Normal Pressure Hydrocephalus (NPH) is a neurological disorder caused by the accumulation of excess cerebrospinal fluid (CSF) in the brain’s ventricles. This buildup causes the ventricles to enlarge, compressing surrounding brain tissue. The condition is often misdiagnosed because its symptoms—difficulty walking, mild cognitive changes, and loss of bladder control—frequently mimic those of common age-related conditions like Alzheimer’s or Parkinson’s disease. Accurately identifying NPH is important because, unlike those other neurodegenerative disorders, NPH is potentially treatable, with many patients seeing significant improvement after intervention. The diagnostic process is a multi-step evaluation designed to correctly identify this reversible cause of dementia-like symptoms.
Initial Clinical Assessment
The diagnostic journey begins with a thorough clinical assessment, where a physician gathers a detailed patient history and performs a neurological examination. This initial step focuses on recognizing the specific pattern of symptoms that characterize NPH and ruling out other obvious causes. The presence of the “classic triad” of symptoms—gait disturbance, mild dementia, and urinary incontinence—is the primary trigger for suspicion of NPH.
The gait disturbance is typically the earliest and most prominent symptom, often described as a “magnetic” gait characterized by short, shuffling steps and difficulty turning. Cognitive changes are usually characterized by slowed thinking, reduced attention, and difficulty with executive functions, rather than the profound memory loss seen in advanced Alzheimer’s disease. Urinary issues often progress from frequency and urgency to frank incontinence, though this symptom usually appears later. The neurological exam provides a baseline for later functional tests and helps differentiate NPH from other conditions.
Structural Imaging Tests
Once NPH is suspected based on the clinical presentation, the next step is non-invasive structural imaging to look for physical evidence within the brain. Computed Tomography (CT) scans and Magnetic Resonance Imaging (MRI) are used to visualize the size and shape of the brain’s fluid-filled spaces. The hallmark finding suggestive of NPH is ventriculomegaly, which is the enlargement of the cerebral ventricles, particularly the lateral and third ventricles.
The key distinction in NPH is that this ventricular enlargement is disproportionate to the cortical atrophy, or brain shrinkage, expected in other forms of dementia. Radiologists use the Evans index, a ratio of the maximum width of the frontal horns of the lateral ventricles to the maximum inner diameter of the skull; a value greater than 0.3 often indicates hydrocephalus. MRI is frequently the preferred imaging modality because it offers better resolution to evaluate subtle signs, such as a reduced callosal angle or a prominent flow void in the cerebral aqueduct, suggesting altered CSF flow dynamics. Imaging strongly suggests the diagnosis but is not definitive, requiring further functional testing for confirmation.
Confirmatory Cerebrospinal Fluid (CSF) Tests
The most definitive phase of the diagnostic process involves functional tests that assess the patient’s symptomatic response to the temporary removal of cerebrospinal fluid. These tests determine if the symptoms are reversible, which is the core principle of NPH treatment. The High-Volume Tap Test, or lumbar puncture, is a common outpatient procedure where a needle is inserted into the lower spine to drain a significant amount of CSF.
Typically, between 30 and 50 milliliters of fluid are removed. The patient’s gait and cognitive function are quantitatively assessed immediately before and within hours after the procedure. A positive result is indicated by a noticeable, temporary improvement in walking speed, balance, or cognitive performance. This temporary relief strongly suggests the patient will respond positively to a permanent surgical shunt, which continuously drains excess CSF. Since the single tap test can sometimes produce a false negative result, a more extended test may be necessary.
For inconclusive cases, continuous external lumbar drainage (ELD) is performed as an extended trial, usually requiring a hospital stay of two to five days. A catheter is inserted into the lumbar spine to drain CSF continuously at a controlled rate, often around 10 milliliters per hour, allowing for a sustained period of reduced pressure. Gait and cognitive assessments are performed daily during the ELD trial. A clear improvement over this extended period is highly predictive of a successful outcome from shunt surgery. The goal of both the tap test and ELD is to simulate the effect of a permanent shunt, providing the most accurate prediction of treatment success.