There are various diseases that can cause LM involvement, which can be categorized into subgroups based on the pathogenesis of the involvement:

Infectious:

Viral: Viruses can cause meningitis and/or encephalitis, with clinical and imaging findings resembling acute pyogenic meningitis but typically milder (1). Imaging is often unremarkable but may reveal subtle, thin linear LM enhancement, with parenchymal enhancement present in cases of associated encephalitis (2) (Fig.1).

Bacterial: Bacterial meningitis is a life-threatening neurological emergency that demands rapid diagnosis and treatment. MRI findings may include thin linear leptomeningeal enhancement, similar to viral meningitis, with additional features depending on the pathogen. Definitive diagnosis is established by identifying the pathogen in CSF (3) (Fig.2).

Fungal: Fungal infections of the CNS primarily affect immunocompromised individuals. They can cause meningitis, cerebritis, abscesses, cryptococcoma, and meningeal vasculitis, often progressing rapidly with overlapping symptoms. Less commonly, it may manifest as granulomas, hydrocephalus, miliary nodules, plexitis, ventriculitis, or pachymeningeal enhancement (4) (Fig.3).

Tuberculosis: Tuberculous meningitis is predominantly caused by the spread of Mycobacterium tuberculosis. The most common finding is thick, nodular leptomeningeal enhancement around the basal cisterns, and progressive hydrocephalus, vasculitis, infarction, and cranial neuropathies. Additionally, concurrent occurrences of tuberculomas, cerebritis, abscesses, rhombencephalitis, and myelitis are possible (5) (Fig. 4).

Inflammatory-Autoimmune:

Neurosarcoidosis: Neurosarcoidosis with leptomeningeal involvement being a common manifestation affecting about 40% of cases. This typically presents as thickening and diffuse or nodular enhancement of the leptomeninges, with a preference for the basilar meninges (6). Other manifestations can include myelopathy, sellar and parenchymal involvement, and small-fiber neuropathy (7) (Fig. 5).

Neuromyelitis optica spectrum disorder (NMOSD): NMOSD is a relapsing autoimmune inflammatory condition of the CNS, frequently associated with blood-brain barrier disruption and leptomeningeal involvement. T1-weighted contrast-enhanced imaging and FLAIR post-contrast sequences demonstrates linear or extensive leptomeningeal and pial enhancement, often involving both cerebral hemispheres, particularly medial surfaces of the cerebral hemispheres (8) (Fig. 6).

Rheumatoid meningitis (RA): CNS involvement of RA is rare. On imaging unilateral or bilateral pachymeningeal and/or leptomeningeal involvement can be observed, predominantly in the frontal (83.3%) and parietal (78.4%) lobes. Lesions may be focal, asymmetric, and may include fluid collections in the meninges, with or without diffusion restriction (9, 10) (Fig. 7).

Cerebral amyloid angiopathy–related inflammation (CAA-RI): CAA-RI is characterized by autoimmune inflammation surrounding vessels or nearby the vessels, caused by the accumulation of amyloid-β. MRI typically shows asymmetric white matter hyperintensities on T2-weighted FLAIR imaging with enhancement in the parenchyma or subarachnoid space (71%), along with the presence of multiple cerebral microbleeds (11) (Fig. 8).

Familial amyloid polyneuropathy (FAP): FAP is a genetic disorder characterized by amyloid fibril deposition in organs and peripheral nerves, most commonly due to transthyretin (ATTR) amyloidosis. Amyloid accumulation often affects the leptomeninges, detectable on MRI as signal changes and contrast enhancement in structures such as the sylvian fissures, cerebral sulci, cisterns, brainstem, cerebellum, spinal cord, and cranial nerves (12) (Fig. 9).

Primary Neoplastic:

Diffuse leptomeningeal glioneuronal tumor (DLGT): DLGT is a pediatric condition characterized by widespread dissemination along the leptomeningeal and CNS surfaces. Imaging typically reveals thick, nodular leptomeningeal enhancement accompanied by subpial cyst formation. Spinal cord involvement is common, often manifesting as peripheral subpial cysts with an intramedullary solid-cystic tumor component (13) (Fig. 10).

Leptomeningeal melanomatosis (LM): LM is a highly rare and aggressive malignant condition characterized by diffuse infiltration of the subarachnoid space by melanoma cells. Imaging features, including nodular lesions disseminating along the cranial and spinal leptomeninges, with T1 hyperintensity, T2 hypointensity, and contrast enhancement, are suggestive of diffuse tumoral infiltration (14) (Fig. 11).

Secondary Neoplastic:

Seeding of primary CNS tumors:

Glioblastoma (GBM): GBM, the most common and aggressive primary malignant brain tumor, often presents with LM spread (LMS). MRI typically shows LMS as linear or nodular lesions in the subarachnoid and subependymal spaces. LMS frequently involves the anterior brainstem and cranial nerves (15) (Fig 12).

Medulloblastoma: Medulloblastoma, a neuroepithelial malignancy of primitive neuroectodermal origin, is the most common malignant brain tumor in children. In approximately one-third of cases at diagnosis, dissemination into the subarachnoid space occurs, presenting as diffuse LM enhancement (16). Although typically associated with a primary tumor mass, cases of isolated LM enhancement have also been documented (17) (Fig. 13).

Pilomyxoid astrocytoma (PMA): PMA is a rare CNS tumor classified as a histologic variant of pilocytic astrocytoma (PA), designated as a grade 2 neoplasm. It represents a more aggressive form of PA, often associated with CSF dissemination, and is most commonly diagnosed in children under the age of two. PMAs typically present as well-defined, bulky space-occupying lesions predominantly located in the suprasellar region, including the hypothalamus and optic chiasm. According to one study, leptomeningeal spread is observed in approximately 20% of cases (18) (Fig. 14).

Non-CNS tumors:

Lymphoma: LM involvement in lymphoma may present as a late complication of systemic lymphoma (6% to 8%) or concurrently with primary CNS lymphoma (extremely rare). MRI findings often include multiple brain masses, cranial or spinal nerve involvement, intramedullary spinal cord lesions, and leptomeningeal enhancement, with the spinal cord and nerve roots being the most commonly affected sites (19) (Fig. 15).

Leptomeningeal carcinomatosis: Leptomeningeal carcinomatosis is a critical diagnosis, as untreated malignant leptomeningeal metastases significantly reduce survival to less than three months. Imaging features of leptomeningeal metastases vary and may include pial lesions, which can present as either diffuse or focal masses, as well as cisternal lesions attached to the dura or cranial nerves (20) (Fig. 16).

Vascular:

Susac’s syndrome: Susac’s syndrome, or retinocochleocerebral vasculopathy, is a rare disorder characterized by acute or subacute encephalopathy, bilateral sensorineural hearing loss, and branch retinal artery occlusions. MRI typically reveals pathognomonic "snowball" lesions in the corpus callosum, indicating multifocal microinfarctions, with additional involvement of white matter, gray matter, cerebellum, and thalamus. Leptomeningeal enhancement, especially in the cerebellum, may also be observed (21) (Fig. 17).

Moyamoya disease: Moyamoya disease is an idiopathic cerebrovascular disorder involving bilateral steno-occlusive changes at the terminal internal carotid arteries, with abnormal vascular proliferation at the brain's base. In pediatric patients with neurofibromatosis type 1 (NF1), it is termed Moyamoya syndrome. Prominent leptomeningeal collaterals cause diffuse leptomeningeal enhancement on contrast-enhanced T1-weighted imaging, while FLAIR MRI often shows the "ivy sign," indicating slow flow along cerebral sulci and the brain surface (22) (Fig. 18).

Posterior reversible encephalopathy syndrome (PRES): PRES is a clinical and radiological entity frequently linked to hypertension, eclampsia, or immunosuppressive therapy, although it can have diverse etiologies. LM enhancement on MRI has been reported in up to 38% of cases, but its clinical relevance remains unclear. This phenomenon is believed to be associated with blood-brain barrier disruption (23) (Fig. 19).

Reversible cerebral vasoconstriction syndrome (RCVS): RCVS is characterized by sudden, severe headache and reversible segmental cerebral artery vasoconstriction. T2/FLAIR imaging may show sulcal hyperintensities indicative of convexity subarachnoid hemorrhage or the hyperintense vessel sign. Post-contrast T2/FLAIR images highlights blood-brain barrier disruption and sulcal enhancement. MRA can detect vascular narrowing, and DWI may reveal watershed infarcts (24) (Fig. 20).

Hyperintense acute reperfusion marker (HARM): HARM is identified as delayed enhancement of the subarachnoid or subpial space on post-contrast FLAIR imaging. It is associated with blood-brain barrier permeability changes in acute stroke and serves as a reperfusion marker in various brain disorders, including infarction (25) (Fig. 21).

Meningioangiomatosis (MA): MA is a rare benign meningovascular malformation or hamartomatous lesion affecting the leptomeninges and cerebral cortex, occurring sporadically or with NF2. It likely arises from cortical vascular malformations inducing meningothelial proliferation in Virchow-Robin spaces, potentially triggered by chronic leptomeningeal stimulation. MA typically presents as a solitary temporal or frontal lesion, causing seizures or headaches, and may extend into the leptomeninges along abnormal vessels (26) (Fig. 22).

Sturge-Weber syndrome (SWS): SWS is a neurocutaneous disorder marked by angiomatosis of the skin, eyes, and meninges. MRI is the diagnostic tool of choice, with post-gadolinium pial enhancement recognized as the gold standard for evaluating disease extent and regarded by some as the key criterion for radiologic diagnosis. This enhancement usually affects the occipital and posterior parietal cortex on the side corresponding to the port-wine stain (27) (Fig. 23).