Normal Pressure Hydrocephalus (NPH) is a neurological disorder primarily affecting older adults, caused by an abnormal accumulation of cerebrospinal fluid (CSF) in the brain’s ventricles. This excess fluid causes the ventricles to enlarge, compressing the surrounding brain tissue. The condition is often misdiagnosed as more common neurodegenerative diseases, such as Alzheimer’s or Parkinson’s disease, because the symptoms overlap significantly. NPH is a treatable, and sometimes reversible, cause of dementia, making a precise and timely diagnosis important for patient outcomes.
Initial Clinical Assessment and Screening
Diagnosis begins with recognizing the characteristic clinical presentation, often described as the classic symptom triad: gait disturbance, cognitive impairment, and urinary problems. Gait disturbance is typically the first and most prominent symptom, often described as a “magnetic gait” where the patient appears to have difficulty lifting their feet. This gait is wide-based, slow, and shuffling, and can resemble Parkinson’s disease but usually lacks the accompanying resting tremor.
The initial neurological examination focuses on observing and quantifying the gait abnormality, sometimes using standardized timed walking tests. Cognitive screening checks for decline involving psychomotor slowing, inattention, and problems with executive functions. Urinary issues generally manifest as urgency or frequency, eventually progressing to incontinence. While the presence of all three symptoms strongly suggests NPH, gait impairment is present in nearly all confirmed cases.
Structural Imaging Evidence
Once NPH is clinically suspected, the next step involves non-invasive structural imaging, typically with a Computed Tomography (CT) scan or Magnetic Resonance Imaging (MRI) scan. The purpose of these scans is to visualize the brain’s anatomy and look for ventriculomegaly, or enlarged ventricles. This enlargement is a prerequisite for an NPH diagnosis and is often quantified using the Evans Index, which compares the width of the ventricles to the inner diameter of the skull.
Crucially, the imaging must show ventriculomegaly that is disproportionate to the amount of cortical atrophy, or brain shrinkage, which is common in other dementias like Alzheimer’s disease. Specific imaging signs suggestive of NPH include a rounded shape of the frontal horns of the lateral ventricles and a small callosal angle. The imaging also serves to rule out other potential causes of symptoms, such as tumors, prior brain hemorrhage, or severe stroke damage.
Confirmatory Diagnostic Procedures
If the clinical and imaging findings align with NPH, functional tests are performed to confirm the diagnosis and predict the likelihood of the patient responding to a surgical shunt placement. The most commonly used procedure is the High-Volume Lumbar Puncture, often referred to as the Tap Test. This procedure involves inserting a needle into the lower back to access the spinal fluid space, from which a large volume of CSF is removed, typically between 30 to 50 milliliters.
The patient’s gait, balance, and cognitive function are assessed before and at timed intervals after the CSF removal. A positive Tap Test is defined by a measurable, temporary improvement in these symptoms, particularly a noticeable improvement in walking speed or balance. This temporary improvement mimics the effect of a permanent shunt and is a strong indicator that the patient will benefit from surgery.
If the Tap Test results are inconclusive, a more intensive procedure called Continuous or External Lumbar Drainage (ELD) may be performed. This involves placing a temporary catheter in the lumbar spine to continuously drain CSF over a period of several days, often three to five days. This sustained drainage allows physicians to monitor for a more prolonged and robust improvement in symptoms, which is considered the gold standard for predicting a positive response to permanent shunt surgery.
During the ELD trial, CSF is drained at a controlled rate, often between 100 to 150 milliliters per day, while the patient’s neurological status is monitored daily. A significant and sustained improvement in gait, cognition, or urinary function during this trial provides a high level of confidence that the patient will benefit from a shunt.