Cerebrospinal fluid (CSF) collection in dogs: to tap or not to tap?

Author

When faced with a neurological case, every clue matters. Cerebrospinal fluid (CSF) collection is often considered a key diagnostic step, offering insight into conditions ranging from infectious and immune-mediated diseases to neoplasia. But is a spinal tap always the right choice?

In this article, Cecilia Danciu, DVM MVetMed PhD DipECVN MRCVS, Lecturer in Veterinary Neurology at the University of Liverpool’s Small Animal Teaching Hospital, explores the role of CSF analysis in veterinary neurology, outlining when it can be invaluable, how it’s performed, and the risks clinicians need to weigh before proceeding.

What is the cerebrospinal fluid?

The cerebrospinal fluid (CSF) is a plasma ultrafiltrate, that surrounds the brain, the spinal cord and fills the ventricular system of the brain. It offers physical support of the neural structures, aids in intracerebral transportation, excretion, and control of the chemical environment of the central nervous system (CNS).

Where is the CSF produced?

Most of the CSF is produced in the choroid plexus of the third and fourth ventricles, then in a lesser extent in the pia-arachnoid vessels, ependymal lining of the ventricles and in the CNS parenchyma.

Where is the CSF absorbed?

The CSF has a predominantly caudal flow, and it gets absorbed in the arachnoid villi, cerebral veins and the in the lymphatic vessels of the cranial and spinal nerves.

Why do we want to collect CSF?

In order to reach a definitive neurological diagnosis, histopathology is required. Taking biopsies of the CNS is invasive and associated with risks. However, performing spinal tap to collect CSF, generally, is considered a relatively safe and non-invasive procedure and can give information on the presence of pathology within the CNS:

infectious: i.e., protozoal, fungal, bacterial meningoencephalitides
non-infectious inflammatory: i.e., immune-mediated meningoencephalitides (meningoencephalitis of unknown aetiology, steroid-responsive meningitis-arteritis)
and neoplastic processes: i.e., round cell tumours such as lymphoma or histiocytic sarcoma

Despite this, presence of changes in the CSF does not correlate with the severity of the lesion. Additionally, absence of CSF changes does not rule out inflammatory or neoplastic disease.

How is the CSF collected?

The decision to proceed with CSF collection is based on the evaluation of clinical and imaging findings, alongside careful consideration of the likely diagnostic yield and overall risk–benefit for the individual patient. The collection of CSF is always performed by a trained veterinarian after a Magnetic Resonance Imaging (MRI) scan, to interpret findings and to aid in decision making.

The CSF is collected under general anaesthesia using an aseptic technique. Because, the CSF has a predominant caudal flow, the aim is to collect caudally to the lesion. An exception to this can be a cervical lesion, where a tap performed in the cerebellomedullary cistern may be of a higher yield. The two locations for the CSF collection would include:

The cerebellomedullary cistern which is located in the dorsal subarachnoid space at the level of the first two cervical vertebrae (atlas and axis)
The lumbar subarachnoid space

For the latter, the CSF collection is performed between L5-L6 in dogs and L6-L7 in cats. A 22 or 20-gauge, 1.5-inch spinal needle will usually reach the subarachnoid space in most dogs and cats. The maximum CSF that can be collected is approximately 1 ml/5 Kg body weight, all by gravitational drop through the spinal needle. The CSF can be collected in plain collection tubes for quantitative analysis, which would include total nucleated cell count, red blood cell count, protein concentration and cytological analysis. The analysis is aimed to be performed within 30 minutes to one hour from collection. Additional testing can be performed (collected in EDTA tubes for ELISA, PCR and biomarkers) as required.

Is it always safe to collect CSF?

It is not always safe to collect CSF!

One contraindication would include the patient being unstable under general anaesthesia. In this scenario, prioritizing patient stability and potentially rescheduling the CSF collection is advisable. Evidence of coagulopathy is another contraindication, as this can result in the development of subcutaneous, epidural or subarachnoid bleeding.

Another contraindication would include suspected or confirmed instability affecting the vertebral column (i.e., atanto-axial instability, or fracture/luxation). Collection of CSF is contraindicated if there is evidence of increased intracranial pressure, that could potentially result in transtentorial or foramen magnum herniation (Figure 1).

Another reason why CSF collection could be contraindicated includes pyoderma at the site of needle placement, because this way infectious agents could be introduced in the subarachnoid space or CNS.

Cerebrospinal fluid (CSF) collection in dog and cat brains — **Figure 1.** A) T2-weighted (W) sagittal MRI image demonstrating a normal dog brain. B) T2-W sagittal MRI image of a 9-year-old male neutered cat diagnosed with a brain meningioma. The cat is having imaging sings of increased intracranial pressure characterised by marked caudal transtentorial herniation of the parahippocampal gyrus (within yellow circle) and foramen magnum herniation of the uvula of the cerebellum (white arrow). C) T2-W sagittal MRI image of the brain of a 9-year-old female neutered cross breed dog who was diagnosed with meningoencephalitis of unknown aetiology with secondary oedema characterised by a multifocal, ill-defined, intra-axial T2-W hyperintense lesion at the level of fronto-parietal area (amber asterisks). This lesion is causing secondary mass effect into caudal transtentorial herniation of the parahippocampal gyrus (amber circle), and moderate compression of the cerebellum and caudal displacement of the cerebellar vermis (amber arrowhead).

Which complications have been associated with CSF collection in dogs?

Minor complications associated with CSF collection have been reported in dogs and include the development of a subcutaneous hematoma in approximately 1%, local dermatitis in approximately 0.6%, or the inability to collect CSF in 7.8% when performed in veterinary referral practice.

More severe complications have been infrequently reported in single case reports, including progressive myelomalacia and hematomyelia after lumbar CSF collection in dogs.

The importance of CSF studies

In a larger study, 0.15% of dogs experienced a major complication from over 7000 CSF collections performed in a veterinary referral practice setting. Complications included cardiopulmonary arrest or severe neurological deterioration leading to death or euthanasia. Clinical signs that were more commonly picked up in these dogs, included obtundation and multifocal neuroanatomical localisation. Changes that were observed on the MRI studies before the CSF collection, included effacement of the cerebral sulci suggesting expansion of brain parenchyma and dilatation of the ventricular system, suggesting increased intraventricular pressure. Repeat MRI after the complication could aid in understanding in the reason of deterioration (Figure 2).

Figure 2. MRI of the brain of a 3.5-year-old male neutered Standard Schnauzer that encountered a major complication following CSF collection (failed to recover spontaneous ventilation). A) T2-weighted (W) sagittal image showing dilation of the third ventricle (white arrows) with reduction of size and angulation of the interthalamic adhesion (white asterisk). Flow artefact is seen in the mesencephalic aqueduct and fourth ventricle (white arrowheads). B) T2-W sagittal image obtained post-CSF collection from the same dog: the third ventricle appears reduced in volume with expansion of the interthalamic adhesion (within the yellow circle) and foramen magnum herniation of the caudal aspect of the cerebellar vermis (yellow arrowhead). All lesions are described relative to normal grey matter (Figure reference: Danciu CG at al., Major Complications Associated With Cerebrospinal Fluid Collection in 11 Dogs: Clinical Presentation and Imaging Characteristics. JVIM, 2025).

CSF collection: key takeaways

The frequency of complications following CSF collection remain low, but when it occurs, the mortality rate can be high. Therefore, prior MRI scan and careful individual evaluation of each patient is indicated, to reduce the risks associated with CSF collection in dogs.

References

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