Normal Pressure Hydrocephalus
MRI of brain with normal pressure hydrocephalus
MRI of normal brain
Hydrocephalus is a condition with excessive accumulation of fluid in the brain. Although hydrocephalus was once known as "water on the brain," the "water" is actually cerebrospinal fluid (CSF), a clear fluid surrounding the brain and the spinal cord. The excessive accumulation of CSF results in an abnormal dilation of the spaces in the brain, called ventricles. This dilation causes potentially harmful pressure on the surrounding tissues of the brain. Hydrocephalus may be congenital or acquired.
Congenital hydrocephalus is present at birth and may be caused by genetic abnormalities or developmental disorders.
Acquired hydrocephalus develops at the time of birth or at some point afterward and can affect individuals of all ages.
This section deals with one specific subtype of acquired hydrocephalus: normal pressure hydrocephalus (NPH), which occurs most often among the elderly. NPH is a condition of enlarged ventricles but with normal opening pressures on lumbar puncture. NPH is distinguished from obstructive or non-communicating hydrocephalus, in which there is a structural blockage of the CSF circulation within the ventricular system. Thus, NPH is a communicating and acquired hydrocephalus.
TRIAD of NPH
NPH is associated with a classic triad of dementia, gait disturbance, and urinary incontinence. NPH usually occurs spontaneously and without an obvious explanation in patients after age 60. This is called an idiopathic condition. But, it may occur secondary to another medical condition, whether current or past. Thus, it may result from a subarachnoid hemorrhage, head trauma, infection, tumor, or complications of surgery.
IDIOPATHIC NPH
Most people develop idiopathic NPH, without an obvious cause. Symptoms of NPH are variable, depending upon age, stage in disease progression when family notices a problem, and the highly individual differences in tolerance to excess CSF. Because the clinical syndrome is potentially reversible by the placement of a ventriculo-peritoneal shunt, it is important to recognize and accurately diagnose the condition. Sadly, there is little consensus regarding the diagnosis of NPH and the selection of patients for shunt placement.
Cerebrospinal fluid (CSF) is produced deep inside the brain in the lateral ventricles by a structure called the choroid plexus. CSF flows from the lateral ventricles down into the third and fourth ventricles and reaches the surrounding spaces that cover the brain and spinal cord. This outlying space is the subarachnoid space. The CSF flows and moves and has to be reabsorbed into the blood circulation, whence it came. This re-absorption occurs through structures close to the venous system of the central nervous system. The structures of reabsorption are known as Pacchionian arachnoid granulations.
If there is decreased CSF resorption into the bloodstream, there will be a gradual accumulation of CSF within the ventricular system. Increased pressure may not be apparent by typical studies, such as lumbar puncture. The elevated pressure occurs locally and affects some brain connections close to the enlarged ventricles and in this way produces the symptoms.
CLINICAL FEATURES
NPH is classically described as having three cardinal features:
- Gait difficulty with the feet being "stuck" to the floor. Steps are characteristically short, with decreased stride length and height, and a broadened base. Patients tend to turn slowly, with several steps. Postural stability is impaired, and a history of falls may be reported.
- Cognitive or intellectual disturbance with psychomotor slowing, decreased attention and concentration, and apathy. The patients may appear depressed, but they lack depressed thought content.
- Urinary incontinence. Initially there is urinary urgency, and the gait disorder may impede getting to the bathroom in time. In later stages, the incontinence is accompanied by a lack of concern.
Other features of NPH may include decreased coordination, hand tremors, and slow movement, so that the appearance resembles Parkinson’s disease.
Diagnosis based on the complaints and clinical presentation can be difficult.
Gait disorders are very common in the elderly and include both neurologic and non-neurologic conditions, for example stroke, arthritis of the spine, vestibular dysfunction with dizziness, degenerative arthritis of hips or knees, visual impairment, and peripheral nerve damage.
Other conditions that can cause cognitive impairment (and gait abnormality) should be considered for the elderly patient suspected of having NPH, for example Alzheimer’s disease and a series of underlying, silent strokes. It is also a problem that NPH can coexist with Alzheimer’s disease, and the two conditions may be present in the same patient.
DIAGNOSTIC TESTS
It is necessary initially to exclude other conditions to support the diagnosis of NPH. Those patients likely to respond to the treatment of NPH should be identified soon in the evaluation. Most importantly, the patient should be examined by a neurologist. Testing is done to evaluate whether there is a pathological impairment of the intellect, using cognitive exams.
Blood testing, review of medications, and family history are important, too. Brain imaging can be done with magnetic resonance imaging (MRI) or computed tomography (CT). MRI is superior to CT for evaluation of NPH. However, a normal CT excludes NPH and is most appropriate for screening purposes. With a normal CT of the brain, there is no further concern about NPH.
The most significant finding on imaging is large ventricles (ventriculomegaly) without other signs of brain shrinkage, such as atrophy, which is a small brain in a normal skull. The neurologist will be looking to determine between the images show a brain with ventriculomegaly as opposed to diffuse brain shrinkage (sometimes called hydrocephalus ex vacuo.)
MRI of brain with marked shrinkage & hydrocephalus ex vacuo
MRI is sensitive to detect blockage of CSF outflow that might cause obstructive hydrocephalus, as opposed to communicative hydrocephalus. There may be signs associated with Alzheimer’s disease. MRI will show large or small strokes, as well as atherosclerotic changes, often described by the radiologist in terms such as: “non-specific small-vessel ischemic white-matter changes, perhaps small infarcts, of uncertain chronicity.”
MRI of brain showing NPH and atherosclerotic abnormalities
LUMBAR PUNCTURE (LP): Diagnostic & Therapeutic
LP is a simple, outpatient procedure that helps to select patients for the specific treatments of NPH (please, see the section on my website about Lumbar Puncture). CSF is removed in the amount of at least 20 ml, up to 40 ml. Changes in gait and cognition are noted in the hours and days after the procedure. Feedback from the patient and family regarding any subjective improvement over the days is important.
Improvement following LP suggests that the patient may have a good outcome after placement of a ventriculo-peritoneal (VP) shunt (see below). Most studies suggest that LP has excellent positive predictive value (above 90 percent), but only limited negative predictive value (30 to 50 percent) with some patients who show no response to removal of CSF but later do improve with surgery.
However, and not surprisingly, there has been at least one report that LP does not add predictive value over clinical and radiographic criteria, and, therefore, may be an unnecessary test. Most neurologists and neurosurgeons find that some type of removal of cerebrospinal fluid is important before considering brain surgery for NPH.
Lumbar drainage is an alternative method to a simple LP, involving continuous CSF drainage (at a rate of 5 to 10 mL/hour) via a temporary catheter in the lumbar CSF space. The drain is placed by a neurosurgeon, an anesthesiologist or a radiologist, but not usually by the neurologist.
Clinicians, the patient, and family members evaluate the clinical response over an observation period in the hospital of two to seven days. Meningitis and subdural hematoma are potential but serious complications of this test, increasing with prolonged lumbar drainage. The complications are rare, but occurred in 2 of 38 patients in one study.
A variety of techniques to evaluate flow of the CSF in the brain has been studied. The purpose is again to select the patients with likely NPH that will respond to treatment.
Cisternography is the injection of a radio-labeled isotope in the lower back by the method of an LP. The needle is placed in the subarachnoid space of the lumbar cistern well below the spinal cord; the distribution of the marked or tagged CSF can now be visualized as it flows fairly slowly through the brain. Imaging in the Nuclear Radiology Department is done at set time periods following the injection, usually at 4, 24, 48, and 72 hours.
NPH is suggested by the continued presence of the isotope at 72 hours, rather than the clearance of it. If the isotope disappears more quickly, the patient will not be called back for later imaging. The initial injection is done by the neurologist, an anesthesiologist, or by an interventional radiologist. The imaging is interpreted by a nuclear radiologist.
Once again, the test has some limitations in predicting surgical success. Complications are very rare and similar to those of simple LP (see section on Lumbar Puncture on this website).
TREATMENT
The treatment of NPH is ventricular shunting. Medication therapies are not effective. Nevertheless, a medical approach is usually also favored in the demented patients, or in a patient who recovering from surgery. They will receive one or several of the medications recommended for Alzheimer’s disease, since this condition often appear also to be present in many patients with NPH.
Most shunts divert cerebrospinal fluid (CSF) from a catheter in the lateral ventricle in the brain into the abdomen (ventriculo-peritoneal shunt). Flow of CSF is controlled via a one-way valve; several models are in use. A programmable valve has been developed that allows pressure adjustments without reoperation. If the patient does not respond clinically to the shunt, the pressure setting can be changed to increase drainage of CSF.
Another treatment option: a series of lumbar puncture can provide temporary benefit during the times that surgical options are considered by the patient and the family. However, repeated high-volume lumbar punctures is not a proven alternative to surgery, and there are no published studies to prove the benefit of this approach.
Gait impairment is probably most responsive to shunting. Urinary incontinence improves in most patients even though it is a late symptom. Cognitive impairment is somewhat more resistant to treatment, particularly if severe at the time of intervention. Complete resolution of all symptoms happens only infrequently after shunting.
The clinical studies on the efficacy of shunting vary in criteria for selection of patients and in the assessment of the pre- and postoperative symptoms. Comparisons with a control group are generally not done. Subsequently, the outcomes are also highly variable. It is difficult for the neurologist and the families to understand exactly what to expect after the shunting.
Disappointingly, progression of neurological deficits may still occur, in part due to the continued aging process, often in combination with another underlying dementing process than NPH, as well as the occurrences of other systemic illnesses that will affect cognition and the general stamina of the elderly patient.
FACTORS THAT PREDICT POOR RESPONSE TO SHUNT PLACEMENT:
- severe dementia
- long duration of symptoms (2-3 years)
- late-presentation
- no gait disorder
- extensive white matter disease
- cortical atrophy
SHUNT COMPLICATIONS
Complications from shunt placement occur at a fairly high rate. Symptoms are variable, and can include malaise, nausea, headaches, confusion, imbalance, vomiting, fever, abdominal swelling. More significant ones with new neurological deficits may occur in 5% of the patients.
SHUNT REVISIONS: are done in approximately 50% of the patients within 6 years. The surgical procedure should be done by a neurosurgeon who has past experience with NPH.
After surgery the patient should be monitored closely by the surgeon and the neurologist. If there is clinical deterioration, CT of the brain (not necessarily MRI) is done to assess for worsening hydrocephalus, development of a hematoma, or change in catheter position. Withdrawal of spinal fluid will assure that an infection has not developed. Patency of the entire shunt system will be tested.
MRI of brain with shunt catheter in right posterior lateral ventricle
PROGNOSIS
There are few studies that have followed the patients many years after shunting. Those studies are in some disagreement about the extent of the benefits. It is clear that a large proportion of patients, maybe 50%, do not have sustained benefit beyond one or two years. However, the effects of aging and of an underlying Alzheimer’s disease would cause a progressive deterioration in the patients.
Leonardo da Vinci, ca. 1492
