Makindo Medical Notes.com

The key to LP is positioning the patient so that the vertebrae open up to allow the needle

About

• The cord ends at L1 so below this the spinal canal holds the cauda equinae lower sacral nerve roots which should not be harmed by a needle. The top of the iliac crest is at L3/4 is a good marker of the level of the L4 interspace where to insert the needle.
• A CT head is not required before all Lumbar punctures but has more or less become a standard. The concern is that removal of CSF leads to movement caudally of the brainstem through the foramen magnum which leads to pressure on the medulla and possibly death.
• CT scan is therefore needed if any symptoms suggest raised ICP - headache, papilloedema or suggest focal neurology

Physiology

• CSF is formed from the choroid plexuses in the lateral, third and fourth ventricles. The epithelium of the choroid plexus forms a barrier between blood and CSF filtering out various substances.
• Flow is from the lateral ventricles through the foramen of Munro into the third ventricle and then through the aqueduct of Sylvius into the fourth ventricle.
• From there it drains through one of three foramina either laterally through the foramina of luschka (lateral) or the medially placed foramen of Magendie.
• The CSF then enters the subarachnoid space. It is finally absorbed by the arachnoid villi into the venous sinuses.
• Obstruction at the level of the aqueduct causes non-communicating hydrocephalus.
• Obstruction to CSF flow at the level of the arachnoid villi causes communicating hydrocephalus.
• Lumbar puncture is not contraindicated, in fact, it may be diagnostic as well as therapeutic in those with communicating hydrocephalus where pressures are freely transmitted through the subarachnoid space.

CSF and Ependymal cells

• CSF is produced by the action of the Na⁺/K⁺ ATPase pump actively moving Na⁺ ions into the lumen of the ventricles which is then followed by H₂.
• This happens in the cuboidal cell epithelial cells that form the choroid plexus.
• The choroid plexus releases about 500 mls of CSF per day into a volume of 100-150 ml.
• The choroid plexus selectively filters the CSF so that its content differs from plasma.
• CSF has lower glucose, very much lower protein, low urea and a low amino acids content.
• CSF protein is 1/1000th that of CSF. CSF means that the brain basically floats such that the effective weight of the brain is reduced from 1400 g to about 50 g.
• The CSF also plays a role in the continual drainage of the ventricular cavities and subarachnoid space.

Technique

• The patient, with a pillow under the head, should curl into a fetal position, placing the lumbar spine in maximal flexion. The patient's back should be at the edge of the bed.
• The site is cleansed with iodinated solution, applied in a circular fashion followed by a careful application of isopropyl alcohol or a chlorhexidine based solution to wash off the iodinated solution.
• Using an orange/blue needle, create a skin wheal with 1% Lignocaine at the insertion site. Infiltrate the subcutaneous and interspinous areas with up to 3 mls of 1% lidocaine solution.
• The bevel is kept pointing up if the patient in the left lateral position to keep the tap as atraumatic as possible so as to cause minimal CSF leak and post LP headache.
• Replace the stylet fully into the spinal needle before withdrawing the needle. Apply pressure on the site with gauze and then apply a band-aid. Place the patient in a comfortable position.
• A delayed CT does not exclude SAH and if suspected an LP should always be done not sooner than 6-12 hours after headache onset looking for xanthochromia - counting cells is inaccurate

Samples

• Tube 1 - cell count
• Tube 2 - gram stain and culture (C+S)
• Tube 3 - glucose and protein
• Tube 4 - cell count (for comparison to Tube 1) to assess if blood
• Tube 5 - PCR

Preventing post LP headache

• Using an atraumatic needle, replacing stylet before withdrawing needle
• Using lateral decubitus position, Orientated bevel

Complications

• Local infection
• Coning
• Accidental puncture of aorta or IVC causing retroperitoneal haematoma
• Post LP headache/Low pressure headache

Exclude symptoms and Signs suggestive of raised ICP: Get CT/MRI

• Severe headache, Vomiting, Rising BP, Papilloedema
• Morning headache, Decreasing level of consciousness
• Focal neurology which could suggest an SOL

Contraindications

• Suspected raised ICP and incipient coning (Check with CT if concerned)
• Obstructive (Non communicating hydrocephalus)
• Bleeding diathesis, Localised skin infection/inflammation

Causes of Increased CSF protein

• Guillan-Barre syndrome (WCC normal), Acoustic neuroma
• Tuberculous meningitis (Lymphocytes)
• Fungal meningitis (Lymphocytes)
• Bacterial meningitis (Neutrophils)
• HSMN III (Dejerine Sottas) levels > 10 g/L
• Multiple sclerosis (elevation of IgG in CSF)
• Viral encephalitis, Cerebral abscess
• Sarcoidosis, Cerebral vasculitis
• Neurosyphilis, Subdural haematoma, Malignancy
• Froin's syndrome - blockage to CSF outflow possibly related to a malignancy

Oligoclonal IgG bands in the CSF

• Multiple Sclerosis, Systemic lupus erythematosus, Neurosyphilis
• Neurosarcoidosis, Behçet's syndrome
• Subacute sclerosing panencephalitis - related to measles infection

Low glucose

• Bacterial meningitis, Tuberculous and fungal infections, Sarcoidosis
• Meningeal dissemination of tumour s

Causes of CSF lymphocytosis

• Viral meningitis - typically well beforehand
• Listeria meningitis - soft cheese + pregnant, neonates, immunocompromised needs Ampicillin
• Tuberculous meningitis - protein high and person in at-risk group with systemic signs
• Syphilitic meningoencephalitis, Fungal meningitis, Lyme disease

Causes of CSF low glucose

• Bacterial meningitis, tuberculous meningitis, fungal meningitis
• Subarachnoid haemorrhage, atypical cases of viral meningitis

Interpretation of CSF findings: Bloody tap subtract 1 WCC for every 1000 RBCs

References

• EBM Lumbar Puncture Procedure
• Ottawa Lumbar Puncture Procedure