Hydrocephalus, Normal Pressure
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- Normal pressure hydrocephalus (NPH) is a clinical triad of gait instability, incontinence, and dementia (mnemonic: wet, wobbly, wacky); originally described by Hakim in 1957
- Two forms of the disorder: idiopathic and obstructive (the latter usually due to physical insult—posttrauma, meningitis, or subarachnoid hemorrhage)
- Absence of papilledema on clinical exam and normal CSF pressures at lumbar puncture
Idiopathic NPH (iNPH) primarily affects persons >60 years; extremely rare before 40 years
- Varies from 1.1–5.5% in various surrogate analysis of shunt data. Arriving at accurate data is hampered by uncertain diagnosis.
- iNPH form primarily affects elderly, at least >40 years of age.
- Secondary form can occur at any age.
- Male = female
- Ranges from 0.2% in the 70 to 79 years age group and increases to 5.9% in those 80 years and older. It is estimated that approximately 700,000 are affected in the United States (compared with 400,000 with MS) (1).
- Estimated to be a contributing factor in 6% of all cases of dementia
Etiology and Pathophysiology
- Idiopathic form is a communicating hydrocephalus, a disorder of decreased CSF absorption (not overproduction). In iNPH, the leading theory suggests that poor venous compliance impairs the subarachnoid granulations’ ability to maintain baseline removal of CSF. In secondary NPH, scarring is likely.
- The result is a pressure gradient between the subarachnoid space and ventricular system.
- CSF production decreases in the face of an increased pressure set point (but still in excess of the amount of CSF absorbed).
- Elevated pressure distends ventricles and compresses the brain parenchyma.
- As a result of compression, ischemic changes occur in the parenchymal vasculature with subsequent tissue damage and loss.
- Some believe that the idiopathic form is a result of persistently insufficient removal of CSF by immature subarachnoid granulations from childhood.
- Secondary NPH may result from the following:
- Head trauma (most common)
- Subarachnoid hemorrhage
- Resolved acute meningitis
- Chronic meningitis (tuberculosis, syphilis)
- Paget disease of the skull
- Idiopathic risk is unknown (case reports suggest a possible genetic link but unsubstantiated).
- Secondary form is due to head trauma, subarachnoid hemorrhage, meningitis, or encephalitis.