3.2.1 Cerebrospinal Fluid

The majority of CSF is produced by the choroid plexus located intracranially within the ventricular system. CSF flows through the ventricles and exits the ventricular system via the foramina of Luschka and Magendie, where it enters the subarachnoid space to surround the brain and spinal cord. CSF is reabsorbed via the arachnoid granulations adjacent to the superior sagittal sinus ¹ (▶ Fig. 3.1). The total volume of CSF at any given point in time is about 150 cc and the rate of CSF production is about 20 to 25 cc/hour, or 450 to 500 cc/day. This results in the resorption and regeneration of the total CSF volume three to four times daily. The ventricles contain only 20% of the total CSF volume at any given time with the remaining amount distributed within the cranial and spinal subarachnoid spaces and cisterns. ² A large amount of CSF resides in the lumbar region; thus, CSF can be effectively accessed in the lumbar spine.

3.2.2 Lumbar Spine

In the adult patient, the spinal cord terminates as the conus medullaris at around the L1 level. ³ Below the conus medullaris, spinal nerve roots travel within the thecal sac surrounded by CSF to exit the spinal column via their respective neural foramina. Due to the termination of the spinal cord at the upper lumbar levels, there is no risk of injuring the spinal cord when the thecal sac is accessed at the lower lumbar levels in the vast majority of patients. ▶ Fig. 3.2 depicts the relevant anatomy of the lumbar spine.

3.3.1 Obtain a CSF Sample

The broadest indication for LP is to obtain a CSF specimen for analysis. A number of neurologic pathologies can be diagnosed with CSF sampling, including viral or bacterial infections, primary central nervous system (CNS) or metastatic malignancies, some neurodegenerative conditions, and autoimmune diseases. Gross examination of CSF includes clarity (clear vs. cloudy) and color (clear, yellow, red, etc.). In patients with a suspected diagnosis of subarachnoid hemorrhage (SAH) but a negative computed tomography (CT) of the head, CSF obtained via LP can be examined for the presence of xanthochromia, which would support the diagnosis of SAH.

3.3.2 Measure Pressure

Many neurologic diseases can result in elevated intracranial pressure (ICP), including meningitis, malignancies, ventriculoperitoneal shunt failure, idiopathic intracranial hypertension, communicating hydrocephalus, and trauma. Obtaining a pressure measurement of the CSF space often guides clinicians on the most appropriate course of treatment, such as the use of hypertonic saline or the need for shunt revision surgery. LP with the patient in the lateral decubitus position can provide a real-time measurement of the pressure within the neuraxis and help guide clinicians on the best course of action.

3.3.3 Therapeutic Drainage

There are a number of indications for therapeutic drainage of CSF. In general, any instance of ICP crisis due to communicating hydrocephalus may be temporarily treated with LP and therapeutic drainage of CSF, such as a patient presenting with shunt failure who requires temporization prior to a definitive operative intervention (i.e., shunt revision surgery) as long as there is no obstruction to flow. Additionally, therapeutic drainage of CSF in patients with normal pressure hydrocephalus (NPH) or idiopathic intracranial hypertension (IIH) can aid in determining if such patients would benefit from permanent CSF shunting. In such patients, high-volume LP results in marked symptom improvement. Patients with cryptococcal meningitis often develop chronic communicating hydrocephalus. In such cases it may be necessary to perform daily LPs as a temporizing bridge to permanent ventriculoperitoneal shunting while antifungals are administered to clear the infection.

LP can be performed to access the CSF space for the purpose of administering medications. The most common scenario in which this would be done would be for the administration of iodinated contrast for the purpose of CT myelography imaging of the spine.

3.4.1 Intracranial Space-Occupying Lesion or Existing Brain Shift

In a patient with an intracranial space-occupying lesion(s) in which brain compression and elevated ICP are suspected or confirmed, there is concern that removal of CSF from the lumbar cistern may result in downward intracranial herniation. As described by van Crevel et al, “the withdrawal of the CSF removes the stopper from below, thus adding to the effects of the compression from above, and increasing the brain shifts already present.” ⁴ This downward herniation may result in compression of the brainstem as well as other critical neurovascular structures, and can result in significant morbidity or mortality. Clinical findings that should prompt CT scan of the head prior to LP include altered mental status, focal neurologic deficits, or comatose state. If the results of the CT are inconclusive or cannot definitively rule out a space-occupying lesion, advanced imaging with magnetic resonance imaging (MRI) should be pursued. Nevertheless, if the benefit outweighs the risk, a low-volume LP may be performed in such patients, taking care to remove the minimum amount of CSF needed and monitoring the patient closely following the procedure. Perhaps most illustrative is the case of acute bacterial meningitis (ABM), in which patients can present with severely altered mental status and ICP is often elevated, but rapid diagnosis is necessary to prevent the morbidity and mortality associated with untreated or delayed treatment of ABM.

Note that elevated ICP and/or papilledema alone, as in idiopathic intracranial hypertension, are not contraindications if present without an intracranial space-occupying mass.

3.4.2 Obstructive Hydrocephalus

Similar to the concern with intracranial space-occupying lesions, obstructive hydrocephalus results in isolated elevated ICP. Removal of CSF from the lumbar cistern can thus lead to downward herniation.

3.4.3 Coagulopathy/Clotting Dysfunction/Bleeding Disorder

There are a number of external and internal causes for bleeding dysfunction in patients that may require LP. These include pre-existing coagulopathies (e.g., hemophilia, von Willebrand disease, and liver failure), medication-induced dysfunction of the clotting cascade (e.g., warfarin, heparin, rivaroxaban, and argatroban) or of platelet function (aspirin, clopidogrel, etc.), recent thrombolytic therapy (e.g., tPA administered for acute ischemic stroke), and thrombocytopenia.

Nevertheless, if LP is determined to be necessary for the appropriate medical treatment of an individual patient, appropriate reversal agents or blood product(s) can be administered prior to the procedure. The incidence of LP-related spinal hematomas reported in the literature is extremely rare, ^{5 , 6} although the true incidence, anecdotally, is likely higher as not all cases are reported. Each case should be evaluated individually to determine the risk and benefit of performing LP in the presence of coagulopathy. If the benefit outweighs the risk of possible hematoma, and the patient and/or family has been counseled and understands the risks and would like to proceed with the procedure, LP can be performed.