32.1 Clinical Presentation

The most common symptom of NPH is that of gait disturbance and is described as “a magnetic” gait as patients take slow, short, shuffling steps with outward rotated feet, a wide base, and diminished step height. Patients have difficulty with tandem gait, display en-bloc turning (requiring 3 or more steps to turn 180 degrees) and often have impaired balance. There is often a history of repeated falls. This is the most likely symptom to improve with treatment.

The cognitive impairment of NPH is likely a result of subcortical/frontal dysfunction. It manifests as psychomotor slowing, daytime sleepiness, inattention, decreased concentration and apathy. While executive functions are impaired, there is an absence of aphasia, agnosia and apraxia. Additionally, in the early stages of the disease process, patients may experience urinary urgency and frequency without incontinence. Later on, however, bladder and even bowel incontinence can occur.

32.2 Pathophysiology

The pathophysiology of normal pressure hydrocephalus remains somewhat unclear, but there is believed to be decreased CSF absorption at the arachnoid villi that transiently result in increased CSF pressures and pulsatility. As greater force is applied to the brain in normal-sized ventricles, ventricular enlargement results and CSF pressures return to normal. With dilatation of the ventricles, periventricular axons and subependymal vessels are stretched causing microvascular ischemia and white matter damage.

32.3 Diagnosis

The diagnostic sensitivity of NPH is reduced by the variability that exists in its clinical presentation and course. It requires convergent date from the clinical history, physical examination and cranial imaging. Clinical symptomatology must include gait/balance disturbance and some degree of impairment in either cognition or urinary continence, or both. The onset of symptoms should be insidious in onset with an origin after age 40 and a minimal duration of at least 3–6 months. Symptoms should progress over time and there should be no antecedent event or other neurological or medical conditions as potential causes of the presenting symptoms.

In addition to the clinical symptoms, an NPH diagnosis requires the documentation of ventricular enlargement on either CT or MRI that is not entirely attributable to cerebral atrophy or a result of CSF obstruction. A commonly used radiographic marker is the Evans’ index, which is the ratio of the maximal width of the frontal horns of the lateral ventricles to the maximal inner diameter of the skull at the same level (▶ Fig. 32.1). Although with the Evan’s index, the ratio can vary with age and gender and is dependent on the location and angle of the CT image. A value greater than 0.3 is considered abnormal.

Potentially incapacitating symptoms of NPH can be improved or even reversed by permanent cerebrospinal fluid diversion with a ventriculoperitoneal shunt (VPS). Ataxia, dementia and incontinence associated with more advanced stages of NPH tend to be less responsive than symptoms of less than two years duration, emphasizing the utility of early diagnosis. Overall up to two-thirds of patients suspected of having NPH based on clinical symptoms and imaging alone will have a favorable response to shunting. Those patients with the complete symptom triad likely will demonstrate the greatest improvement. As the complication rate with shunting of NPH patients has been reported to be as high as 35% in some series, adjunctive testing should be performed to improve the diagnostic accuracy.

With supplemental testing, the ability to predict a positive response to VPS for NPH can potentially be increased to greater than 80%. This testing usually replicates the removal of CSF in some form. The performance of a high volume (35–50 ml) lumbar puncture (i.e., cerebrospinal fluid tap test) and the subsequent assessment of symptomatic improvement can improve the sensitivity of the testing to almost 80%. A “pre-tap” and “post-tap” objective assessment of cognitive and gait function yields a better determination of surgical candidacy than relying on subjective improvement alone.

Placement of a lumbar drain and an inpatient trial of continual CSF removal over a period of up to three days may increase the sensitivity closer to 90%. ² This technique, however, is associated with a much higher cost associated with an inpatient hospital stay and complication rate (e.g., infection, extraaxial hematoma formation, drain disconnection) in this elderly population and may not be worth the additional risk. For more difficult cases, CSF outflow resistance can be assessed through infusion studies, but these can often be difficult to interpret and may be of questionable clinical utility. ³

We advocate for a more structured outpatient assessment in which an initial consultation is performed with a comprehensive history, physical exam, and review of all available cranial imaging. The examination focuses on cognitive function and ambulation, as it is difficult to reliably assess urinary incontinence in the outpatient neurosurgical office. Our cognitive evaluation consists of rapid and delayed memory testing, the Folstein mini-mental status exam (FMMSE), and timed cognitive testing. Ambulation status is assessed with a modified Timed Up and Go (TUG) test and a 25-foot walk. The patient then has a high volume (ideally 40 ml CSF removal) lumbar puncture with a follow-up appointment approximately 24 hours for repeat testing. Pre- and post-LP scores are compared and VPS is recommended if there is 1) a > 20% overall improvement, 2) a > 25% improvement in any two single parameters (excluding FMMSE), and 3) a > 50% improvement in a single parameter (excluding FMMSE), or 4) the FMMSE score improves by ≥ 8 points.