Pathogenesis and Pathophysiology. NPH is the result of an imbalance between production and resorption of the CSF, usually around the brain convexities. Whereas there is full communication between the ventricles and the subarachnoid space (communicating hydrocephalus), the communication between the subarachnoid space and the arachnoid villi and granulations is not intact, so fluid is not transferred efficiently to the superior sagittal sinus. Subarachnoid hemorrhage, meningitis, head trauma, and elevated levels of CSF protein all cause thickening of the arachnoid or obstruction of the subarachnoid space. A relatively large number of patients do not have an identifiable cause for hydrocephalus, and, pathophysiologically, their lesion best fits with reduced resorption of CSF, based on studies of increased resistance to CSF outflow. y Less frequently, intracranial tumors, stenosis of the aqueduct of Sylvius, and arachnoid cysts (causes mainly discussed in the previous section) can cause NPH due to obstruction within the ventricular system, and normal pressure is maintained as a result of apparent compensations.
In spite of its name, it appears that intracranial pressure in NPH is not always normal. Physiological experiments suggest that a transient elevation of pressure may increase ventricular size and a new fluid balance is reached with normal pressure but with a higher force, based on Pascal's law of pressure in fluids. Other mechanisms may include abnormal ventricular distensibility, increased CSF pulse amplitude, and increased transmantle pressure gradients between the intraventricular and subdural spaces that lead to expansion of the ventricles.
Epidemiology and Risk Factors. NPH has its highest prevalence in the late middle-aged and elderly groups, and is rare in patients younger than 60 years of age. On the other hand, neonates, infants, children, adolescents, and young adults can develop primary or secondary NPH. It accounts for 5 percent of demented patients in the older age group, but its prevalence in the general population is unknown.
Clinical Findings and Associated Disorders. The triad of progressive dementia, gait disturbance, and urinary incontinence was originally described by Adams and colleagues.^ In most instances, gait disturbance is the first sign, followed by dementia, and, later, urinary dysfunction. The gait is slow, unsteady, and wide based. Steps are usually short, and patients have difficulty picking their feet off the ground to ambulate (so-called magnetic gait). Turning is difficult and takes several steps. On examination, there is bradykinesia, and the legs may be spastic with increased reflexes. Patients may have difficulty in handwriting and dressing, and may appear to be mildly parkinsonian, but their tremor, if present, is usually postural, not resting. One particular feature is the discrepancy between leg function during walking and simulated walking when sitting. Patients can usually move their legs well and imitate walking while in a chair, but they become awkward and severely impaired as soon as they attempt to walk. This dyspraxia for gait eliminates a pyramidal lesion.
Cognitive decline is clinically multidimensioned and involves impaired attention, poor memory, and poor executive function. y Apathy and bradyphrenia are common, whereas aphasia and upper extremity limb dyspraxia are uncommon.
Neuroanatomically, the gait dysfunction and urinary impairment are believed to relate to the stretched fibers innervating the legs and sphincters that project through the vicinity of the frontal horns of the ventricular system. Compromised microcirculation, due to increased intraparenchymal pressure, may contribute to the cognitive impairment. Positron-emission tomography (PET) data suggest that glucose utilization defects are widespread in NPH and involve subcortical and cortical regions.
Evaluation and Differential Diagnosis. Because the clinical picture of NPH is not specific, especially in the younger patient, several diagnostic tests play a crucial diagnostic role. CT or MRI scans are important to establish the presence of hydrocephalus. The diagnosis of NPH is supported when the anterior ventricular horns measure greater than 30 percent of the diameter of the cranial cavity and the inferior horns are wider than 2 mm. On T 2 - weighted MRI, increased signal in the periventricular areas occurs due to presumed transependymal exudation of CSF. Other findings can suggest alternate diagnoses: for example, a normal-sized fourth ventricle with dilated third and lateral ventricles usually indicates aqueductal stenosis.
Clinically, dementing illnesses of all types must be differentiated
from NPH (see Chapter33 ). Alzheimer's disease is not associated with marked gait dysfunction, and a patient with multi-infarct dementia or Binswanger's disease may have a similar clinical presentation to a patient with NPH but has MRI signs of multiple strokes. In younger patients, in whom the classic clinical triad may not be met, the differential diagnosis is wider.
A lumbar puncture usually reveals normal CSF pressure. Furthermore, removal of 20 to 50 ml of CSF may cause clinical improvement in cognitive and gait dysfunction supporting the diagnosis of NPH. Intermittent pressure B- waves detected by intracranial monitoring devices suggest decreased brain compliance and NPH. Radioisotope cisternography is used most often but is not particularly specific. Whereas in normal subjects isotope injected intrathecally can be seen around the brain convexity within 48 hours, reflux into the ventricles and stasis beyond 48 hours are often seen with NPH. However, this finding has relatively low predictive accuracy in the diagnosis of NPH and the response to therapy by shunting procedures. y
Management and Prognosis. The primary treatment is large-volume lumbar puncture and eventual ventricular shunting. However, selection of patients for shunting is controversial; most reports agree that dementia of less than 2 years' duration and typical gait and urinary dysfunction should be considered for shunting as long as there is no evidence for a multi-infarct state on MRI scans. Medical management can include acetazolamide or digoxin to decrease CSF production. The rate of complications of CSF- diverting procedures is approximately 30 percent, with 5 percent being serious with long-term sequelae. y Subdural hematomas, intracranial infections, stroke, and failure of the shunt are the major complications. The proportion of patients who experience long-term benefit from this treatment range from 25 to 80 percent.
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