Etiology
The underlying etiology is still contentious. Many theories have been proposed, including:
Abnormal absorption of cerebrospinal fluid (CSF)
Buildup of toxic metabolites in the CSF
Abnormal arterial pulsatility compressing the venous vasculature.
While often quoted in the literature, these theories are considered implausible as they do not take into account the physiologic function of CSF.
Pathophysiology
The pathophysiologic mechanism is contentious. The most plausible mechanism involves restriction of blood perfusion of the basal ganglia.
Because fluid is incompressible, blood entering the skull must be balanced exactly by fluid leaving the skull; otherwise there will be no net flow of blood, only a change in intracerebral pressure. The shape of the venous pulse wave as blood leaves the skull is very different from that of the arterial pressure wave during the cardiac cycle. In order to balance the difference between arterial blood entering and venous blood leaving the skull, CSF moves in and out of the skull during the cardiac cycle. Thus, bidirectional movement of CSF is essential in order to maintain efficient blood flow to the brain.[10][Figure caption and citation for the preceding image starts]: Approximately 0.5 mL of cerebrospinal fluid is produced by the choroid plexus and is reabsorbed at the arachnoid granulations every minute. However, more important is the bidirectional flow of CSF at the foramen magnum, which in a healthy individual peaks at 4 mL every second and facilitates efficient perfusion of blood in the brainPersonal collection of Richard Adam Grünewald [Citation ends].
In aging patients, especially those with risk factors for vascular disease, there is a gradual increase in pulse pressure in the major arteries as they become impregnated with atheroma and calcify, which renders the vessels less elastic. Abnormal arterial compliance is characteristic of patients with NPH.[11] The increased pulse pressure of arterial blood in the aorta is transmitted to the arteries supplying blood to the brain. This augmented pulsatility increases the mismatch between arterial supply of blood to, and venous drainage of blood from, the cranium, and increases the rate at which CSF must flow in and out of the cranium in order to maintain blood flow to the brain.
In some individuals, especially those with congenitally smaller aqueducts (i.e., high CSF outflow resistance) and/or higher arterial pulse pressure of the cerebral vasculature, CSF movement gradually becomes the limiting factor in cerebral perfusion. This accounts for the observed increase in CSF flow in patients with NPH and the relationship between CSF outflow obstruction and response to CSF shunting.[12][13]
Inefficient brain perfusion results in the development of ischemia in the parts of the brain with the most tenuous arterial blood supply: the periventricular end arteries and basal ganglia. Dysfunction of these parts of the brain produces the characteristic clinical features seen in NPH.
Classification
Related conditions
Communicating hydrocephalus:
A term that encompasses all causes of hydrocephalus not associated with structural abnormalities of the cerebrospinal fluid pathways, and that includes NPH.
Idiopathic (benign) intracranial hypertension:
A syndrome characterized by raised intracranial pressure without motor, cognitive, or urinary symptoms of hydrocephalus, or abnormality on structural imaging of the brain.
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