Lumbar Puncture Explained: How It Helps Diagnose NPH

Lumbar Puncture and Its Role in Normal Pressure Hydrocephalus (NPH)

A lumbar puncture (LP), or spinal tap, is a common diagnostic procedure where a small amount of cerebrospinal fluid (CSF) is removed from the spinal canal.  In neurology, this procedure is commonly used to establish a diagnosis when meningitis is suspected.  In the context of normal pressure hydrocephalus (NPH), a larger amount of fluid is drained in combination with tests of gait.  This is often referred to as a “tap test”.  In this setting, the LP is performed both to confirm a diagnosis of NPH and to predict if symptomatic relief will be achieved with ventricular shunt surgery.

The Tap Test and Its Significance:

The “tap test” is an outpatient procedure that is typically performed in the office of a neurologist or neurosurgeon.   Sometimes, however, it is performed by a radiologist using fluoroscopy or CT guidance. After numbing the lower spine with lidocaine, a needle is inserted into the sac within the spinal canal that contains CSF.  The fluid is then allowed to drain by gravity into a collection container.  Usually, between 1 and 2 fluid ounces of fluid is obtained. The needle is then withdrawn.  Prior to the procedure, the patient’s gait is quantitatively assessed.  This may involve a video recording of the patient walking a defined distance with measurements of speed and step length, but in some situations, more elaborate testing using computerized techniques are employed.  These gait tests are then repeated, generally within 1 hour after draining CSF.  If there is a clear improvement in gait following the tap, a shunt operation may be recommended.

CSF obtained during the tap test is usually sent to a laboratory to look for abnormalities that may point to a co-morbid or alternative neurologic disease.  For example, tests are now available to assess for Alzheimer’s disease and other forms of neuropathology.

It is important to recognize that gait improvement following a tap test is short-lived.  Typically, the benefit lasts for only a few hours.  Sometimes the effect will last into the following day, but it is rare for it to last longer than that.  The procedure is performed to estimate whether long-term benefit will be achieved with ventricular shunt surgery.  It is not used as a replacement for shunt surgery.

It is also important to appreciate that while clear gait improvement following a tap test may confidently predict a favorable response to shunt surgery, many patients with NPH who do not improve with a lumbar puncture may still benefit from a shunt.  In medical parlance, the tap test is considered to be of high specificity but relatively low sensitivity.

In some cases, continuous lumbar CSF drainage is recommended as an alternative to the tap test or when no improvement is observed after a tap test.

Tap test vs. Continuous Lumber CSF Drainage (CLD):

Pros of Tap Test:

• Minimally invasive
• Out-patient procedure
• Can be performed in an office setting
• Very low risk of complications

Cons of Tap Test:

• The effect is often very brief (a few hours)
• The effect is typically not as robust as seen after CLD
• Lower sensitivity than CLD

Pros of CLD:

• More closely replicates the physiologic effect of a shunt operation
• More likely to result in observable gait improvement (greater test sensitivity)
• Improvement, when it occurs, will often last for several days:
  • This gives patients and family members an opportunity to observe for improvement in
  • a home setting and while performing common tasks of daily living.

Cons of CLD:

• More invasive
• Greater risk of complications
• Must be performed in the setting of a several-day hospital admission

Risks and Considerations of Lumbar Puncture

The most common adverse effect of a lumbar puncture is a headache, and sometimes nausea.  This is because the procedure is performed to lower pressure within the head, which can impart a stretch on membranes within the cranium that contain pain fibers.  The headache is experienced when a patient is seated or standing, and is relieved when a patient lies flat in bed.  Usually, the headache resolves within a few hours and is treated with only Tylenol and hydration.  Very rarely, it can persist for several days and require an additional corrective procedure.  Other risks, including infection, bleeding, or nerve injury, are exceedingly uncommon.

Diagnosis Beyond the Tap Test

Sometimes the diagnosis of NPH is clearly evident after reviewing MRI or CT results, neurologic examination, neuropsychologic testing, and consideration of the patient’s history.  A tap test is often recommended when the diagnosis is not certain or if a patient (or patient’s family) requires a very high degree of confidence that shunt surgery will be helpful before consenting to neurosurgical intervention.

Long-Term Outlook for NPH Patients

For patients diagnosed with NPH, the long-term outlook can vary.  Intervention with treatments like ventriculoperitoneal (VP) shunting, or sometimes endoscopic third ventriculostomy (ETV), can lead to significant improvements in symptoms, particularly in gait and bladder control. Cognitive benefits can also sometimes be evident.  Many patients experience substantial relief, allowing them to maintain a more independent lifestyle.

The effectiveness of treatment can depend on the timing of diagnosis.  Current evidence suggests that early detection and intervention may improve prognosis.  While ventricular shunt surgery can be highly effective, some patients may still experience ongoing challenges, particularly with cognitive function, even after shunting.  This may reflect the presence of a co-morbid neurologic process such as stroke, microvascular disease, Alzheimer’s disease, or other neurodegenerative pathologies.  In addition, shunts may fail over time, requiring revision surgery and other forms of adjustment.  It is also not uncommon for patients who exhibit pronounced initial benefit from shunt surgery to deteriorate over subsequent years, even when their shunts are functioning optimally.  The explanation for this phenomenon is sometimes not clear.

In conclusion, lumbar puncture remains an important diagnostic and treatment-planning tool for NPH. It helps identify patients who may benefit from surgical interventions, providing valuable insights for guiding optimal care.

Information you can trust! This article was produced by the Hydrocephalus Association, copyright 2024. We would like to thank James Golomb, MD, for his valuable contribution and expert input.

This article is designed to provide helpful information on the subjects discussed. It is not intended as a substitute for treatment advice from a medical professional. For diagnosis or treatment of any medical condition, consult your doctor.