Neurology - Hydrocephalus Flashcards
What is hydrocephalus?
CSF is secreted by the choroid plexus epithelium. Adults have a total volume of CSF of around 140ml, this volume is renewed several times a day.
CSF resorption occurs primarily by the arachnoid villi. Hydrocephalus is a term that means there is an excess quantity of CSF within the cranial cavity.
How is hydrocephalus classified?
Hydrocephalus is classified as either primary (usually accompanied by increased intracranial pressure) and secondary hydrocephalus (occurring due to loss of cerebral tissue).
What is primary hydrocephalus?
Primary hydrocephalus includes any disorder in which the accumulation of CSF is usually accompanied by an increase in intracranial pressure. It can be due to:
- obstruction to CSF flow (non-communicating hydrocephalus)
- impaired CSF absorption at the arachnoid villi (rare; communicating hydrocephalus)
- excess CSF production by choroid plexus papilloma (very rare)
What is obstructive hydrocephalus?
Obstructive hydrocephalus is caused by blockage to CSF flow within the ventricular system and is the most common cause of primary hydrocephalus. The impaired flow causes ventricular dilatation proximal to the block.
Obstructive hydrocephalus is either congenital or acquired. Congenital causes include Chiari I and Chiari II malformations (the later associated with spina bifida) and Dandy Walker syndrome (atresia of the foramina of Lushka and Magendie).
Acquired hydrocephalus can be caused by any lesion that obstructs the CSF pathway. Expanding lesions in the posterior fossa are prone to cause hydrocephalus as the 4th ventricle and aqueduct are easily obstructed. Others include colloid cyst, blood clots, inflammatory exudates or aqueduct stenosis caused by viral infection.
What is communicating hydrocephalus?
This is another cause of primary hydrocephalus, but usually results from a failure of CSF absorption due to inflammatory processes, or malignancy in the subarachnoid space.
Causes include SAH, meningitis, and carcinomatous meningitis. Very rarely, a CSF secreting choroid plexus papilloma can cause communicating hydrocephalus through overproduction of CSF.
What is normal pressure hydrocephalus (NAH)?
This usually occurs in the elderly and patients have normal intracranial pressure. It is characterised by the triad of dementia, ataxia and urinary incontinence.
It may be secondary - i.e. a low grade communicating hydrocephalus, or idiopathic associated with cerebrovascular disease in the elderly.
What are associated risk factors for hydrocephalus?
There is a high incidence in patients with spina bifida with open myelomeningocele - 80% require a shunt.
Number of genetic syndromes.
Late deterioration in functioning following head injury can occur due to post traumatic hydrocephalus.
Describe CSF production and reabsorption.
CSF is produced in the cerebral ventricles at a rate of approximately 500-750ml/day. 80% is secreted by the choiroid plexus (composed of vascular pia and epdenymal cells) and 20% by transependymal bulk flow of brain interstitial fluid.
CSF flows through the ventricular system beginning in the lateral ventricles. These are connected to the 3rd ventricle by the foramen of monroe. CSF then flows from the 3rd to the 4th ventricle via the cerebral aqueduct (of Sylvius) before exiting via the lateral foramina of Lushka and the medial foramen of Magendie to enter the subarachnoid space and cisterns.
CSF absorption occurs via the arachnoid granulations, nerve sheaths, nasal lymphatics and through “reverse bulk flow” into the brain capillaries.
How is CSF related to ICP?
CSF production is independent of ICP but the absorption of CSF rises linearly with ICP. Thus, if flow is obstructed or if scarring in the subarachnoid space results in reduced reabsorption, CSF production results in continualy expansion of the ventricles and raised ICP.
What symptoms of hydrocephalus can patients present with?
In most cases, you will encounter patients who already have a diagnosis of hydrocephalus but where the treatment has failed and they re-present with recurrent symtpoms.
Presentation depends on age group:
Infants:
- abnormal head growth with crossing centiles on charts
- irritability, vomiting, apnoeic episodes and delayed development
Children and younger adults:
- symptoms of raised intracranial pressure: headache, nausea, vomiting and visual disturbances
- seizures: these are relatively uncommon and can be confused with “hydrocephalic attacks” - abrupt collapse with loss of consciousness due to abrupt rise in ICP
Elderly:
- triad of NPH
Shunt infection:
- if a shunt has been inserted in the last year there is a risk it can become infected
- ask about history of fever, abdo pain, and redness along the course of the shunt as it passes subcutaneously
Examination findings in a patient with hydrocephalus
Again, depends on age group.
Infants:
- head circumferance over 98th centile, tense or “full” fontanelle, dilated scalp veins
- loss of upgaze or “sunsetting” (usually due to obstructive hydrocephalus) due to downward pressure on the tectum
Children and younger adults:
- signs of raised ICP (reduced GCS, headache, papillodema, bilateral VI nerve palsy)
- gait ataxia
- visual field defects/ acuity defects
Elderly:
- usually do NOT have signs of raised ICP, typically gait ataxia and cognitive impairment only
How should hydrocephalus be investigated?
CT head:
- look for enlargement of the ventricles (Evan’s ratio > 30%)
- pattern of ventricular enlargement can help identify the level of obstruction (e.g. in aqueduct stenosis, the IV ventricle is small but the III and lateral ventricles are large)
- VP shunt catheter is visible as a white linear structure
