Mucormycosis is the second most frequent invasive mold disease after aspergillosis [1–3], with rising incidence reported in some countries [4–7]. The term mucormycosis refers to invasive disease caused by filamentous fungi belonging to the order Mucorales. Patients at risk are those with poorly controlled diabetes mellitus, immunosuppressed patients such as those undergoing treatment for hematological cancer or recipients of solid organ and hematopoietic stem cell transplantation, and persons who have sustained severe trauma to soft tissues, often with direct inoculation of organic matter. Importantly, the clinical presentation of mucormycosis is shaped by the immune characteristics of the host. Thus, patients with diabetes mellitus usually develop rhino-orbital-cerebral mucormycosis (ROCM), patients with hematological neoplasms tend to develop sino-pulmonary disease, and trauma patients present with necrotizing skin and soft tissue infections [8,9]. In all anatomical locations, relentless tissue invasion and infarction secondary to angioinvasion are the hallmarks of mucormycosis [10–13].

Involvement of the central nervous system (CNS) represents one of the most severe manifestations of mucormycosis, and often determines the survival and functional outcome of the patient [8,9,14].

Epidemiology

Current estimates of the frequency of central nervous system (CNS) mucormycosis are largely based on published case reports and small series [8], which are prone to publication bias. Population-based surveys provide estimates of the overall incidence of mucormycosis, ranging from 0.43 per 1 million population in Spain [15], to 1.73 per million population in the San Francisco Bay area [16]. Incidence rates in France increased from 0.7 per million in 1997 to 1.2 per million in 2006 [4]. Traditionally, autopsy studies have been the gold standard for assessing the organ involvement of invasive mycoses [5,6]. However, autopsy rates have dropped steadily over the past three decades [5,6]. Moreover, autopsy-based studies on organ involvement in invasive mycoses do not provide information on brain involvement, as autopsies frequently exclude the brain or require separate consent from the family for brain inspection [5,17].

The frequency of CNS mucormycosis is highly dependent on the population studied (Table 1) [8,18–20]. In areas where diabetes mellitus is the predominant risk factor, for example on the Indian subcontinent, sinus disease predominates and the CNS is the third most common site of infection after the paranasal sinuses and orbit [14]. CNS infection is relatively less frequent in case series dominated by patients with hematologic malignancies [7,21], but cancer has nevertheless become the most significant risk factor for CNS mucormycosis.

Table 1. Patterns of Central Nervous System (CNS) mucormycosis in different patient populations.

Diabetes Mellitus

Patients with diabetes mellitus are at risk for rhino-orbital-cerebral mucormycosis (ROCM), specifically in the context of uncontrolled glycemia and diabetic ketoacidosis (DKA) [8]. Of 337 mucormycosis patients with diabetes mellitus reviewed by Roden et al. [8], 66% had disease of the paranasal sinuses, and most of these (43% of the total) had cerebral extension [8]. Similarly, rhinocerebral disease was strongly associated with an underlying diagnosis of diabetes mellitus (11 of 21 patients, 52%) in a European registry of 230 patients with mucormycosis [18], and in the French RetroZygo cohort (16 of 23 patients, 70%) [9].

Unlike the situation in developed countries, where malignancy has emerged as the main risk factor for mucormycosis (see below), uncontrolled diabetes mellitus continues to be the dominant driver of disease in developing nations [22–24]. Moreover, in developing countries mucormycosis is frequently reported as the presenting event of diabetes mellitus [23]. For example, in a report from India, as many as 23% of patients with mucormycosis had previously undiagnosed diabetes mellitus [23].

Malignancy

Malignancy, particularly hematological, has replaced diabetes mellitus as the most frequent risk factor associated with mucormycosis of all kinds in developed countries [8,9,25,26]. This epidemiological trend is likely explained by the increasing number of patients receiving intensive chemotherapy, declining rates of uncontrolled diabetes mellitus [1,4,9,25,27–30], and possibly other treatment-related factors [31]. Referral bias favoring reports from tertiary-level medical centers caring for hematological malignancy patients over community hospitals may also partially explain this trend. In a review of published reports from the 1940s to 1990s, cancer patients accounted for 17% (154/929) of mucormycosis cases [8]. In contrast, patients with hematological cancer accounted for 50% of cases (50/101) in the more contemporary French RetroZygo cohort (2005–2007) [9]. Marr et al. described a two-fold increase in the incidence of mucormycosis in stem cell transplant recipients from 1985 to 1999 [30]. Remarkably, in a recent multicenter analysis of 46 patients with mucormycosis, 42 (92%) had hematological malignancies [25].

Mucormycosis of the CNS seems to be less common among patients with malignancy and recipients of stem cell transplantation, as compared to those with diabetes mellitus [8]. Only sporadic cases were reported in a series of CNS mycoses in cancer patients  published in 1994 [21]. This may be due to the propensity of patients with neoplasms for pulmonary mucormycosis, and the relative rarity of rhino-orbital disease [8]. In the review by Roden et al. [8], CNS involvement was noted in 4% of patients with cancer and 11% of stem cell transplantation recipients. Contemporary data suggests that the frequency of CNS mucormycosis has increased [12,32]. In one review of CNS mycoses, Mucorales accounted for 22% of CNS fungal infections, second only to Aspergillus spp. [32]. Pagano et al. identified CNS involvement in 11 of 59 patients (19%) with hematologic malignancy and mucormycosis [19]. In a retrospective study performed at a tertiary cancer center, 35% of culture-positive brain mycoses were attributed to Mucorales [12]. Interestingly, Muggeo et al. [33] retrospectively reviewed pediatric cancer patients with mucormycosis, and found rhino-orbital disease in the majority of cases (14/15; 93%) and CNS involvement in 53% (8/15). These findings suggest age-related differences in the pathobiology of cancer-related mucormycosis, and imply that a high index of suspicion of cerebral extension should be maintained, especially in children.

The Bruton tyrosine kinase (Btk) inhibitor ibrutinib, used in the treatment of mantle cell lymphoma and chronic lymphocytic leukemia, has been associated with invasive fungal diseases with frequent involvement of the CNS. Specifically, intracerebral invasive aspergillosis and, to a lesser extent, cryptococcosis and Pneumocystis jirovecii pneumonia were reported [34–39]. Few cases of mucormycosis have been described so far in patients receiving ibrutinib [40–42]. Of 33 patients with invasive fungal disease (IFD) following treatment with ibrutinib reported by Ghez et al. [37], only one had mucormycosis; the site of involvement was not reported. No cases were reported among 16 patients with ibrutinib-related IFD described by Verughese et al. [43]. The role of Btk inhibitors in antifungal immunity is probably dependent on the interplay among multiple disease- and treatment-related factors [36]. Thus, the contribution of Btk inhibitors to the risk of mucormycosis is presently poorly understood.

Trauma

Mucormycosis after trauma typically involves soft tissues of the limbs, and less frequently the abdominal or thoracic wall, head and neck [20]. Dissemination to the CNS is rare, even in cases related to head trauma. Reviewing 122 cases of post-traumatic mucormycosis reported in the literature, Lelievre et al. found only one case of dissemination to the brain [20]. There were no cases of CNS infection among 16 patients with post-traumatic mucormycosis in the RetroZygo cohort [20]. Cerebral mucormycosis due to direct inoculation of the brain as a result of penetrating head trauma or neurosurgery is extremely rare [44–46]. In one reported case, a diabetic woman sustained blunt injury to the forehead triggering sino-orbital mucormycosis with cavernous sinus invasion [46].

Injection Drug Use

Injectable drug use is associated with a unique form of isolated cerebral mucormycosis [8], presumably due to hematogenous spread of sporangiospores present in contaminated drugs and drug injection paraphernalia. Overall, 67% of intravenous drug injectors with mucormycosis have CNS involvement, with the majority of those (97%) having isolated cerebral disease [8]. Kerezoudis et al. found 68 reported cases that met their definition for isolated cerebral mucormycosis [47]. The majority of these patients (82%) had a history of intravenous drug abuse and 20% were infected with HIV.

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