Introduction

The multifocal dissemination of cancer cells from its primary site to subarachnoid, pia mater, and CSF in the brain and spinal cord is referred to as carcinomatous meningitis (CM). It is also termed as ‘leptomeningeal meningitis,’ ‘leptomeningeal carcinomatosis,’ ‘leptomeningeal metastasis,’ or ‘neoplastic meningitis.’ Beerman, in 1912, used the term ‘carcinomatous meningitis’ for a condition in which cancer cells metastasized to the meninges without involving brain parenchyma.[1] 

CM can occur in the advanced stage of many malignancies when cancer cells seed through CSF and deposit in the meninges. Involvement is usually multifocal and can cause a wide variety of symptoms. CSF analysis, along with imaging techniques, aid in the diagnosis. Treatment options include radiotherapy and chemotherapy. However, early diagnosis is challenging, and prognosis generally remains dismal. The development of newer techniques for diagnosis and novel therapeutic strategies may improve prognosis and limit morbidity.

Etiology

Primary cancers that metastasize to the leptomeninges are intracranial or extracranial. The most common extracranial malignancies include breast cancer, lung cancer, and melanoma. Intracranial malignancies include cerebellar medulloblastoma, malignant ependymomas, neuromas, and gliomas.

Epidemiology

The incidence of leptomeningeal metastasis from solid tumors is 5% to 8%, and from hematological malignancies is 5 to 15%.[2] Among hematological malignancies, B-cell lymphoma is the most common etiology. However, the incidence may be as high as 20%, given the instances of undiagnosed cases, which are only reported at autopsies.[3]

The most common solid organ tumors causing carcinomatous meningitis are lung cancer, melanoma, and breast cancer, with a varying incidence reported in several studies.[4][5] Parenchymal brain malignancies causing carcinomatous meningitis include gliomas, ependymomas, and medulloblastomas. CM is usually associated with underlying parenchymal involvement in about 60% of the cases.[5] In 1 to 7% of the cases, the primary site of the disease cannot be located despite extensive evaluation.[6]

Pathophysiology

Cancer is an abnormal and uncontrolled proliferation of cells at a certain location that has the potential to spread to a distant site. These malignant cells develop vascularity, which in turn increases its ability to proliferate. About 14.1 million cases of cancers are registered every year, out of which about 8.8 million are eventually fatal, making cancer one of the leading causes of death.[7][8] Though the preferential metastatic site is not definite, certain cancers are more likely to metastasize to specific locations.

The central nervous system (CNS) has three layers of meninges called the dura mater, the arachnoid, and the pia mater. The dura mater is known as the patchy-meningeal layer. The arachnoid and pia meningeal layers are called the leptomeninges. The cerebrospinal fluid (CSF) lies in the space between the arachnoid and pia mater, known as the subarachnoid space.

The CSF is the main route for the malignant cells to gain access to the leptomeninges. However, to do so, the cancer cells must cross the blood-brain barrier (BBB). The BBB is an anatomical structure that consists of brain microvascular endothelial cells (BMECs), which form tight intercellular junctions with astrocyte foot processes and regulate transport from blood to CSF. It is hypothesized that tumor cells bind to endothelial cells via receptors and confer changes in the endothelium. This exposes the vascular basement membrane to tumor cells, which then binds to specific components and enters the CSF. Certain genomic functions mediate this whole process of extravasation of cancer cells. This metastatic potential is acquired over time. The Massague’ group suggested that overlapped genes like COX-2, EGFR ligand, ANGPTL4, and LTBP1 are responsible for causing disruption of BBB and tumor proliferation in different parts of the brain.[9]

Refer to figure 1:[9] Schematic presentation of tumor cell penetration across BBB.

Routes of Spread

The routes of spread to BBB from primary tumor site includes:

1. Hematogenous spread
2. Direct invasion from primary tumors in the brain parenchyma and meninges
3. Choroid plexus metastasis into CSF
4. Epidural, subdural, and vertebral metastasis
5. Perineural and endoneurial spread from the brain and spinal cord in a retrograde manner
6. Iatrogenic spread via neurosurgical procedures

CSF is known to be rich in high amounts of oxygen and glucose, enough to support the high metabolic demands of the rapidly multiplying malignant cells. Therefore, tumor proliferation in leptomeninges is not dependent upon angiogenesis, a limiting factor for tumors in other locations, including brain parenchymal metastasis.[10]

Risk Factors

The major risk factors associated with CM are:

1. Incomplete surgical resection of brain metastasis
2. The Omission of postoperative radiation therapy
3. Presence of parenchymal brain metastasis in young (age <40 years)
4. Advanced systemic disease

Neurosurgical resection is the preferred treatment option for solitary brain metastasis. Many studies indicate that the risk of CM is 2.81 times more with piecemeal resection than in en-bloc resection.[11]

Due to the selective permeability of BBB, conventional systemic therapy may not be effective in brain metastasis. Radiotherapy (RT) is a feasible option for radiosensitive tumors, especially if the primary site is unknown. Adjuvant RT is recommended after neurosurgical resection. Whole-brain radiation therapy (WBRT) reduces the risk of recurrence but is associated with increased adverse effects, and therefore stereotactic radiosurgery (SRS) is increasingly becoming the preferred option. Studies have shown that the risk of CM after prior neurosurgical resection is 6.5 times higher than SRS without previous surgery.[12]

Location of brain metastasis and tumor volume has not been found to increase the possibility of occurrence of CM. It is the chance of CSF exposure for the spilling of the tumor cells that is rather significant.[13] In patients receiving SRS for brain metastases, certain independent factors that influence the risk of CM are the type of primary tumor, number of intracranial metastases, young age, and distant brain failure (defined as development of new metastatic lesion outside of the treated radiation field).[14]

History and Physical

Clinical features of carcinomatous meningitis are pleomorphic because of the extensive involvement of neuraxis. Different pathophysiological mechanisms include:

1. Increased intracranial pressure (ICP) caused by obstruction to CSF flow; if this obstruction occurs at convexities, communicating hydrocephalus emanates; if the blockage occurs at the ventricles, non-communicating hydrocephalus ensues.
2. Disruption of the blood-brain barrier causing cerebral edema.
3. Direct tumor cell invasion into cranial and spinal nerves from subarachnoid space.
4. Malignant cells invade the brain parenchyma, usually via Virchow-Robin spaces, which are pia matter lined perivascular spaces in different regions of the brain and utilize the oxygen and glucose required for neuronal cell growth and functioning. This competition can cause widespread depression of metabolism in the cerebral cortex, causing encephalopathy.[10] The tumor burden in the cerebral vessels of the spaces can also cause stroke-like symptoms in patients with CM.
5. The coexistence of leptomeningeal metastasis with brain parenchymal metastatic lesions present with focal signs of intraparenchymal disease.

The most common presenting symptom is a headache and is found in about 39% of patients.[15][16] Headache is caused by either meningeal irritation or by raised ICP. In meningeal irritation, headaches are associated with nuchal rigidity worsened by leg flexion (Kernig sign). In raised ICP, headaches are accompanied by nausea, which is worse in the morning. Confusion is the second most common presentation and occurs because of the metabolic depression of the cortex by active malignant cells. This results in an altered state of consciousness or cognitive impairment. It can also cause temporal lobe seizures presenting as deja vu, stereotypical movements, euphoria, hallucinations, and amnesia.

Posterior fossa involvement causes both cerebellar signs and cranial neuropathies. These are observed in 65% of the cases and incorporate nausea, vomiting, dizziness, ataxia, and diplopia.[16][17] Cranial neuropathies present as diplopia, visual loss, facial weakness, hearing loss, balance disturbances, dysphagia, dysarthria, and hoarseness. Among cranial nerves, CN 6, 7, and 8 are more frequently impacted.

CM may also involve the spinal cord and its exiting nerve roots. The spinal nerve roots that are affected produce symptoms in the anatomically associated regions. Symptoms may include segmental numbness, dysesthesia, pain, and lower motor neuron pattern limb weakness. The involvement of sacral nerve roots may cause bowel and bladder dysfunction. Cauda equina and conus medullaris syndromes can be the only presentation in CM patients. Leg weakness (28%) and back pain/paralysis (18%) are the initial presenting symptoms of leptomeningeal metastases of the spinal cord.[18]

CM can be asymptomatic in 2% of the patients.[16]

Evaluation

The first and foremost step in the evaluation of carcinomatous meningitis is obtaining a comprehensive history and physical exam. An accurate neurological exam helps determine whether the involvement is focal or multifocal and leads the clinician to suspect leptomeningeal involvement. CSF evaluation and neuroimaging study is the next step in the diagnosis. A pragmatic approach for both diagnosis and prognosis may require testing of tumor markers as well.

Cerebral Spinal Fluid

CSF findings typically include the following:

1. High CSF pressure (greater than 25 cmH2O) is observed in about 50% of CM patients.[19]
2. Pleocytosis is detected in 33-79% of CM.[20] White blood cells are mostly lymphocytes, but eosinophils are also identified in lymphomas and leukemia. The presence of RBCs or xanthochromia in CM may develop due to a traumatic tap.
3. The protein level in CSF is elevated in about 80% of the cases.[21] The normal range of protein in CSF is between 15-45 mg/dl, and elevated values are due to proteins produced by the tumor cells or breakdown of the blood-brain barrier.
4. The glucose level in CSF in CM are decreased in about 25-40% of the cases and is called hypoglycorrhachia.[19] Normal glucose levels in CSF range between 50-80 mg/dl. Hypoglycorrhachia occurs because of the metabolism of tumor cells or defective transport mechanisms. This is also seen in infective meningitis, and cytology is necessary to differentiate between the two in such cases.
5. Initial CSF cytology is positive in 55% of the cases.[22] Repeat CSF cytology is done if the first sample is negative and if clinical suspicion remains high. This increases the sensitivity to 80%.[23] Moreover, higher volumes of CSF (>10 ml) also improves yield. The sensitivity of CSF cytology for the diagnosis of CM ranges from 80 to 95%, but the specificity is very high.

The standardization of the method for CSF collection is an indispensable requirement. An appropriate site (producing symptoms preferably) and sufficient volume (minimum 10 ml) are the prerequisites for the spinal tap. Analysis of CSF via lumbar puncture (LP) and CSF tested from an indwelling catheter produces varying results.

Although CSF cytology is the gold standard for diagnosis, immunohistochemical staining of the cells is recommended, particularly if the primary site of the tumor remains unknown. Flow cytometry is a rapid, quantifiable measure of a specific type of cell with certain characteristics such as cancer cells that are associated with specific antigens. It provides a highly sensitive method for the detection of hematological malignancies. FISH (fluorescent in situ hybridization) is a cytogenetic technique for localizing particular DNA sequences or genes and may yield information on metastatic lesions but with a lower sensitivity.[2] PCR (polymerase chain reaction) is used to identify immunoglobulin gene arrangements in malignant cells, specifically for lymphoma. Hence the study of cytological, morphological, molecular, and cytogenetic characteristics is vital for identification of the tumor type and, at times, the prognosis.[24]

Tumor markers have a role in making an early diagnosis and monitoring response to treatment. However, it may also be imperative in determining the prognosis. The nonspecific markers include LDH (lactate dehydrogenase), beta-2 microglobulin, and specific markers include AFP (alpha-fetoprotein), BHCG (beta-human chorionic gonadotropin), and CEA (carcinoembryonic antigen). CSF value of more than 1% of the serum level of these markers is diagnostic.[2] A study done at Memorial Sloan-Kettering Cancer Center demonstrated that patients with CM from breast cancer, lung cancer, and melanoma have abnormal levels of at least one tumor marker in CSF in 74-90% of the cases.[25] CEA is helpful in adenocarcinoma of the lung, AFP in germ cell tumors, and Beta-2 microglobulin in hematological malignancies. The pro-angiogenic factor VEGF (vascular endothelial growth factor) in CSF also has a diagnostic significance. The VEGF index, defined as the ratio of CSF/serum VEGF concentration to CSF/serum albumin levels, is found to be higher in CM and is a promising reliable diagnostic tool.[26] Moreover, the detection of several tumor markers in CSF can increase sensitivity, specificity, and predictive value.[25]

The glucose levels in CSF also have a prognostic value. The probability of CM is less with hyperglycorrhachia, and levels greater than 2.7 mmol/L are associated with better outcomes.[27][28]

CSF Flow Studies

In CM, obstructions in CSF flow are common because of the adhesion of tumor cells in the subarachnoid space and occur in 30 to 70% of the cases.[3] Obstruction to CSF flow is hazardous as it decreases treatment efficacy and increases toxicity and mortality by hindering chemotherapy drug distribution. Hence, it is essential to ensure appropriate CSF flow during chemotherapy. CSF dynamics are monitored by 111Indium-DPTA ventriculography or Technetium-99m-DPTA flow studies before starting intrathecal therapy.[29] Furthermore, it will decrease treatment-related mortality and improve survival by reducing tumor progression, neurological, and systemic toxicity.[30]

Neuroimaging

Gadolinium-enhanced MRI scan is a pertinent test for the diagnosis of CM. Radiographic findings may include hydrocephalus and leptomeningeal enhancement of the brain in the T1 weighted image with contrast. T2/FLAIR hyperintensities may be seen in the subarachnoid space. The cerebral convexities, cerebellar folia, basal cisterns, and ventricular ependymal regions are the common areas that display enhancement or nodular deposits in CM.[20] In the spinal cord, linear and nodular leptomeningeal enhancements may occur along the nerve roots, particularly in the cauda equina.[31]

For solid tumors, the sensitivity of MRI is 76%, and it increases to 95% for spinal MRI in patients presenting with cauda equina syndrome.[32]

MRI shows enhancement in both inflammatory and carcinomatous meningitis. Hence, it is imperative to pair it with CSF cytology. False-positive leptomeningeal enhancement may be seen in patients undergoing treatment due to therapy-induced inflammatory reaction, as seen with radiotherapy and immunotherapy. The prevalence is low (<1%) but still should be considered as a possibility in patients.[33]

Refer to Figure 2:[34] MRI T1 weighted images showing leptomeningeal enhancement, and

Figure 3: [35] MRI T1 weighted images showing leptomeningeal enhancement in lower portions of the spinal cord and post-treatment images.

Differential Diagnosis

Numerous conditions can present with similar symptomatology as carcinomatous meningitis and should be taken into consideration whenever patients present with indistinguishable signs and symptoms.

1. Intraparenchymal primary brain lesions or metastasis can present with similar symptoms. Calvarial and dura matter involvement should be ruled out. Leptomeningeal metastasis can present separately or co-exist with the involvement of these regions.
2. Chronic or recurrent meningitis caused by a variety of bacterial, fungal, viral, or protozoal organisms can also resemble CM. Autoimmune and drug-induced causes of meningitis should also be considered.
3. Paraneoplastic syndromes, including Lambert Eaton syndrome, Myasthenic crises, cerebellar degeneration, encephalomyelitis, neuropathies, and limbic encephalitis, can mimic CM and are associated with advanced-stage disease.

Prognosis

Carcinomatous meningitis represents advanced-stage disease and usually has a dismal prognosis. The median time of survival is 2 to 4 months, even with treatment.[5] Early diagnosis, normal CSF protein levels, and low disease burden are factors associated with a better prognosis.[51] Patients with KPS score >60, minimal systemic disease, minimal neurological deficits, and options for effective systemic therapy are considered to have a good-risk disease. In these patients, systemic therapy combined with IT or RT may be beneficial. In poor-risk patients, palliative care is the best option. CM, due to lymphomas and leukemias, has a better prognosis compared to solid tumors.[52]

Enhancing Healthcare Team Outcomes

Several future directions may help improve survival in patients with carcinomatous meningitis. This includes early diagnosis by analyzing circulating tumor DNA in CSF (CSF-ctDNA). These are shed from the cancer cells as a result of apoptosis and necrosis. Identification can help avoid subjecting the patient to high-risk surgeries or biopsies. The amount of CSF-ct DNA can provide information about tumor burden, recurrence, and treatment response. The identification of novel mutations in CSF-ctDNA can also provide newer therapeutic targets. There is a need for discovery of non-invasive methods like ultrasonography or CT scan guided drug delivery. Novel biological agents and immunomodulatory drugs may help in achieving better survival and improved quality of life.

The multidisciplinary team comprising of neurologists, neurosurgeons, medical oncologists, radiation oncologists, psychiatrists, and nurses must manage the biological, psychological, and social aspects of this disease.