A lumbar puncture (LP, also known as a spinal tap) is a diagnostic and at times, therapeutic medical procedure.
Diagnostic indications
The main diagnostic indications of performing a LP is for collection and evaluation of cerebrospinal fluid (CSF) for diagnosis and exclusion of infectious, inflammatory and neoplastic diseases affecting the central nervous system.
For example, a LP would be indicated in a patient who presents with a few day history of fever, headache and altered mental status. The patient fits into the clinical picture of meningitis, but this would need further confirmation by evaluation of the CSF, which could be obtained via a LP.
Another example is a young woman presenting with recurrent episodes of weakness and numbness. In this case, demyelinating diseases of the central nervous system e.g. multiple sclerosis, is one of the differential diagnosis. Obtaining CSF via a LP would help, as presence of oligoclonal bands in the CSF would support such a diagnosis.
A LP is also indicated in patients whom subarachnoid haemorrhage is strongly suspected, but the non-contrast cranial CT scan is non-revealing. Although a non-contrast cranial CT scan is very sensitive in patients with a subarachnoid haemorrhage (93-100% if performed within 24 hours of onset), the sensitivity decreases with time, such that at about 5 days after onset of subarachnoid haemorrhage, the sensitivity of a non-contrast CT scan in detecting blood drops to 85%, and at around 1 week after onset of subarachnoid haemorrhage, the sensitivity further drops to 50%. If subarachnoid haemorrhage is strongly suspected despite a negative CT, the patient should undergo a lumbar puncture to delineate whether the presence of xanthrochromia is present. If present, this means that there are presence of red cells in the CSF. With time, the red cells breakdown, releasing heme, which is subsequently degraded into the yellow-green pigment bilirubin causing the characteristic yellow colour in patients with xanthochromia. In contrast, if red blood cells enter the CSF due to a “traumatic tap” (i.e. a small blood vessel was damaged during the LP), there is inadequate time for the red cells to degrade and thus xanthochromia would not be present.
Therapeutic indications
Sometimes, a LP is also performed as a therapeutic measure. Medications can be injected “intrathecally” via a LP. For example, in patients with hematological malignancies affecting the central nervous system, chemotherapeutic agents can be injected intrathecally via a LP to maximise its effect on the central nervous system. Anesthetic agents are also commonly injected intrathecally in patients who require spinal anesthesia.
In patients who have a communicating hydrocephalus due to an excessive accumulation of CSF, LPs are often performed to therapeutically remove CSF. Examples would include patients with normal pressure hydrocephalus, where an excessive accumulation of CSF results in classical symptoms of gait disturbances, cognitive impairment and incontinence. One of the diagnostic tests of this condition is called the “tap test” where the patient’s functional status is assessed before and after large amounts of CSF is removed via a therapeutic LP. In other patients with marked increased intracranial pressure due to chronic meningitis, excessive CSF could also be removed to lower the intracranial pressure via a therapeutic LP.
