Normal pressure hydrocephalus

Pathophysiology

CSF is produced by the ependymal cells of the choroid plexus in the lateral ventricles

CSF flows via foramina of Monro >> third ventricle via cerebral aqueduct >> fourth ventricle via foramina of Luschka and Magendie >> basal cisterns >> subarachnoid space >> arachnoid granulations>> absorbed into sagittal sinus.

NPH requires a “communicating hydrocephalus” on imaging, i.e. no macroscopic narrowing of the communicating structures (foramen of Monro, aqueduct of Sylvius, fourth ventricle).

NPH can confusingly be primary of secondary. If used without a qualifier, NPH is almost always referring to primary NPH:

  • Primary: Idiopathic (i.e. no identifiable cause)
  • Secondary: previous IVH/SAH, scarring from meningitis (especially TB meningitis), leptomeningeal carcinomatosis

The result is ventricular enlargement with a normal LP opening pressure (when collected in the lateral decubitus position – 5-18 mmHg/ 70-245 mmH2O).

Despite normal pressure, a pressure effect is thought to act on periventricular white matter tracts fibres of corona radiata.

Epidemiology

Age usually > 60 yrs. Slight male preponderance.

Symptoms/Signs

  • Gait difficulty – usually referred to as a “magnetic gait”, referring to the patient’s feet being stuck to the floor, similar to the freezing seen in patients with Parkinson’s disease. Importantly symptoms are likely to be bilateral, distinguishing it from idiopathic PD.
  • Cognitive disturbance – usually presents as deficits in psychomotor behaviours and “bradyphrenia”. MMSE may not be sensitive to these deficits. Considered to be frontal deficits with apathy prominent in later stages.
  • Urinary urgency, progressing to incontinence.

Diagnosis

Differentials include:

  • normal ageing brain +/- delirium
  • Alzheimer’s disease
  • Obstructive hydrocephalus
  • Lewy body dementia
  • Parkinson disease

Firm diagnosis cannot be made, but rather patients are categorised as either probably, possible or unlikely NPH.

MRI head: enlarged lateral and third ventricles out of proportion to sulcal enlargement (thus unlikely to be atrophy).

CSF flow studies may help in diagnosis: increased aqueductal CSF stroke volume.

Treatments

Referral to neurosurgery for consideration of VP shunt. Incontinence is the symptoms that is most likely to improve.

Note, complication rates of neurosurgery in these patients (who tend to be frail at baseline) are high (up to 35%) – patients are at high risk for subdural haemorrhage.