According to the aetiology, we classified the acute conditions involving CNS in adult Patients affected by haematologic malignancies into three main categories; therefore we have analyzed the imaging findings of cases belonging to each of these groups.

MASS EFFECT

Focal or generalized neurological disorders like headache, motor or sensory impairment, cranial nerves palsies, changes in mental status, seizures and coma, may acutely arise as a clinical manifestation of tumoral growth within the CNS or in the surrounding anatomical structures.

The most frequent hematological entities which can involve CNS by the effect of solid neoplastic tissue are listed below [2].

Primary CNS Lymphoma

It represents about 5% of all extra-nodal Non-Hodgkin lymphomas (mainly diffuse large B-cell lymphoma and high-grade Burkitt-like B-cell lymphoma) and appears as solitary or multiple lesions with a predilection for the periventricular and superficial white matter in the supratentorial brain ( Fig. 1 ).

Secondary CNS Lymphoma

It is more common than the primary form, typically originating from a systemic diffusion of Non-Hodgkin lymphomas. It may involve the brain tissue, with single or multiple enhancing lesions, or the meninges, showing dural, leptomeningeal, subependymal or cranial nerve enhancement (Fig. 2 , Fig. 3 and Fig. 4).

Angiocentric Intravascular Lymphoma

In this rare form of CNS Lymphoma in which there is an angiotropic growth and intravascular proliferation of malignant cells often causing multifocal ischemic alterations.

Plasm Cell Neoplasms

Multiple myeloma is the most common primary malignant bone neoplasm in adults, characterized by proliferation of monoclonal plasma cells in the bone marrow producing lytic lesions and other systemic manifestations. Among the most common sites of involvement are the skull (Fig. 5 and Fig. 6) and the vertebrae (Fig. 7), often leading to pathologic fractures. Compression or infiltration of the CNS structures can cause acute sensory-motor impairment at various levels.

Plasmacytoma has similar pathological features to multiple myeloma but presents as a solitary tumor and lacks a systemic involvement (Fig. 8).

VASCULAR

Onco-hematological patients are exposed to a higher risk of developing cerebro-vascular acute conditions due to their often-associated state of impaired coagulation. Hypercoagulability state or bleeding diathesis can be consequent to the alteration of coagulation factors by interference with production, accelerated break down, and side effects of chemotherapeutic agents.

The main types of the said conditions are the following.

Hemorrhage

Intra-axial or extra-axial hemorrhage is favored by shortage of normal coagulation factors and/or platelets (Fig. 9) given by systemic involvement of different forms of hematological malignancies (Fig. 10). Acute Promyelocytic Leukemia, among the others, classically causes thrombocytopenia and disseminated intravascular coagulopathy, with a high risk of intracerebral hemorrhage as a major cause of death. Moreover, drugs like Ibrutinib used for Chronic Lymphocytic Leukemia can also cause thrombocytopenia as a side effect, concurring to the hemorrhagic complications (Fig. 11).

Cerebral infarction

Less common than hemorrhagic events, it can be caused by intravascular coagulation, tumor emboli, septic emboli, and artery compression/infiltration by tumor cells.

Drugs like L-asparaginase, used to treat acute lymphoblastic leukemia, acute myeloid leukemia, and non-Hodgkin's lymphoma is known to produce numerous hemostatic defects potentially leading to cerebro-vascular accidents [3] (Fig. 12).

Cerebral venous thrombosis

Prothrombotic hematological conditions like thrombocythemia, leukostasis, clotting factor deficiencies and chemotherapeutic effect (particularly L-asparaginase) may lead to occlusion of venous channels in the cranial cavity, including dural venous thrombosis, cortical vein thrombosis and deep cerebral vein thrombosis [3] (Fig. 12 and Fig. 13). Hemorrhagic or ischemic evolution of the adjacent brain structures may also be present accordingly. Frequent neurological sign and symptoms consist in headaches, altered conscious state, papilledema, focal neurological deficits and seizures [4].

IMMUNE-RELATED

Immunodeficiency often occurs in patients affected by hematological malignancies, as a consequence of systemic involvement by the pathologic entities and the side effects of antineoplastic or immunosuppressive drugs [5, 6]. Immunocompromised patients have a higher risk of developing CNS infections and disorders like immune reconstitution inflammatory syndrome, causing a worsening of the clinical conditions, in some cases with an acute onset of neurological signs and symptoms.

CNS infections

Aspergillus Spp and Toxoplasma Gondii are the predominant causative agents of CNS infections in immunocompromised patients, majorly in that undergoing allogeneic hematopoietic stem-cell transplantation. Aspergillus infections, in particular, are associated with bone destruction, vascular invasion, and rapid intracranial spread, causing meningitis, brain abscess (Fig. 14), ischemic or hemorrhagic infarction.

JC virus-related progressive multifocal leukencephalopathy (PML)  is a rare but frequently fatal CNS disease that typically affects severely immunocompromised hosts.

Bacterial CNS infections are rarely diagnosed in patients with hematological disorders, more frequently occurring after neurosurgical interventions.

Immune reconstitution inflammatory syndrome (IRIS)

IRIS is a paradoxical worsening of the symptoms of a previously acquired opportunistic infection, such as JC-related PML (Fig. 15), caused by restoration of immunity. The immune system abruptly responds to the infection itself, producing an excessive inflammatory state [7, 8].