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. Leblanc [4] 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 not diagnostic and if the clinical history and presentation are atypical. A lumbar puncture should not be performed without prior CT if the patient has any focal neurologic deficits because the neurologic abnormality might indicate the presence of an intracranial hematoma. Intracranial mass lesions can increase the likelihood of downward transtentorial herniation following a lumbar puncture. A SAH can also cause acute obstructive hydrocephalus by intraventricular extension, thereby causing obstruction to CSF flow or by obstruction of the resorptive mechanisms at the arachnoid granulations. In such a case, the CT scan would demonstrate ventriculomegaly, which is best treated by the placement of 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, or by the use of a spectrophotometer. 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. Another method to differentiate between blood due to intracranial hemorrhage and that due to a traumatic spinal tap has been demonstrated in neonates [5]. The mean corpuscular volume of erythrocytes in the CSF can be compared with that in peripheral blood. The mean corpuscular volume in CSF will be lower than in venous blood in cases of SAH, but the values are similar if the hemorrhage was a traumatic lumbar puncture.

CSF evaluation is the single most important aspect of the laboratory diagnosis of meningitis. The analysis usually includes a Gram’s 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, Hemophilus 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 tests exist 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, antibiotic therapy should be initiated without delay following CSF collection [7].

A ventriculoperitoneal shunt is the most commonly encountered system. The hardware varies but commonly consists of a ventricular catheter connected to a reservoir and valve mechanism at the skull and a subcutaneous catheter that passes in the neck and anterior chest wall to the peritoneum. The distal tubing may also have been inserted in the jugular vein, the pleura, or even the urinary bladder. Proximal shunt failure of the ventricular catheter may be 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 intraabdominal pseudocyst.

The clinical presentation of an obstructed shunt is variable; it may be slowly progressive, intermittent, or there may be a rapid decline in mentation progressing into a coma. A CT scan should be performed immediately and compared with previous studies because the ventricular system in a shunted patient is often congenitally or chronically abnormal. Ventriculomegaly is a good indicator of a malfunctioning shunt, but noncompliant ventricles may remain small and not vary significantly in size.

Aspiration from the reservoir or valve system of a shunt can be performed to determine patency and to collect CSF to diagnose an infectious process. However, one should remember that shunt aspiration is an invasive procedure that carries a risk of contaminating the system with skin flora. 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 formal shunt revision can be performed. The determination of the need for a shunt tap, as well as the procedure, is best left to a neurosurgeon.

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 and 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. 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 [11]. Some authors have proposed a broader definition of the “pseudotumor cerebri syndrome” based on 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 demonstrate an increase in outflow resistance. Serial daily punctures can be therapeutic, with CSF aspirated until closing pressure is within normal limits (less than 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. A generous amount of CSF or multiple samples may be required for diagnosis. Cisternal puncture may enhance the diagnosis if the lumbar CSF is nondiagnostic [13]. 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]. Individual proliferating T and B lymphocytes can also be detected in the CSF and may aid in the differentiation of an opportunistic infection from a leukemic infiltration [15]. CSF analysis for auto antibodies could play a role in the diagnosis of some paraneoplastic syndromes (e.g., anti-Yo titers in paraneoplastic cerebellar degeneration) [16].

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 may be the usual access route for cervical myelography but 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 multiple sclerosis. 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 [17]. Antibodies against cardiolipin synthetic lecithin, a lectin protein involved in the structural stabilization of myelin, have been detected in the CSF of patients with multiple sclerosis and may constitute a very sensitive and specific diagnostic test [18].

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.

CSF leaks occur for a variety of reasons, including nontraumatic and traumatic etiologies. Orthostatic headaches are a characteristic symptom, and CSF 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 with potential wound breakdown. Leaks following lumbar surgery are unusual but may occur as a result of recent myelography, dural tear, or inadequate dural closure [19]. 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.

The most common presentation of a CSF fistula follows trauma. Basilar skull fractures that traverse the ethmoid or frontal sinuses can cause CSF rhinorrhea. Fractures along the long axis of the petrous bone usually involve the middle ear, causing the hemotympanum noted on examination and CSF otorrhea if the tympanic membrane is ruptured. Delayed leaks are not uncommon because the fistula can be occluded with adhesions, hematoma, or herniated brain tissue, which temporarily tamponades the defect.