“Spinal Tap” Testing for Horses: A Veterinarian’s Guide to Cerebrospinal Fluid Analysis

Neurologic disease in horses often presents a diagnostic challenge, as clinical signs can be vague or non-specific. A valuable tool veterinarians use to diagnose neurologic cases is cerebrospinal fluid (CSF) analysis, also known as a “spinal tap.”

By directly assessing the biochemical and cytologic environment of the central nervous system (CNS), CSF analysis offers insight into disease processes that cannot be obtained through routine bloodwork or imaging alone.

CSF samples are typically collected from either the lumbosacral or atlanto-occipital spaces, two reservoirs of cerebrospinal fluid that are relatively easy to access. The choice of site depends on the horse’s clinical condition, practitioner experience, and available resources.

Once obtained, CSF is evaluated for clarity, color, protein concentration, and cellular content. This information provides important clues about the nature of the neurologic disorder. CSF analysis is best integrated with a neurologic examination, advanced imaging when available, and targeted serologic or molecular testing to reach a definitive diagnosis.

The following article addresses common questions related to CSF analysis in horses, including indications for testing, collection techniques, interpretation of findings, and the role of CSF in the broader diagnostic workup of equine neurologic disease.

Cerebrospinal Fluid Testing for Horses

Cerebrospinal fluid (CSF) is the clear fluid that surrounds the horse’s brain and spinal cord. It provides cushioning and physical protection for delicate nervous tissues in the event of trauma. ^[1]

CSF also helps transport signaling molecules, nutrients, and other essential substances that support normal nervous system function. ^[1]

Diseases affecting the brain or spinal cord can cause changes in the composition of the CSF, which veterinarians can assess through fluid analysis. ^[1]

Veterinarians may collect CSF as part of the diagnostic process for neurologic diseases in a procedure commonly referred to as a spinal tap. The results can provide important clues about the underlying condition when interpreted alongside other diagnostic tests such as imaging, bloodwork, and additional laboratory analyses. ^[2]

Signs of neurologic disease in horses that may warrant cerebrospinal fluid analysis include: ^[1]

• Ataxia (uncoordinated movement)
• Weakness
• Collapse
• Behavioral changes
• Seizures
• Blindness
• Paralysis of the limbs or tail
• Severe lethargy
• Fever of unknown origin

Equipment

CSF collection, commonly referred to as a “spinal tap,” requires sterile equipment to prevent introduction of bacteria or foreign material into the subarachnoid space, the space where CSF sits.

Equipment commonly used for an equine CSF tap includes: ^[3]

• Spinal needles up to 20 cm (8 in) long
• Sterile gloves
• Local anesthetic to numb the needle insertion site
• A syringe to collect fluid

Depending on the location of CSF collection and the practitioner’s preference, they either place the horse under general anesthesia or standing sedation.

If using general anesthesia, they need a large, open space with a non-slip floor to lay down the horse and recover them safely. For standing sedation, a set of stocks or another method of restraining the horse are necessary. ^[3]

Procedure

Veterinarians typically collect CSF from either the horse’s atlanto-occipital joint or lumbosacral space. The procedure for the two locations differs.

Atlanto-Occipital Joint

The atlanto-occipital (AO) joint is the joint between the skull and the first cervical (neck) vertebra. ^[1] Most veterinarians perform AO CSF collection under general anesthesia. ^[1]

After anesthetizing the horse, the veterinary team clips the horse’s poll and performs a surgical scrub to remove bacteria and other foreign contaminants from the skin. ^[1] They flex the horse’s head inward to open the atlanto-occipital joint as much as possible, then the veterinarian inserts a long needle into the joint space. ^[1]

Some veterinarians may use ultrasound to help guide the needle into the joint space. Once the needle enters the joint, CSF should flow out of the needle. ^[1] The team uses a syringe to collect the fluid for analysis.

Once the sample is collected, the veterinary team carefully allows the horse to wake up from general anesthesia. The recovery period has a high risk of horses injuring themselves or their handlers as they attempt to stand on unsteady feet, especially if their condition is complicated by neurologic deficits. ^[1]

Lumbosacral Space

The lumbosacral space is an area between the last lumbar (lower back) vertebra and the sacrum, a fused set of vertebrae in the lower back. ^[1] Collection of CSF from this location can be performed under standing sedation. ^[1]

After sedating the horse, the veterinary team prepares the needle injection site by clipping and thoroughly cleaning the area. As the horse is only sedated, they typically inject local anesthetic under the skin in the area of the injection site to minimize the horse’s reaction to the collection process. ^[1]

Once the site is ready, the veterinarian introduces a large spinal needle into the area. They may use ultrasound guidance in some cases. Horses may react strongly to the needle entering the joint space by flicking their tail, stomping their feet, or kicking out. ^[1]

With the needle in the lumbosacral space, the veterinarian uses a syringe to withdraw CSF for further analysis. ^[1]

Interpreting Results

After fluid collection, veterinarians examine several aspects of the fluid’s composition to make a diagnosis.

Features assessed include: ^[2]

• Appearance
• Protein concentration
• Cell counts
• Biochemical composition
• Infectious disease testing

Interpreting the results of CSF analysis can be challenging, as analysis frequently returns false negative results.

Reasons why a CSF test may return a negative result in the presence of neurologic disease include: ^[1]

• Timing of the collection relative to the progression of the disease
• Sample taken in a location distant from the site of origin of the disease
• The disease process does not produce significant changes in CSF composition

For these reasons, CSF analysis is best for detecting inflammatory diseases that may cause tissue death in the brain or spinal cord. These diseases are most likely to cause a widespread change in CSF composition that is detectable on analysis.

Identifying inflammatory disease can help veterinarians narrow down their list of potential diseases to come to a diagnosis. ^[1]

Appearance

Healthy CSF is clear and colorless. ^[2] Samples with high protein or cell content may become cloudy, indicating there is an abnormality. A color change towards yellow may indicate previous bleeding into the subarachnoid space. ^[2]

Causes of xanthochromia (yellow hue) include: ^[2]

• Parasitic infections, such as equine protozoal myeloencephalitis
• Viral encephalitis, such as Eastern equine encephalomyelitis
• Traumatic injury to the nervous system
• Bleeding disorders

Protein Concentration

There is typically very little protein within the CSF. If protein levels increase, it may indicate damage to the blood-brain barrier, the filtration system that prevents blood components from entering the CSF. ^[2]

This type of damage can occur due to inflammatory, infectious, or traumatic diseases affecting the nervous system. ^[2] Elevated protein may also indicate tissue degeneration, activation of the immune system, or blocked CSF flow in some cases. ^[2]

Cell Counts

Cell counts involve counting the numbers of different types of cells within the fluid sample under a microscope. ^[2]

Three different values are reported on a cell count: ^[2]

• Total cell count: A count of all cells present in the sample
• Red blood cell count: Total number of red blood cells
• Differential cell counts: Counts of individual types of cells, such as specific white blood cells like lymphocytes and neutrophils

Normal equine CSF has low cell counts. If total cell count increases, it indicates there is an inflammatory, infectious, or traumatic disease present. From there, differential cell counts can help further narrow down the list of potential causes by identifying which cells are increased. ^[2]

Monocytes are a cell population found in low numbers in healthy CSF. They are responsible for immune monitoring of the fluid and identify any foreign contaminants early. ^[3] Elevated monocyte counts can occur when there is any disturbance to the CSF, including traumatic injuries, infections, or inflammation. ^[3]

After a traumatic injury, diagnosticians may note erythrophagia in monocytes. This is where the monocytes consume red blood cells as part of the clean-up process after bleeding into the subarachnoid space.

Neutrophils in the CSF typically indicate there is an active infection. These immune cells primarily target bacteria and fungi, however their numbers can also increase with viral infections.

Causes of CSF neutrophilia (elevated neutrophils) in horses include: ^[2]

• Bacterial meningitis
• Abscesses of the brain or spinal cord
• Eastern equine myeloencephalitis
• Fungal infections

Lymphocytes produce antibodies that target specific disease-causing agents. ^[2]

Elevated lymphocyte counts are most commonly associated with infections, such as: ^[2][4]

• West Nile virus
• Rabies
• Equine protozoal myeloencephalitis
• Lyme disease

A large population of lymphocytes that look abnormal can also indicate lymphoma or leukemia. Elevated lymphocytes may also be a sign of polyneuritis equi. ^[4]

Eosinophils are immune cells that target parasitic or fungal infections. These types of nervous system infections are rare in horses, so eosinophils are not expected on CSF cell counts. ^[4]

Biochemical Composition

The concentrations of enzymes, electrolytes, pH, and other biochemical compounds can be evaluated on CSF fluid. For example, decreased glucose (sugar) in the CSF is an indicator of meningitis. ^[4]

These tests can provide additional information that helps guide the diagnostic process.

Infectious Disease Testing

Veterinarians can submit CSF for infectious disease testing. These tests either identify the infectious agent itself, or antibodies against the agent. ^[4]

Infectious disease tests commonly performed on CSF include: ^[4]

• Bacterial culture: Diagnosticians place the CSF sample on a bacterial growth medium and see if bacterial colonies form. They can then identify the species of bacteria.
• Serology: The laboratory introduces the infectious agent to the sample, then identifies whether there are antibodies against the organism present. If antibodies are present, it indicates there was a previous exposure to the organism. It does not necessarily indicate active infection in the horse.
• M-capture serology: This test specifically looks for IgM, antibodies that are produced early in an infection. IgM should not be present in the subarachnoid space. If any is detected, it indicates an active infection in the horse.
• PCR: This test looks for fragments of an infectious organism’s genetic material. PCR tests are available for equine herpesvirus, equine protozoal myeloencephalitis, and Lyme disease.

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Aftercare for Horses Following CSF Collection

Following the CSF sampling procedure, your veterinarian may have specific aftercare instructions for you to follow.

The needle insertion site must be kept clean and dry to reduce the risk of infection. Additionally, pain medication may be recommended for your horse to help ease discomfort and reduce swelling.

Horses undergoing an atlanto-occipital collection may require stall rest and elevated feeding to help keep them from lowering their head too frequently. Grazing or standing with the head lowered appears to increase neck soreness after the procedure. ^[1]

Complications

Complication rates after CSF collection have not been widely studied, but are considered rare. ^[1][3] The most common complications are pain and swelling at the site of needle entry, and injuries associated with recovery from general anesthesia. ^[1] Some horses’ neurologic signs get worse after the procedure, but return to baseline within a few days.

Rarely, CSF collection can introduce foreign materials into the subarachnoid space, potentially causing bacterial infections and septic meningitis. ^[1]

For this reason, veterinarians use aseptic technique when performing a CSF collection. This includes surgically preparing the needle insertion site and using sterilized needles and equipment during the procedure.

Frequently Asked Questions

Here are some frequently asked questions about cerebrospinal fluid (CSF) analysis in horses:

Summary

Cerebrospinal fluid (CSF) analysis is a key diagnostic tool for evaluating neurologic disease in horses, offering information that complements clinical examination and other diagnostics.

• CSF analysis helps detect inflammation, infection, trauma, hemorrhage, or neoplasia within the central nervous system
• Collection can be performed at the lumbosacral (standing sedation) or atlanto-occipital (general anesthesia) site
• Normal equine CSF is clear, colorless, and has low protein and cell counts
• Testing includes gross appearance, protein levels, cytology, and advanced diagnostics like PCR or antibody titers
• CSF results should always be interpreted in conjunction with the neurologic exam, imaging, and serology for accurate diagnosis