Description

• 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)
• There is an absence of papilledema on clinical exam and normal cerebrospinal fluid (CSF) pressures at lumbar puncture

Epidemiology

• iNPH form primarily affects elderly, at least >40 years of age.
• Secondary form can occur at any age.
• Estimated to be a contributing factor in 6% of all cases of dementia

Incidence
Varies from 1.1% to 2.2%. Arriving at accurate data is hampered by uncertain diagnosis (2).

Prevalence
Ranges from 1.3% in the >65 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 multiple sclerosis) and 2,000,000 in Europe (2).

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, disrupting the normal cerebral blood flow with resulting ischemic changes leading to 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

Risk Factors

Secondary form is due to head trauma, subarachnoid hemorrhage, meningitis, or encephalitis.