Methotrexate (MTX) is a folate analog, which inhibits the enzyme dihydrofolate reductase, blocking the de novo production of purines and thymidine. Moreover, it blocks the conversion of homocysteine to methionine and S-adenosyl-methionine, an important pathway in CNS myelination. Overall, it disrupts the important role of folate in synthesis and repair of DNA and CNS myelination. This drug is highly lipophobic; thus, it reaches the CNS only when given at high intravenous (i.v.) doses (>1 g/m²) or directly into the subarachnoid space. Intrathecal (i.t.) administration is mainly used in treatment or with prophylactic purpose of leptomeningeal spread of acute leukemia (AL), mainly acute lymphoblastic leukemia (ALL), or lymphoma. Several polymorphisms involving MTX transport and metabolism have been investigated as potential markers of heterogeneous toxicity and response to MTX. The non-synonymous C677T and A1298C variants in the 5,10-methylenetetrahydrofolate reductase are among the most widely studied. However, results are conflicting, main reasons being small and heterogeneous populations and differences in protocols and criteria defining toxicity (70, 71). Prevention of MTX neurotoxicity based on leucovorin rescue has been adopted in most protocols; however, its use is limited by rescue effect exerted even on leukemic cells and its efficacy in preventing neurotoxicity is partial (20).

Methotrexate neurotoxicity may be divided into acute, subacute, and delayed forms, either transient or chronic. i.t. administration is associated with the development of acute chemical meningitis in about 5–40% of patients, usually starting few hours after treatment and lasting up to 3 days. This usually consists of headache, stiff neck, fever, nausea, vomiting, and lethargy, generally self-limited. It is more common with no concomitant cranial irradiation. Adhesive arachnoiditis is the most severe form, which results in scarred tissue compressing nerve roots and their blood supply (20, 21).

Acute–subacute encephalopathy typically arises within few days to few weeks after i.t. or i.v. MTX administration. It consists of stroke-like episodes with transient neurologic symptoms such as hemiparesis, speech impairment, dysphagia, diplopia, hemisensory deficits, and sometimes seizure followed by complete recovery in a few days. Patients may be successfully rechallenged, but toxicity may recur. It is thought to be related to acute neuronal swelling caused by excessive stimulation of NMDA receptors by the high levels of homocysteine in cerebrospinal fluid (CSF). Dextromethorphan is a non-competitive antagonist of NMDA receptors and appears to be a promising agent in ameliorating symptoms and fastening recovery. DWI appears to be the most sensitive technique, revealing transient restricted diffusion, compatible with cytotoxic edema (27–31). In some patients, after repeated courses of MTX, radiologic evidence of leukoencephalopathy (LE) may be present, with white matter hyperintensity on T2-weighted and FLAIR MRI, as shown in Figure 2. of note, LE during active therapy also develops in about 20% of asymptomatic patients. These findings often persist at the end of therapy (22, 32, 33). Transverse myelopathy is another subacute complication arising few days to several months after i.t. MTX. Back or leg pain followed by paraplegia and sensory loss, flaccid paresis, and fecal and urinary incontinence/retention are typical features. This seems related to a non-inflammatory vacuolar demyelination and necrosis of the spinal cord, starting from the surface and progressing centrally. Demyelination is most prominent in the posterior funiculus, but it can also involve both the lateral and anterior funiculi. Elevation of protein level in CSF also occurs. T2-weighted MRI shows signal hyperintensity of the lateral and dorsal columns, with enhancement when contrast is used. Clinical course is often rapidly progressive, and recovery is often only partial (24, 25).

Methotrexate chronic LE is the major delayed complication following repeated courses of i.t. or i.v. MTX, but most commonly occurs with the combination of the two and radiotherapy. It may develop after months or even years from treatment. White matter damage leads to changes in mental status usually with preserved language. Main features are progressive cognitive impairment, loss of memory and concentration, changes in personality, dementia, seizures, paresis, and incontinence. Clinical course is variable but may be severe, also leading to coma and death. Neuroimaging shows white matter hyperintensity with possible temporary focal enhancement, usually involving periventricular areas. Ventriculomegaly with concomitant cortical atrophy is also seen (34, 35).

Fludarabine is a purine analog, mainly used in ALs and non-Hondgkin lymphoma, and in reduced intensity conditioning regimens prior to HSCT. Fludarabine-induced LE is peculiar because of its delayed onset (21–60 days from exposure), which can lead to difficulties in diagnosis. Pathophysiology is unclear but is probably due to direct cytotoxicity on axons and oligodendrocytes. Neurotoxicity appears to be strictly dose related, varying from very high incidence (36%) with high doses (>96 mg/m²/day × 5 days) to <1% with standard low doses (25 mg/m²/day × 5 days). Association of low doses of fludarabine with cytarabine is linked to a higher risk (3.6%) of neurotoxicity, particularly peripheral neuropathy. Clinical manifestations of LE include peripheral neuropathy, motor weakness, paralysis, pyramidal tract dysfunction, altered mental status, hallucinations, seizure, or extremely severe forms with several deficiencies worsening progressively to coma and death in a few weeks or months. Clinical course is characterized by progressive degeneration with short survival (median of 2.2 months). Resolution is rare and, when occurs, several dysfunctions are usually left over. No prophylaxis or treatment is known to date. A classic feature is visual disturbances resulting from cortical blindness, visual pathway demyelination, and/or retinal damage. Pathology reveals axonal and myelin damage in the periventricular matter of the brain and spinal cord, with vacuolization and macrophage infiltration. MRI abnormalities are relatively mild compared with clinical severity and usually do not show spinal cord involvement. T2 and FLAIR sequences show mildly hyperintense lesions in the periventricular cerebral white matter, with restricted diffusion but no enhancement. Sequential MRI images reveal increasing size and intensity of the lesions (36, 37). PML caused by JC virus after standard-dose fludarabine is another possible complication. Only few cases have been described. Lymphopenia lasting up to a year after therapy increases the risk for opportunistic infections, which seem a plausible cause for PML. The resulting syndrome is similar to other causes of LE (see Brentuximab Vedotin and Rituximab), with a rapid worsening course conducting to death. Up to date, no treatment is available (38, 39).

Cytarabine (Ara-C) is a nucleotide analog frequently used in the treatment of leukemia and lymphoma, especially in case of CNS involvement and brain tumors. When given i.v., it is fast eliminated by hepatic cytidine deaminase, but this enzyme is not present in the CNS, so cytotoxic levels in CNS persist longer when given i.t. or i.v. at high doses (>3 g/m²). Liposomal Ara-C is a slow release formulation, which allows maintenance of therapeutically effective concentrations up to 40 times longer than standard Ara-C, with a better response rate. However, it has to be administered in association with corticosteroid in order to reduce the development of arachnoiditis. Neurological side effects of i.t. liposomal Ara-C are similar to that of i.t. MTX. Despite prophylactic treatment with dexamethasone, chemical meningitis with nausea, fever, vomiting, and back pain may develop in 20% of patients. More severe complications are spinal cord lesions (cauda equina syndrome, paraplegia, or tetraplegia) and papilledema, occurring at a median of 10 days (1–22, 24–59, 62, 63, 70–100) after a variable number of courses. For these, recovery is often incomplete with severe neurologic deficits after many months and no effective therapy available. Risk is particularly high when i.t. liposomal Ara-C is administered in association with i.v. high dose of MTX and/or Ara-C or in combination with i.t. MTX and steroids (“triple therapy”). MRI may show contrast enhancement of lateral columns and cauda equina but may also be non-informative. Finally, altered mental status and seizure are further possible complication of i.t. liposomal cytarabine (42–46, 72). Acute cerebellar toxicity is a typical complication of high dose i.v. cytarabine due to direct damage of Purkinje cells. Cerebellar signs such as nystagmus, ataxia, dysarthria, and oculomotor impairment, rarely with confusion and drowsiness (40). Symptoms are usually mild and reversible with in two weeks of discontinuing the drug but in some cases, permanent impairment has been described. CT and MRI images are usually normal. The most important risk factors, besides cumulative dose (>36 g/m²), are age (>50 years) and renal dysfunction (41).

Blinatumomab is a bispecific T-cell-engaging antibody which allows the interaction between normal CD3+ T cells and CD19+ acute lymphocytic leukemia cells, leading to a lysis of the tumor cells. It is usually administered as continuous i.v. infusion over 4 weeks. Clinical trials have proven significant response rate in refractory or relapsed B-ALL. Neurotoxicity is rather common, reaching 52% (13% of grade 3 or 4 neurologic events), but it is usually mild and reversible. Moreover, usually it can be managed with dexamethasone treatment without infusion discontinuation. CNS toxicity may consist of headache, somnolence, confusion, aphasia, syncope, seizure, or cerebellar symptoms such as ataxia or tremor with a median time of onset of 7 days. The underlying mechanisms remain unclear (47–49, 73).

Progressive multifocal leukoencephalopathy is a rare, often fatal, demyelinating disease of the CNS resulting from infection of oligodendrocytes by JC polyoma virus (JCV). This is a complication associated with immunosuppressive condition, such as lymphoproliferative disorders, autoimmune diseases, human immunodeficiency virus infection or, as described in the last decade, treatment with monoclonal antibodies (mAbs) such as brentuximab vedotin and rituximab. Brentuximab vedotin is a CD30-specific antibody drug conjugated with the microtubule inhibitor monomethyl auristatin E employed in treatment of relapsed-refractory Hodgkin lymphoma and anaplastic large cell lymphoma. Rituximab is a chimeric anti-CD20 mAb approved for CD20-positive B cell non-Hodgkin’s lymphoma and chronic lymphocytic leukemia. Pathogenesis of PML is uncertain, probably related to reactivation of the virus in a setting of altered immune surveillance: while brentuximab causes CD30+ activated T-cells depletion, rituximab does not result in T-lymphocyte depletion and PML may result from expansion of pre-B-lymphocytes harboring latent JC virus after rituximab mediated B-lymphocyte depletion (50, 51). Clinical presentations include aphasia, hemiparesis, hemianopsia, memory loss, gait dysfunction, and confusion with a rapid and progressive worsening leading to death over a period of a couple of months. Time of onset is relatively short in brentuximab-related PML (days to weeks after the administration of two to six doses) compared to rituximab-related PML (median of 16 months after median of six doses). Case fatality rate is extremely high (80% for brentuximab, 90% for rituximab), with death in few months from presentation (52–55). Many foci of demyelination are seen as increasingly confluent white matter abnormalities on MRI, especially in the subcortical areas of cerebral hemispheres. JCV DNA is often not detectable in CSF. There is no standardized approach to PML. Discontinuation of the immunosuppressive agent or cytarabine is often adopted, although benefit is rarely observed. An immune reconstitution inflammatory syndrome characterized by rapid infiltrates of T-cells may develop. This can not only be life saving, due to the reconstitution of the immune system, but also lethal for excessive inflammation (55).

Ifosfamide is an alkylating agent mainly used in treatment of relapsed ALL, lymphoma, sarcoma, and other solid tumors. CNS toxicity is frequently observed (10–40%) with high dose treatment. It happens more frequently after oral than after i.v. administration, or, when given i.v., it occurs more frequently with a short infusion time. Risk factors to develop ifosfamide-induced encephalopathy (IIE) appear to be related to altered renal function, hepatic metabolism, and, as recently reported, to specific ifosfamide formulation (74). The more widely accepted explanation is toxicity of one or more of the ifosfamide metabolites, particularly chloroacetaldehyde. This crosses the blood–brain barrier and exerts direct neurotoxic effect, depletes CNS glutathione, and inhibits mitochondrial oxidative phosphorylation. IIE ranges from mild confusion (the most frequent symptom) or somnolence to more severe forms with memory loss, disorientation, hallucinations, seizure, delirium, or, rarely, even coma and death. Cerebellar and cranial nerve dysfunction or other movement disorders have rarely been described (56, 57). Symptoms occur during infusion or after few hours, followed by complete recovery in 20–72 h. Additional treatment generally is not necessary but i.v. methylene blue, 50 mg every 4–8 h, has been used. However, it is unclear if clinical symptoms resolved independently of treatment. There seems to be little advantage in using methylene blue during subsequent infusions of ifosfamide, mainly consisting of shorter duration and lower grade of toxicity but not in reducing the risk of recurrence. CT and MRI imaging, when performed, do not reveal any abnormalities (58, 59).

Busulfan is an alkylating agent, largely used in conditioning regimens prior to HSCT. High doses (>16 mg/kg) are a good alternative to total body irradiation in myeloablating regimens, while lower doses (3.2–6.4 mg/kg) are used in combination with melphalan and fludarabine in reduced intensity conditioning regimes. Neurotoxicity is a well-known side effect, mainly consisting of generalized seizure in the third or fourth day of administration without any sequela. This complication appeared to be dose dependent and related to the high CSF concentrations of this small, lipophilic molecule during systemic administration. Anticonvulsant prophylaxis is now considered the standard of care in pediatrics, although the real utility is not fully understood (62, 75).

The predominant fungal pathogen is Aspergillus spp., with Aspergillus fumigatus prevailing over other species. Candida spp., and Mucorales, may less frequently be encountered (83). In the past decade, annual incidence of invasive aspergillosis was 0.4% of hospitalized immunocompromised children (84). Although CNS aspergillosis has traditionally been associated with a dismal prognosis, during the last 25 years, there has been a drastic mortality reduction from more than 80% to less than a half. This decrease may be explained by CNS aspergillosis earlier diagnosis as well as the development of amphotericin B lipid formulations and the introduction of azoles (85–87). Prognosis correlates primarily with immune status, with allo-HSCT patients having sixfold greater odds of death (88). Aspergillar fungal hyphae have a characteristic angiotropism: they form broad hyphae that invade medium to large arterial and vein vessels, leading to acute infarction and hemorrhage. Afterward they extend into surrounding tissue inducing an infectious cerebritis or progressing into an abscess. Rarely, CNS aspergillosis presents with overt meningitis or cause granuloma (89). The clinical picture is usually characterized by fever, seizures, mental status alteration, visual deficits, and focal neurologic signs (89).

Prompt diagnosis is important given the high morbidity and mortality associated with this infection, but many children with suspected invasive mold infections do not have an identified pathogen, and are treated empirically. Although culture from a sterile site remain the gold standard for diagnosis and provide antifungal susceptibilities information, invasive procedures are needed and the diagnostic yield is low. In addition, unfortunately Aspergillus is rarely recovered from blood cultures (90). Thus in many patients the diagnosis relies on indirect findings including fungal markers (galactomannan antigen and 1,3-β-d-glucan) and radiology. Typical neuroimaging features are shown in Figure 3 and include ring-enhancing lesions, with a central T2 hypointensity area, associated with hemorrhagic foci and peculiar intracavitary projection at the diffusion study, representing fungal hypae (91). The degree of immunocompetence of the host is tightly connected with the level of contrast enhancement of the lesion. As Aspergillus has a peculiar “philia” for perforating arteries, thalami, basal ganglia, and corpus callosum, as well as subcortical regions represent usual sites of involvement. Due to its angioinvasive nature, arterial infarction (lacunar and territorial), vasculitis, and mycotic aneurysm can be encountered (92). Candidiasis has a different appearance, vasculitis, hemorrhage, and thrombotic infarction are less commonly encountered, and it comprises multiple ring-enhancing micro-abscesses at the cortico/medullary junction, cerebellum, and basal ganglia (93).

Viral infection caused by herpesviridae (herpes simplex virus, Epstein–Barr virus, varicella zoster virus, cytomegalovirus—CMV, and HHV-6) and polyomaviridae (JC polyoma virus—JCV) families are more frequently encountered in allo-HSCT recipients (94). Viral PCR assays on CSF samples have high sensitivity and specificity, thus regarded as a gold standard for diagnosis (95).

HHV-6 primary infection may be asymptomatic or may cause roseola infantum (sixth disease), a febrile exanthema. The virus achieves latency in the salivary gland, white blood cells, and the brain. The clinical presentation of HHV-6 encephalitis is characterized by temporal lobe seizures, fever, and mental status changes that include insomnia. Diagnosis is reliably established by PCR analysis for HHV-6 in CSF, and immunohistochemistry is used to confirm the presence of HHV-6B protein antigens. Prompt therapy with ganciclovir or foscarnet significantly improves outcome (96). Neuroimaging studies may be normal in the early stage of symptomatic infection. The most commonly reported MRI pattern consists of generally confluent lesions, involving the mesial temporal lobes and limbic system, that are hyperintense on T2-weighted images. The lesions usually do not enhance in T1-weighted images (97). Unusual but devastating form of primary HHV-6 infection reported in infants is characterized by acute necrotizing encephalopathy with symmetric involvement of the basal ganglia, thalami, and brainstem (98). Diffusion restriction may affect the mesial temporal lobe and the basal ganglia (98).

Cytomegalovirus, also termed human herpesvirus-5, is one of the most ubiquitous members of the Herpesviridae family. An estimated 90% of the population is infected worldwide. Endogenous reactivation may occur in transplanted patients. CMV primary infection in a seronegative host after receiving a CMV-seropositive transfusion or HSCT represents the most adverse presentation (99, 100). The virus can affect both the CNS and the peripheral nervous system. Chorioretinitis and myelitis–radiculitis, followed by encephalitis and ventriculitis are the most common forms of neurologic involvement. The virus can affect both the CNS and the peripheral nervous system. Chorioretinitis and myelitis–radiculitis, followed by encephalitis and ventriculitis are the most common forms of neurologic involvement. CMV ventriculitis is characterized by thin, smooth ependymal enhancement with associated debris within the ventricles. Despite not specific, retinal involvement or polyradicular or ependymal enhancement should raise suspicion of CMV infection (101). Ganciclovir (and its orally available prodrug, valganciclovir) and foscarnet are the most effective first-line agent for CMV treatment after HSCT.

JCV is a neurotropic polyomavirus whose only known reservoir is humans. It is the causative agents of PML, a demyelinating disease of the CNS with focal or multifocal neurologic deficits due to virus reactivation (102). JCV seroprevalence rises from about 16% in children 1–5 years of age to 34% by age 21–50 years, which may explain why PML is relatively uncommon among children (103). PML has been discussed in more details in section dedicated to brentuximab vedotin and rituximab toxicity.

Toxoplasma gondii is an intracellular protozoan that is found worldwide and represents the pathogenetic agent of CNS Toxoplasmosis. The mortality rate is extremely high (60–90%), despite specific therapy (104). Clinical manifestations may span from fulminant, disseminated infection to disease confined to the brain. In patients receiving HSCT, the incidence of toxoplasmosis has been reported up to 6% (105). The vast majority of cases are reactivation of a latent infection in patients receiving allo-HSCT, while is rare after autologous HSCT. Cord blood transplantation may be a risk factor (106). MRI is characterized by ring-enhancing lesions—often multiple, but solitary in 30% of the cases—involving more commonly thalami and basal ganglia. An eccentric target sign has been described on post-contrast images, which presents high specificity (95%) and a rather low sensitivity (30%) (107). MR spectroscopy reveals elevated lipid/lactate peak. CNS lymphoma represents the main differential diagnosis in patients suspected of having CNS toxoplasmosis. CNS Lymphoma represents the main differential diagnosis in patients suspected of having CNS toxoplasmosis. Positron emission tomography with fluoro-deoxy-glucose commonly demonstrates absent glucose consumption in Toxoplasma lesion, characteristic of its non-malignant behavior, thus differentiating it from lymphoma (108). Pyrimethamine and sulfadiazine are usually given for treatment of cerebral toxoplasmosis and the occurrence can be significantly reduced by the use of trimethoprim–sulfamethoxazole prophylaxis.

Intracranial hemorrhage is the most frequent and severe hemorrhagic complication in patients with AL, representing one of the main causes of mortality of these patients. The highest incidence is observed in acute myeloid leukemia, especially the promyelocytic subtype (111, 112). Often it occurs at the onset or during induction therapy as a consequence of the disease itself, generally associated with thrombocytopenia and coagulopathy. Among described risk factors are prolonged prothrombin time, female gender, thrombocytopenia, and above all hyperleukocytosis. Also, asparaginase (ASP) treatment has been associated with increasing risk of ICH in some reports (113). ICH is more often localized in the supratentorium but could also involve basal ganglion, cerebellum, and brainstem, often with multiple sites. Cortical hemorrhage is the most frequent with not only parietal involvement as typical site but also subarachnoid hemorrhage, subdural hemorrhage, and epidural hemorrhage can occur (113). Brainstem, subarachnoid, and epidural hemorrhages are strongly related to early death. A CT scan of a patient with multiple hemorrhagic foci is shown in Figure 4. Seizures, focal neurological deficits and impaired mental status are major clinical manifestations depending on cerebral area of stroke. Prompt CT scan at the onset of signs/symptoms could be very useful in the diagnosis and management of ICH, allowing a rapid neurosurgical approach if needed. In AL, controlled decrease in the circulating leukemic blast cells and correction of coagulopathy should be appropriate treatment to avoid this complication (110).

Incidence of symptomatic deep venous thrombosis in pediatric oncology patients is described between 7 and 14%, while asymptomatic ones are estimated in over 40% of cases. Different risk factors are reported including age (<2 years or >10 years), non 0-blood group, presence of central venous cine, immobility, infections, obesity, underlying thrombophilia, corticosteroids, and ASP therapy. The thrombosis can occur both in central and peripheral veins, occasionally involving the arterial system. Cerebral sinus venous represents the main CNS localization (Figure 5), accounting for about 8% of all thrombotic events (114). As regard ALL, the Prophylactic Antithrombin Replacement in Kids with ALL treated with Asparaginase study reported a prevalence of asymptomatic thrombosis of 36.7% as opposed to only 5% of symptomatic events (115). Other studies confirm that incidence of symptomatic thrombotic events is about 5%, with CNS and upper extremity being the most more frequent districts involved (69, 116, 117). Clinical manifestations of cerebral sinus venous thrombosis is altered mental status, headache, vomiting, and diplopia. Seizures and other focal neurological deficits could also occur. In particular, the use of ASP in patients with ALL is associated with a risk of thrombosis. Usually, this complication occurs during induction phase. An imbalance of the pro- and anticoagulating systems with reduction in fibrinogen, V, VII, VIII, and IX factors but also antithrombin III (AT), protein S, protein C, plasminogen, and alpha-2-antiplasmin has been reported in patients treated with this drug (118). So far, although recent paper highlighted a lower thrombin generation profile with pegylated Escherichia coli asparaginase (PEG-ASP) than with native ASP, no difference has been demonstrated in influencing thrombotic complications in clinical trials between the different ASP formulations, as native Escherichia coli-ASP and PEG-ASP. Few data are available about adverse effects of Erwinia ASP, but no significant differences with E. Coli ASP profile were found (119–121). Concomitant use of corticosteroids and ASP has been investigated as a worsening factor for thrombotic events but no agreement was achieved (122).

Several strategies have been investigated to reduce the risk of developing thrombotic complications during treatment with ASP with controversial results. Prevention therapy with fresh frozen plasma or cryoprecipitate replacement was proposed without successful results (123). Prophylaxis using low molecular weight heparin (LMWH) was also examined with positive effects reported in a non-randomized study of 41 pediatric patients with ALL (124). However, additional study did not prove the same results (125, 126). Trials with preventive supplementation of AT have been trying with promising outcomes but consistent data are lacking (127). Prophylaxis with combination of LMWH and AT replacement also appears as a valid therapeutic strategy (128). As concern, management of thrombosis with LMWH is usually the first choice, even if unfractionated heparin is more appropriate in patients at a high risk of hemorrhage or with renal failure. Monitoring of AT levels and replacement when needed should be useful to maintain heparin anticoagulation. Efficacy and safety of new oral drugs such as rivaroxaban or dabigatran could be evaluated, although their irreversibility is a potential contraindication in this kind of patients.