenic fever in the setting of recent chemotherapy. Empiric antimicrobial therapy was initiated for a presumed central-line associated infection, and fluconazole was continued for antifungal prophylaxis. Blood cultures grew Bacillus cereus. The patient continued to experience persistent intermittent fevers and was transferred to our institution for access to IV voriconazole. Upon arrival a chest radiograph revealed consolidation of the left lung. In addition, CT of the head showed a nonspecific hypodensity in the right frontal lobe, and subsequent MRI showed acute to subacute infarcts in the right frontal and occipital lobes. There was clinical concern for cardioembolic strokes secondary to infective endocarditis, with differential considerations including an angioinvasive pulmonary infection with hematogenous spread. An echocardiogram showed no evidence of valve vegetations. The patient’s condition acutely worsened and she was transitioned to comfort care. A complete autopsy was requested.

Autopsy examination revealed no evidence of endocarditis. There was hemorrhagic consolidation of the left lung (Figure 5B). The kidneys were pale with multiple hemorrhagic and targetoid infarcts (Figure 5A and C). There was geographic yellow-red discoloration of the liver (Figure 5D). Examination of the brain showed multifocal acute to subacute cerebral infarcts. Microscopic examination revealed the presence of innumerable angioinvasive hyphae morphologically consistent with a mucormycete, with involvement of the lungs, kidneys, liver, and brain. Tissue culture of the left lung was performed but only grew incidental Candida glabrata. The cause of death was disseminated mucormycosis as a complication of chemotherapy for myelodysplastic syndrome. Although an angioinvasive infection had been considered clinically, the patient did not receive amphotericin B.

Figure 5:

Case 3. Hemorrhagic, targetoid infarcts were identified in the kidneys (A and C). There was also hemorrhagic consolidation of the left lung (B) and geographic discoloration of the liver (D).

Case 4

An adolescent boy was admitted with a new diagnosis of T-lymphoblastic leukemia after initially presenting with respiratory failure. Induction chemotherapy was initiated with a rapid reduction in circulating lymphoblasts. He subsequently developed severe pancytopenia secondary to chemotherapy. His prolonged admission was complicated by development of tumor lysis syndrome, multisystem organ failure, acute respiratory distress syndrome, and mixed bacteremia with Bacteroides fragilis and Granulicatella adiacens. In addition, he developed Candida albicans and Candida glabrata fungemia, for which he received voriconazole, fluconazole, caspofungin, and ultimately IV amphotericin B. CT of the abdomen, performed approximately one month after admission, identified thickening and pneumatosis of the small intestinal wall consistent with neutropenic enterocolitis. Multiple granulocyte transfusions were administered without improvement. His condition continued to worsen despite aggressive therapies, and he was ultimately transitioned to comfort care. A limited, chest and abdomen-only, autopsy was requested.

External examination revealed jaundice and a distended abdomen. Internal examination revealed diffuse petechial hemorrhages and dark red mottled discoloration of the pulmonary parenchyma. There were multiple pale, wedge-shaped, cavitary infarcts in the left lung (Figure 6). The peritoneal cavity contained 1800 mL of dark yellow-brown translucent fluid. Examination of the esophagus and stomach revealed ulcerated and hemorrhagic mucosal plaques. The wall of the small intestine was markedly thickened and there was diffuse mucosal hemorrhage. Microscopic examination revealed the presence of angioinvasive hyphae morphologically consistent with a mucormycete, with extensive involvement of the left lung, esophagus, stomach, small intestine, and cecum. There were also innumerable small, yellow, round nodules throughout the lungs, liver, and kidneys, secondary to disseminated candidiasis. The cause of death was disseminated mixed fungal infection (mucormycosis and candidiasis) due to neutropenia due to chemotherapy for T-lymphoblastic leukemia. Mucormycosis had not been suspected clinically.

Figure 6:

Case 4. There was diffuse petechial hemorrhage and dark red mottled discoloration of the lungs. Multiple wedge-shaped infarcts were identified.

Discussion

Autopsy has been referred to as the gold standard for diagnosis of mucormycosis given that the disease can be challenging to diagnose clinically; for example, only 25% of gastrointestinal mucormycosis is estimated to be diagnosed antemortem (2). In our series of four cases of mucormycosis, only one case was diagnosed antemortem. The frequency of mucormycosis at autopsy in the United States is unknown, although it has been reported in 0.01% to 0.16% of autopsies in Japan (9 -11). Approximately 2% of bone marrow transplant recipients have been found to have mucormycosis at autopsy (12).

The pathology visualized during autopsy examination in cases of mucormycosis is caused by the proliferation of angioinvasive hyphae, which leads to thrombosis and subsequent tissue necrosis. This process can produce a spectrum of gross pathology, ranging from nonspecific petechial hemorrhages to hemorrhagic geographic necrosis and well-circumscribed necrotic mass lesions, as illustrated by this case series. The variably targetoid “bull’s-eye” appearance of lesions is secondary to zonal, vasocentric necrosis (1,13). A mass-like or nodular infarct can form in instances of marked localized edema and/or hemorrhage.

A high degree of suspicion for atypical infections, including invasive fungal infections, should be maintained during the autopsy of an immunocompromised patient. The presence of unusual or hemorrhagic lesions should heighten this suspicion. Disseminated mucormycosis can involve any organ or tissue; in this case series, mucormycetes were identified in the lungs, liver, gallbladder, colon, small intestine, kidney, intra-abdominal soft tissue, and brain. A postmortem diagnosis of mucormycosis can be made histologically or by tissue culture. The classic histologic appearance of Mucorales is broad, pleomorphic, ribbon-like, and pauciseptate hyphae (1,14,15). Although often described as having obtuse-angle branching, the Mucorales in fact have variable-angle branching, ranging from 45° to 90°. The thin-walled hyphae are prone to twisting and distortion in tissue and on sectioning, which can create folds that have the appearance of septae (15). Additionally, Lichtheimia can have more septae than other mucormycetes. The hyphae demonstrate angioinvasion and perineural invasion and have paradoxically relatively weak Grocott methenamine silver (GMS) and Periodic acid-Schiff staining (14,15).

Tissue culture may allow for identification of a mucormycete, but requires a pretest suspicion for mucormycosis, given that special processing is necessary. If the tissue is homogenized, the pauciseptate hyphae of Mucorales may be disrupted, which prevents growth in culture (14). Therefore, a negative tissue culture does not preclude a diagnosis of mucormycosis. Unlike other invasive fungal infections such as aspergillosis, there is no antigen-based testing currently available for diagnosing Mucorales infection. Diagnostic molecular testing for mucormycosis is not currently routine or readily available; however, DNA sequencing for Mucorales may become more common in the future (16).

The main differential diagnoses for mucormycosis at autopsy are other invasive fungal infections, including, but not limited to, Aspergillus, Fusarium, and Scedosporium species. Invasive aspergillosis is the primary diagnostic consideration (14). Aspergillus is also an angioinvasive mold and can therefore produce targetoid, geographic, or nodular infarcts, like those seen in mucormycosis. Aspergillus and other non-Mucorales can be distinguished histologically based upon the presence of septate, narrow hyphae with acute angle branching (Figure 7A-D) (14,15). However, a folded or distorted Mucorales hyphal form can have pseudoseptae and a branching pattern that could be mistaken for Aspergillus. Strong GMS staining favors a diagnosis of aspergillosis. Additionally, positive antemortem serologic testing for components of the Aspergillus cell wall, ( 1,3 ) beta-d-glucan and galactomannan, favor a diagnosis of aspergillosis (14). However, the presence of ( 1,3 ) beta-d-glucan alone is not specific for Aspergillus. The narrow septate hyphae of Candida can also be mistaken for a mucormycete; however, the presence of pseudohyphae and small yeast forms argues in favor of Candida and against mucormycosis (14). Additionally, as illustrated by Case 4, candidiasis often presents as multiple yellow-white nodules at autopsy. Candida is not angioinvasive and thus does not form vasocentric, hemorrhagic infarcts.

Figure 7:

Mucorales can be distinguished from Aspergillus by the presence of large, pauciseptate, ribbon-like hyphae (A). In contrast, narrow, septate hyphae with acute angle branching are characteristic of Aspergillus (C). Mucorales may have less intense Grocott methenamine silver (GMS) positivity compared to Aspergillus (B and D). (Hematoxylin-eosin, magnification ×400 [A and C], and GMS, magnification ×400 [B and D]).

Conclusion

These cases serve as informative examples of the gross and microscopic pathology that can be seen in mucormycosis and underscore the importance of autopsy examination in the diagnosis. Mucormycosis can present with a spectrum of gross pathology including hemorrhagic, targetoid infarcts or nodules, petechial hemorrhage, or geographic tissue necrosis. It is critical for pathologists to maintain a high degree of suspicion for opportunistic fungal infections such as mucormycosis in immunocompromised patients.