Lumbar puncture
From WardWiki - Foundation Doctor Helper
A lumbar puncture is most commonly performed to investigate suspected meningitis or subarachnoid haemorrhage. When using surface anatomy, anatomy in children WILL differ from adults. according to age so consider LP for children, as for any procedure, to require separate training.
Contents |
Indications
This is more frequently done in the younger as the work up for fever of unknown origin. Major indications include:
- Meningitis
- Subarachnoid haemorrhage
- Hydrocephalus
- Benign intracranial hypertension
- Intrathecal injection
- Carcinomatous meningitis
- Medulloblastoma.
Contraindications
Included in this list are some signs of raised intercranial hypertension. Except in cases of bacterial meningitis, a CT head is usually performed prior to LP to prevent coning.
- Prolonged or focal seizures
- Focal neurological signs (including ocular palsies)
- Widespread purpuric rash in ill patient - Give antibiotics and stabilise patient first and correct any coagulopathy. When safe to do so carry out an LP
- Glasgow Coma Scale Score < 13
- Pupillary dilatation
- Impaired ocular-cephalic reflexes
- Abnormal posture
- RICP – inappropriately low pulse, elevated BP and irregular respirations (indicating impending brain herniation)
- Coagulopathy
- Current anticoagulation (warfarin or heparin etc)
- Papilloedema
- Uncontrolled Hypertension
- A history of spina bifida or skin appearances of spina bifida occulta (hair patch over lumbarregion)
- Local infection or pressure sore over lumbar area
Do not delay treatment in possible meningococcal septicaemia or meningitis in order to perform an LP.
Preparation
Explain procedure and reason for LP to patient. Always tell the patient the possible complications including headache and infection. It is worth mentioning that when the LP needle reaches the correct place, it is extremely common for a patient to experience a sharp shoot of pain down one leg; patients need to be reassured prior to the procedure that if this happens they should let the operator know and that it does not mean something has gone wrong. The patient needs to be aware that the needle will not enter the spinal cord. It is almost impossible to put a needle through one of the nerve roots and the pain is typically caused by the needle touching the nerve root (e.g. like a cold drink touching an exposed nerve root in a bad tooth). Give the patient a copy of patient information sheet. Make sure you are clear what samples need to be sent and have relevant tubes ready and inform microbiologist/lab of intended procedure.
Patient positioning
The patient is best positioned lying on their side, at the edge of the bed, flexed, with the spine horizontal and perpendicular to the couch throughout its entire length. It is important to ensure the patient is as flexed as possible, but shoulders should be perpendicular to couch, and knees and ankles should be symmetrical and together. Careful attention to positioning and explanation to patient before starting significantly increases the chance of success.
Cleaning
To clean the skin, chorhexidine or iodine aqueous solution may be used. Start in the centre and wipe in circular motion to outside area. Iodine is neurotoxic so be careful to wipe the skin with a swab after its use.
Finding the site
The needle is usually introduced at L3/4 interspace which is indicated by a line drawn joining the tips of the iliac crests. (In adults the spinal cord usually ends at the lower border of L1 so a needle inserted into the sub-ararachnoid space below this level will enter the sac containing the cauda equina floating in CSF).
Please use local anaesthetic. Local anaesthetic is used for the skin and immediate tissues. Up to 5ml of 2% lignocaine should be infiltrated starting with an orange needle subcutaneously to infiltrate skin, waiting for it to take effect and then infiltrating deeper with a green needle. Aspirate prior to injection of lignocaine to ensure not in blood vessel or CSF.
Needle insertion
- Please use local anaesthetic. Local anaesthetic is used for the skin and immediate tissues. Up to 5ml of 2% lignocaine should be infiltrated starting with an orange needle intradermally / subcutaneously to infiltrate skin, waiting for it to take effect and then infiltrating deeper with a green needle. Aspirate prior to injection of lignocaine to ensure not in blood vessel or CSF.
- A sharp disposable fine LP needle (e.g. gauge 22) with a stylet in position is introduced through the skin and advanced through the space between the two spinous processes. The top of the bevel should always be parallel to the back (eg for a patient on their side, the bevel will point to the sky). The needle point needs to be directed slightly forwards (anteriorly).
- At a depth of about 4-7 cm more firm resistance may be encountered as the ligamentum flavumis reached. Beyond this there is a slight ‘give’ as the needle punctures the dura. The stylet is removed and clear CSF will drip out of the needle if this has been correctly positioned.
- Once in position and CSF is obtained, the stylet should be placed in the centre of the sterile trolley and kept sterile as it will need to be reinserted prior to taking the LP needl eout at the end of the examination.
- Measure and record CSF pressure (in lying position) using manometry.
- If no fluid appears or bone is encountered, it is probable that the needle is not in the correct position. The stylet should be reinserted, the needle partially withdrawn and then advanced with a slightly different angle.The commonest causes of failure are that the needle is not in the midline, the patient’s back is not perpendicular to the bed (eg twisted at shoulders, or legs not together) or is at too great an angle with the skin.
- If unsuccessful after 2 attempts, it is advised that another doctor tries. If that is unsuccessful, X-ray screening may be used. If LP is to evaluate presence of xanthochromia, then further attempts need to be performed at this time and not delayed by more than 2-4 hours, otherwise altered blood may be found as a consequence of traumatic tap and the test becomes unhelpful if xanthochromia is found.
Sample collection
- It is good practice to take 4 tubes and to fill them each by at least 5ml. Up to 40-50ml CSF can be safely removed during a lumbar puncture, assuming there are no contraindications to performing this in the first place. All bottles need to be carefully labelled with correct patient details. They should be numbered 1 to 4 according to the order they were filled.
- Send CSF for basic testing for microscopy and culture, protein, paired glucose (blood + CSF) – fluoride bottles), and cytology. If there are focal neurological symptoms or signs and there is any possibility of CNS infection. If there is diagnostic doubt it is often helpful to ask the lab to put a sample aside temporarily in case further tests are needed. In this situation, a paired serum tube may also be saved.
Special situations
Sub arachnoid haemorhage
- If a subarachnoid haemorrhage is suspected take 3 sequential tubes of CSF to establish whether the fluid is uniformly blood stained or whether initial blood stained CSF clears as a result of a traumatic tap. When a subarachnoid or other haemorrhage is suspected the CSF should always be analysed by spectrophotometry to look for xanthochromia which indicates pathological bleeding rather than a traumatic tap. It is important not to perform the LP within 12 hours of the suspected bleed, as time is required to assess presence of xanthochromia.
- NEVER proceed to LP on a suspected SAH without a CT head: SAH can cause raised intercranial pressure that can lead to coning on LP.
Meningitis
- If infection is suspected the CSF sugar must be measured with a paired serum glucose taken at the same time. In bacterial and fungal meningitis the CSF sugar is usually < 2 mmol/l or <40% blood glucose level. In bacterial and TB meningitis CSF lactate is raised > 3.3 mmol/l. Lactate samples need to go the lab on ice and it is important to inform the lab if they are going to be sent so that they can analyse them quickly. If there are a significant number of red cells from a traumatic tap as a guide about 10 white cells/7000 red cells would normally be expected. Paired blood samples need to be taken on the same day to evaluate OCB’s. The CSF needs to be examined for bacteria, fungi, cryptosporidium or mycobacterium tuberculosis (with AAFB testing) as indicated clinically. TB PCR can also be checked but false positives may sometimes be seen. The immunodetection of specific bacterial antigens and antibodies can also aid diagnosis, particularly when the patient has already been on antibiotics. Latex agglutination can detect Haemophilus influenzae B, Strep pneumoniae and Neisseria meningitidis in >75% of affected patients. For suspected encephalitis, samples are sent for viral PCR (e.g. HSV, VZV, CMV, etc.).
Autoimmune and neoplastic
- Specific abnormalities of CSF protein occur. Patients with MS or other forms of CNS inflammation often have oligoclonal bands of gamma globulins on electrophoresis. To detect oligoclonal bands, a paired serum sample is needed, taken on the same day as the LP. The synthesis of immunoglobulins in the central nervous system may occur in other inflammatory/infective processes. Send CSF for cytology for possible malignant cells if indicated (a minimum of 10mls is required in a single tube labeled for cytology).
The lab should be contacted to ensure that cytology is carried out there and then; samples left unattended or overnight may be useless for purposes of cytology. If malignant meningitis is suspected, it is important to perform up to 3 LP’s, each with at least 10mls dedicated for cytology (i.e. if negative on first attempts). Samples may also be sent for ACE levels if sarcoidosis is suspected and VDRL (only if blood tests positive and syphilis suspected).
Finishing the procedure and aftercare
When the CSF samples have been collected, it is essential to replace the sterile stylet before removing the LP needle. Failure to replace this is one of the main causes of post LP headache. If iodine has been used to clean the skin, it is good practice to swab the LP site with a saline soaked gauze to remove iodine, if left it may cause skin rashes or burns.
There is no good evidence to suggest that lying down following an LP is helpful at preventing post LP headache though it is common poractice for a patient to lie down for four to six hours post LP. Good hydration is sensible though this need not be excessive. Oral caffeine is not to be recommended as it does not prevent post LP headache and may actually induce perpetual caffeine-related headache in those predisposed. If a post LP headache occurs, this will be a headache that is very severe and always occurs soon after sitting or standing – it should completely disappear on lying flat (or significantly diminish if patient already had headache prior to LP). Whilst this may settle, if it continues then the longer that treatment is delayed, the less chance of success. Analgesics are to be avoided as they may perpetuate headache. An infusion of IV fluids is the most appropriate first line management of post LP headache (e.g. 500mls normal saline over 2 hours). An ECG is recommended prior to this infusion and patients should be warned of potential for palpitations to occur. If this is unsuccessful, it may be repeated before considering a blood patch as second line treatment.
Potential Complications
- Low pressure headache – postural, worse erect, occur c. 10%
- Backache
- Introduction of infection - this should be minimal with good aseptic technique
- Precipitation of coning with a cranial or spinal mass lesion or any cause of raised intercranial pressure; often lethal.
- Subarachnoid or epidural haemorrhage (increased risk with anticoagulants, bleeding disorders)
- Cranial nerve palsies – diplopia from CN VI
- Lumbosacral nerve palsies (extremely rare)
- Dermoid formation
- Insufficient information obtained (e.g. failure to measure opening pressure, failure to ask for sample to be saved in case needed).
Interpretation of results
See main article: Cerebrospinal fluid
