Hydrocephalus

Imbalance of CSF formation and reabsorption of sufficient magnitude to produce a net accumulation of fluid within the cerebral ventricle.

Background/Epidemiology

  • Hydrocephalus can be congenital (1:2000 births) or acquired (prevalence- 1-1.5%).

CSF volume in adults is around 150ml.  CSF production in a young adult is around 450ml/day.  Therefore, there are 3 full exchanges of CSF fluid in one day.

  • CSF production is an active process of specialised cells in the choroid plexuses of the lateral, 3rd and 4th ventricles.
  • CSF absorption is a passive process through arachnoid villi into the dural sinuses via bulk flow and facilitated diffusion (there is also a small amount of absorption from the nerve root sheathes, paranasal sinuses and lymphatics).

Aetiology/Causes

  • Congenital causes include X-linked cerebral aqueduct stenosis, spina bifida and Chiari hindbrain malformation (i.e. where there is a degree of cerebellar tonsillar herniation through the foramen magnum).
  • Acquired causes include:
    • blockage of the arachnoid villi due to meningitis; blood clots;
    • Blockage due to pressure effects of mass lesions e.g. tumours, abscesses, haematoma (particularly posterior fossa masses)
    • blockage of the dural venous sinuses due to pregnancy or pro-thrombotic states.

Classification

  • Communicating (or non-obstructive) hydrocephalus is due to blockage of CSF from outwith the ventricular system (usually due to impaired function of the arachnoid granulations and impaired CSF drainage)
  • Non-communicating (or obstructive) hydrocephalus is due to blockage of CSF from within the ventricles, commonly due to bleeding e.g. subarachnoid haemorrhage
  • NB In reality, both types are physiological obstructions (this is why communicating and non-communicating is preferably used).

NB see also Normal pressure hydrocephalus

Symptoms and Signs

  • Headache- worse on coughing
  • Vomiting
  • Diplopia
  • Papilloedema
  • Decreased GCS
  • Sunset eyes (caused by pressure on the superior colliculus which helps control eye movements)-
    • other eye problems (parinaud syndrome) include failure of eye convergence; convergence spasm; dissociation of pupillary near-light responses; supranuclear upward gaze palsy or down gaze palsy (the latter is usually a feature of sylvian aquaduct blockage) and nystagmus
  • In babies(congenital causes)
    • bulging fontanelle and splaying of the bony sutures (increased head circumference)

Diagnosis

  • On CT, an important measurement to know is the Evan’s ratio.  That is, the ratio of the width of the lateral ventricles to the width of the skull at that point is <0.4

Treatment

  • Removal of the source of obstruction should be done where possible (i.e. if there is an obstruction).
  • Drugs like acetazolomide and diuretics may have a small effect.
  • A ventricular tap or external ventricular drainage device may be inserted to more quickly reduce the pressure.
  • The most common procedure, however, is a ventriculo-peritoneal shunt.
    • Problems with shunts are that they can fail (block).  30% fail within 6 months and 50% within 2 years, the majority of which are mechanical failures.  However, infection can also be a cause (common organisms are Staph epidermidis; Staph aureus and gram negative organisms).
  • Very rarely, an endoscopic third ventriculostomy is performed (mainly for primary hydrocephalus that is causing problems).

Monitoring

ICP Monitoring

Invasive procedure that can measure the pressure of the CSF in the ventricles and the pressure in the brain.  It is useful for critical patients, but does carry the risk of infection, bleeding etc and it does not give instantaneous results (may take 2-3 days).

  • P waves are normal (P1-3)
    1. Percussion wave (arterial pressure)
    2. Tidal wave (intracranial compliance)
    3. Dicrotic wave (aortic valve closure)
  • Lundberg waves are always pathological
    1. ‘A’ waves or plateau waves are steep increases in ICP lasting for 5-10mins.  Represents reduced intracranial hypertension indicative of brain herniation.
    2. ‘B’ waves are unstable waves likely due to vasospasm

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