Leptomeningeal Metastases

Leptomeningeal metastases (LM) represent a severe and often terminal complication of cancer, marked by high morbidity and mortality. It can arise at any point during the cancer course—either as an initial symptom or, more commonly, as a manifestation of disease relapse.

LM involves the spread of cancer cells into the subarachnoid space of the central nervous system (CNS), where they multiply. This invasion can result from several mechanisms: hematogenous spread, direct perineural extension (commonly seen in head and neck or gastric cancers), or “drop metastases” descending from a primary tumor located higher up in the neuraxis.

The leptomeninges, composed of the arachnoid and pia mater, enclose the cerebrospinal fluid (CSF). Tumor cells entering the CSF—via direct invasion (as in brain tumors) or through the bloodstream (as seen in leukemia) are dispersed throughout the CNS by CSF circulation. This results in either widespread or patchy infiltration of the leptomeninges. The condition is referred to as leptomeningeal carcinomatosis when caused by solid tumors and as lymphomatous or leukemic meningitis when hematologic cancers are involved. Despite the term “meningitis,” an inflammatory response is not always present.

Originally described by Eberth in 1870, LM has remained underrecognized but is now diagnosed more frequently due to advancements in imaging (notably MRI), improved treatment options, and greater clinical awareness. It presents in diverse pathological forms—sometimes alongside CNS metastases or intraventricular involvement—leading to varied clinical features depending on the pattern and extent of spread.

LM is identified in approximately 1% to 8% of individuals with cancer. Among patients who exhibit neurological symptoms, LM is found in nearly 19% of autopsy cases, particularly in those with advanced or widespread disease. The prevalence varies by cancer type: LM affects 1–5% of patients with solid tumors, 5–15% with leukemia, and 1–2% with primary brain tumors. In some cases (5–10%), LM may be the initial sign of malignancy, although its true incidence is likely underreported due to challenges in detecting multifocal spread or sampling unaffected CNS regions during autopsy.

Adenocarcinomas are the leading cause of LM, although any systemic cancer can potentially spread to the leptomeninges. Small-cell lung cancer has a leptomeningeal involvement rate between 9–25%, melanoma around 23%, and breast cancer roughly 5%. Due to the high global incidence of breast cancer, it remains the most common primary source of LM, followed by lung cancer.

Rare malignancies like embryonal rhabdomyosarcoma and retinoblastoma also demonstrate a tendency to involve the leptomeninges, while sarcomas generally do not. In head and neck cancers, particularly squamous cell carcinoma, spread to the meninges may occur along cranial nerves. In pediatric populations, LM is uncommon but is a known complication of medulloblastoma.

The likelihood of LM increases with longer cancer duration, and it is frequently accompanied by other CNS metastases—seen in 98% of non-leukemic patients with LM. As cancer treatments improve and patient survival increases, the incidence of LM appears to be rising. Some tumors, such as ALK-positive non-small cell lung cancers treated with crizotinib, have a propensity to relapse in the central nervous system, making the CNS a potential sanctuary site for specific cancer subtypes.

LM develop when cancer cells infiltrate the leptomeninges and cerebrospinal fluid (CSF), leading to widespread dissemination throughout the central nervous system. This spread can occur through several proposed pathways, including hematogenous dissemination to the choroid plexus or directly through leptomeningeal vessels, retrograde flow via the Batson venous plexus, extension along perineural lymphatics, centripetal spread from axial lymph nodes through intervertebral or cranial foramina, or direct invasion from adjacent tumors. Once malignant cells enter the CSF, they are distributed by its flow, with accumulation often seen in regions such as the basilar cisterns, dorsal spinal cord, and cauda equina. LM is often accompanied by parenchymal or dural metastases. The resulting clinical manifestations are primarily due to obstruction of CSF pathways, direct tumor infiltration, metabolic disturbances in neural tissue, or vascular occlusion. These pathological changes can lead to increased intracranial pressure, hydrocephalus, cranial nerve palsies, radiculopathies, seizures, encephalopathy, focal neurological deficits, and, in some cases, infarcts due to compromised blood flow in the subarachnoid space.

It is also referred to as carcinomatous meningitis or leptomeningeal disease, arise when malignant cells infiltrate the leptomeninges—the pia mater and arachnoid layers—and spread through the cerebrospinal fluid (CSF). This condition typically occurs as a late manifestation of systemic malignancies, most frequently associated with breast cancer, lung cancer, and melanoma. The spread of tumor cells to the leptomeninges can occur through several pathways. One common route is hematogenous dissemination, where cancer cells travel via the bloodstream and invade the leptomeningeal space. Another mechanism involves direct extension, in which tumors situated near the brain or spinal cord grow into the meningeal layers. Additionally, cancer cells may enter the CSF through shedding from the primary tumor and then disseminate widely throughout the central nervous system via CSF flow. These mechanisms collectively contribute to the development of LM in patients with advanced cancer.

The prognosis for LM is generally poor, with survival heavily influenced by the primary cancer type and disease extent within and beyond the CNS. Without treatment, survival is just 4–6 weeks, but therapeutic interventions may extend it to 2–4 months. Leukemic and lymphomatous meningitis respond better to agents like methotrexate and cytarabine, whereas solid tumors show variable outcomes. Among them, breast cancer-associated LM has the best response, with 60% stabilizing or improving and median survival of 7 months; 15% may survive over a year.

In contrast, LM from small-cell lung cancer and melanoma have worse outcomes, with median survival of 4 months and 3.6 months, respectively. Prognosis is poorer in patients with low Karnofsky scores, severe neurologic symptoms, extensive systemic disease, impaired CSF flow, or bulky CNS lesions. A Karnofsky score ≥70 is linked to significantly longer survival (313 days vs. 36 days for scores ≤60). Early treatment response within two weeks suggests a better outcome, while rapid deterioration and extensive CNS involvement predict poor survival. A study of 135 older patients found that poor performance status, early LM onset post-cancer diagnosis, and primary lung cancer or melanoma were associated with shorter survival, while systemic chemotherapy was the key factor improving life expectancy.