Sunday, 27 April 2014

Blocked VP Shunts

Normal Patho-physiology
- CSF produced at 0.35 ml/min (20 ml/hour or 500 ml/24 hours)
- Recycled over three times each day
- Capacity of normal lateral and third ventricles is approximately 20 ml
- ICP rises if production of CSF exceeds absorption. CSF production falls as ICP increases.

Shunts
Most shunting systems drain according to the differential pressure gradient between the ventricle and the tip of the distal catheter. Most neurosurgeons use medium pressure valves, that will drain CSF continuously if the differential pressure is over about 10 mm Hg. The ventricular catheter of a shunt is normally inserted through a burr-hole in the right parieto-occipital region and the valve will sit usually behind the right ear. The distal catheter is tunnelled subcutaneously down to another incision in the abdomen where it is then placed into the peritoneal cavity. It is not usually helpful for non-neurosurgeons to palpate or flush the shunt valve, as their contours and characteristics are so variable as to make interpretation notoriously inaccurate.

Blocked shunts:
blood or debris in the proximal site (choroid plexus within the ventricle can get stuck in it
fracture of the tubing along its course
infection of the tubing along its course
something like a piece of omentum getting stuck in the distal port in the abdomen

Hydrocephalus features
Young adults:
- Symptoms—headache, vomiting, failing vision, drowsiness, “muzziness of the head”, fatigue
- Signs—papilloedema, enlarged blind spots on visual field analysis or reduced visual acuity, failure of upward gaze, general clumsiness, dyspraxic gait, large head

Older adults/elderly:
- Symptoms—slowing of mental capacity, unsteady on feet/frequent falls, incontinence, drowsiness, headaches less frequently
- Signs—gait dyspraxia (slow, hesitant shuffling gait), dementia (reduced mini-mental score), rarely papilloedema

Clinical features of shunt malfunction:
Drowsiness and general malaise
Headaches
Vomiting
Papilloedema with or without failing vision
Occasionally failure of upward gaze
Neck stiffness
Thoracic back pain in patients with spina bifida
Abdominal tenderness or distension

Investigations
CT scan has sensitivity of 80% and shunt series (plain films of neck and abdo) has sens of 20% but you still need both.
Head CT (to help define if there is hydrocephalus vs overdrainage)
Shunt Series (to image the apparatus for obvious kinks and breaks)Peripheral blood for C reactive protein, white cell count if there has been any recent surgery
ICP monitoring/lumbar infusion test


http://jnnp.bmj.com/content/73/suppl_1/i17.full
http://adc.bmj.com/content/87/3/198
http://emergencymedicineireland.com/2012/04/raised-icp-and-intracranial-shunts-some-notes/
http://pedemmorsels.com/tapping-a-vp-shunt/

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