A nontraumatic SAH in the adult population may be due to a ruptured aneurysm. A paroxysmal severe headache is the classic symptom of aneurysm rupture, but atypical headaches reminiscent of migraine are not uncommon. Warning leaks or a sentinel headache occurring at least 4 weeks prior to the diagnosis of SAH was reported by Beck et al. [4] in 17.3% of patients with subsequent diagnosis of SAH.

Leblanc [5] reported that up to 50% of patients with a warning “leak” headache are undiagnosed after evaluation by their physician and 55% of patients with premonitory warning headaches had normal CT findings, but all had a positive finding of SAH on lumbar puncture. Lumbar puncture is indicated with such presenting headache if the head CT is normal and if the clinical history and presentation are typical for aneurysm rupture.

A lumbar puncture should not be performed without prior CT if the patient has any focal neurologic deficit. The neurologic abnormality might indicate the presence of an intracranial mass lesion, and lumbar puncture can increase the likelihood of downward transtentorial herniation. SAH can also cause acute obstructive hydrocephalus by intraventricular extension or
obstruction to CSF resorptive mechanisms at the arachnoid granulations. The CT scan would demonstrate ventriculomegaly, which is best treated by CSF access and diversion using a ventricular catheter.

A traumatic lumbar puncture presents a diagnostic dilemma, especially in the context of diagnosing suspected SAH. Differentiating characteristics include a decreasing red blood cell count in tubes collected serially during the procedure, the presence of a fibrinous clot in the sample, and a typical ratio of about 1 leukocyte per 700 red blood cells. Xanthochromia is more indicative of SAH and is quickly evaluated by spinning a fresh CSF sample and comparing the color of the supernatant to that of water. In performing this test, the use of a spectrophotometer is much more sensitive than by visual inspection. Spinal fluid accelerates red blood cell hemolysis, and hemoglobin products are released within 2 hours of the initial hemorrhage, creating the xanthochromia. Associated findings, such as a slightly depressed glucose level, increased protein, and an elevated opening pressure, are also more suggestive of the presence of an SAH.

CSF evaluation is the single most important aspect of the laboratory diagnosis of meningitis. The analysis usually includes a Gram stain, blood cell count with white cell differential, protein and glucose levels, and aerobic and anaerobic cultures with antibiotic sensitivities. With suspicion of tuberculosis or fungal meningitis, the fluid is analyzed by acid-fast stain, India ink preparation, cryptococcal antigen, and culture in appropriate media. More extensive cultures may be performed in the immunocompromised patient.

Immunoprecipitation tests to identify bacterial antigens for Streptococcus pneumoniae, streptococcus group B, Haemophilus influenzae, and Neisseria meningitidis (meningococcus) allow rapid diagnosis and early specific treatment. Polymerase chain reaction testing can be performed on CSF for rapid identification of several viruses, particularly those commonly responsible for CNS infections in patients with acquired immunodeficiency syndrome. Polymerase chain reaction testing exists for herpes, varicella zoster, cytomegalovirus, and Epstein–Barr virus, as well as toxoplasmosis and Mycobacterium tuberculosis [6]. If the clinical suspicion is high for meningitis, administration of broad-spectrum antibiotic therapy should be initiated without delay following CSF collection [7].

A ventriculoperitoneal shunt is the most commonly encountered implanted system for CSF diversion. The system consists of a ventricular catheter connected to a reservoir and valve mechanism at the skull and a catheter that passes in the subcutaneous soft tissue in the neck and anterior chest wall to the peritoneum. The distal tubing can be alternatively inserted in the jugular vein, the pleura, or even the urinary bladder. Proximal shunt failure of the ventricular catheter may occur due to choroid plexus obstruction or cellular debris from CSF infection. Valve or distal tubing obstruction occurs also from cellular debris, from disconnection, poor CSF absorption, or formation of an intra-abdominal pseudocyst.

The clinical presentation of an obstructed shunt is variable. It may be slowly progressive and intermittent, or there may be a rapid decline in mentation progressing into a coma. A CT scan should be performed immediately to determine ventricular size. Ventriculomegaly is a reliable indicator of a malfunctioning shunt; however, the CT scan should be compared with previous studies because the ventricular system in a shunted patient is often congenitally or chronically abnormal.

Aspiration from the reservoir or valve system of a shunt can be performed to determine patency and collect CSF to diagnose an infectious process. The necessity of and procedure for a shunt tap is best left to a neurosurgeon. Shunt aspiration is an invasive procedure that carries a risk of contaminating the system with skin flora, and the resultant shunt infection requires a lengthy hospitalization for shunt externalization, antibiotic treatment, and replacement of all hardware. Therefore, CSF collection by shunt tap should be performed very selectively and after other potential sources of infection have been evaluated. When shunt failure is due to distal obstruction, aspiration of CSF may temper neurologic impairment and even be lifesaving until surgical revision can be performed.

Serial lumbar punctures or continuous CSF drainage via a lumbar subarachnoid catheter can be used as provocative diagnostic tests to select patients who would benefit from a shunt for CSF diversion. The results have a positive predictive value if the patient’s gait improves. Lumbar CSF access may also be used for infusion tests, measurement of CSF production rate, pressure–volume index, and outflow resistance or absorption. Some studies suggest that these values are also predictive of therapeutic CSF diversion [8,9,10].

Benign intracranial hypertension occurs in young persons, often obese young women. Intracranial pressure (ICP) is elevated without focal deficits and in the absence of ventriculomegaly or intracranial mass lesions [11]. The condition causes blindness, and most patients demonstrate some visual loss. Etiologic factors for childhood presentation include chronic middle ear infection, dural sinus thrombosis, head injury, vitamin A overdosage, tetracycline exposure, internal jugular venous thrombosis, and idiopathic causes. Some authors have proposed a broader definition of the “pseudotumor cerebri syndrome” on the basis of the underlying pathophysiologic mechanism of the presumed CSF circulation disorder [12].

Lumbar puncture demonstrates an elevated ICP (up to 40 cm H₂O), and CSF dynamics demonstrates an increase in outflow resistance. Serial daily punctures can be therapeutic, with CSF aspirated until closing pressure is within normal limits (< 20 cm H₂O). In some cases, this can restore the balance between CSF formation and absorption; other cases require medical therapy, such as weight loss, steroids, acetazolamide, diuretics, and glycerol. If all these therapeutic interventions fail, placement of a permanent shunting system may be necessary.

The subarachnoid space can be infiltrated by various primary or secondary tumors, giving rise to symptoms of meningeal irritation. CSF cytology can determine the presence of neoplastic cells, although their complete identification is not always possible. Systemic neoplasms, such as melanoma or breast cancer, have a greater propensity to metastasize into the CSF spaces than do primary CNS tumors and may even present primarily as meningeal carcinomatosis. Ependymoma, medulloblastoma or primitive neuroectodermal tumor, germinoma, and high-grade glioma are the most commonly disseminated primary tumors. Hematopoietic cancers such as leukemia and lymphoma also frequently infiltrate the subarachnoid spaces with little or no parenchymal involvement. CSF sampling is useful for an initial diagnostic and screening tool in the neurologically intact patient who harbors a tumor type with high risk of CNS relapse. Lymphoma cells in primary CNS lymphoma
are present in increased number and pleocytosis correlates with positive cytology [13]. A generous amount of CSF or multiple samples may be required for diagnosis and cisternal puncture may enhance the diagnosis if the lumbar CSF is nondiagnostic. Acute leukemias that tend to invade the CNS include acute lymphocytic leukemia, acute nonlymphocytic leukemia, acute myelogenous leukemia, acute myelomonocytic leukemia, and acute undifferentiated leukemia [14].

Lumbar puncture is the most common means of access for lumbar and cervical myelography because the density of contrast material is higher than CSF and may be directed by gravity to the area of interest. Cervical C1–2 puncture had been a usual access route for cervical myelography, but now, it is often reserved for patients in whom a successful lumbar puncture is not possible due to extensive arachnoiditis, epidural tumor, severe spinal stenosis, or CSF block.

There is extensive literature on CSF changes in demyelinating diseases, including MS. Typical lumbar puncture findings are normal ICP, normal glucose levels, mononuclear pleocytosis, and elevated protein levels due to increased endothelial permeability. Immunoelectrophoresis reveals elevated IgG and oligoclonal bands that suggest inflammation in the CNS and may be a sign of MS [15,16].

CSF findings described in other disease states include elevated tau protein and decreased β-amyloid precursor protein in Alzheimer’s disease and the presence of anti-GM1 antibodies and cytoalbumin dissociation in Guillain–Barré syndrome [17].

CSF leaks occur due to a variety of nontraumatic and traumatic etiologies. Orthostatic headaches are a characteristic symptom of CSF leak, and rhinorrhea may be evident. Iatrogenic postoperative CSF leaks may occur following surgery at the skull base as a result of dural or bony defects. CSF fistulas following middle cranial fossa or cerebellopontine angle surgery occur infrequently, and CSF usually leaks through the auditory tube to the nasopharynx. Dural closure in the posterior fossa following suboccipital craniectomy is often difficult and not watertight. A fistula in that area usually results in a pseudomeningocele, which is clinically apparent as subcutaneous swelling at the incision site. Leaks following lumbar surgery are unusual, but they may occur as a result of recent myelography, dural tear, or inadequate dural closure [18]. In pediatric patients, repair of meningoceles or other spina bifida defects are more likely to present with a CSF leak because of dural or fascial defects.