The cerebrospinal fluid (CSF) is an ultra-filtrate of plasma, produced mostly by the choroid plexi within the ventricular system. It surrounds the brain and spinal cord, flowing caudally from the ventricular system through the central canal of the spinal cord toward the cauda equina.
Indications for CSF collection:
- Encephalopathies – any disease affecting the brain, including seizures. Infectious and non-infectious inflammatory disorders can lead to an accumulation of different types of cells in the CSF and affect the amount and type of present proteins. Degenerative, metabolic, traumatic and neoplastic brain lesions may also alter the CSF.
- Any spinal cord lesion or myelopathy that is not diagnosed on spinal imaging. Focal, multifocal or diffuse lesions can lead to changes in the CSF.
- Lesions that affect the spinal nerve roots (radiculopathies) may also alter the CSF because the meninges enclose the distal nerve roots until they become peripheral nerves.
CSF collection is relatively safe when performed with care and appropriate precautions. But it is an invasive procedure with certain significant risks and contraindications:
- CSF tap is contraindicated in cases of suspected increased intracranial pressure, intracranial haemorrhage, severe hydrocephalus, severe cerebral oedema and large intracranial masses.
- Any general anaesthesia contraindication is also a contraindication for CSF tap.
- Cisternal tap is contraindicated in cases of atlantoaxial luxation or other cervical vertebral instability (danger associated with head positioning).
- Elevated intracranial pressure due to intracranial lesions (particularly space-occupying lesions, severe inflammatory disease or following head trauma) increases the risk of brain herniation at CSF tap. Mannitol, corticosteroids and hyperventilation may decrease the risk and CT or MRI should be performed before a CSF tap when elevated intracranial pressure is suspected.
- Accidental puncture of the parenchyma at the cerebellomedullary angle may lead to brainstem trauma and temporary signs of brainstem disease. Vestibular dysfunction may take a long time to subside or even be permanent.
One millilitre of CSF per 5 kg body weight can be safely removed at once in most patients, but the sample of 1-1,5 ml is enough for a complete analysis. A 22- or 20-gauge (larger patients) spinal needle is used to collect CSF by allowing it to drip from a needle into a sterile tube without anticoagulant. CSF should NEVER be aspirated with a syringe attached to the needle! Aspiration may cause a rapid decrease in the CSF pressure and trigger intracranial haemorrhage or cerebral and/or cerebellar herniation. EDTA may cause falsely elevated protein concentration or falsely low cell concentration in small samples. It is bactericidal and may interfere with culture results in cases of bacterial infection. But because EDTA helps preserve cellular morphology, also both – plain and EDTA samples are useful to collect. EDTA will also prevent clothing in haemorrhagic samples.
The procedure requires general anaesthesia and a non-collapsing endotracheal tube should be used to avoid occluding airflow during the cisternal tap. The patient is placed in lateral recumbency and the area of a tap is shaved and aseptically prepared.
There are two sites for CSF collection. Because CSF flows in cranial to caudal direction, it is more likely to be abnormal when collected caudal to the lesion. For brain and cervical diseases, the cisternal tap is more representative, and in lesions involving the spinal cord or canal, the caudal lumbar region should be chosen.
Cisternal tap (cerebellomedullary cistern, atlanto-occipital site)
This is the most common site of CSF collection in small animals; the collection from this site is easier, usually results in a larger sample and less iatrogenic blood contamination than lumbar tap.
Patient’s neck is fully flexed by an assistant. The animal’s nose is elevated parallel to the table. The spine of the axis and the external occipital protuberance should be in line. Anatomic landmarks used for cisternal tap are the occipital protuberance, the cranial aspect of the spine of the axis and the transverse processes (wings) of the atlas.
There are more ways to identify the atlanto-occipital space and location for needle insertion:
- Locate the cranial aspect of the axis spine with an index finger, then press and advance finger cranially; a cranial aspect of the arch of the atlas can usually be palpated as a ridge approximately one-third of the distance between the cranial aspect of the axis spine and the external occipital protuberance. Inserting the needle just cranial to the ridge should allow entry into the atlanto-occipital space.
- Palpate a triangle of landmarks formed by the occipital protuberance and the most prominent points of lateral wings of the atlas. The location for needle insertion is in the middle of the triangle (on the midline between the wings of the atlas, one-third to one-half of the way caudal to the occipital protuberance).
- Another method is to draw an imaginary line across the cranial limits of the wings of atlas and a perpendicular line from the external occipital protuberance caudally. Insert the needle at the intersection of these lines.
The needle is positioned perpendicular to the neck and advanced slowly 1-2 mm at the time. Observe for CSF flow every few millimetres – once the subarachnoid space has been entered, CSF will appear in the needle hub. If blood is obtained, a few drops of fluid should be allowed to flow. If the fluid clears, it can be collected, if not, the needle should be removed, and the procedure repeated. If the needle hits bone, redirection of the tip slightly cranial or caudal may allow entry into the dorsal subarachnoid space.
Lumbar tap (caudal lumbar site)
Lumbar tap is usually performed at the L5/L6 space (possible also L4/L5 in large dogs and L6/L7 in cats). The patient’s pelvic limbs are advanced cranially to flex the lumbar spine and open up the interarcuate space. The vertebral spinous process immediately cranial to ilial crests is that of L6. The spinal needle is inserted on the midline, just cranial to the L6 vertebral spinous process, at a 45°angle with the needle point directed cranially. The needle is advanced the same as in cisternal tap. After the interarcuate space is entered, the needle will pass through the cauda equina, which can elicit a muscle twitch of the pelvic limbs and/or tail. The needle is advanced to the floor of the vertebral canal and the fluid is collected from the ventral subarachnoid space. Although the spinal needle penetrates the spinal cord during the lumbar tap, this doesn’t appear to cause clinical problems.
Dewey C. W., Da Costa R. C.: Practical guide to Canine and Feline Neurology, 3rd Ed. Wiley Blackwell, 2016
Platt S. R., Olby N. J.: BSAVA Manual of Canine and Feline Neurology, 3rd Ed. BSAVA, 2004