Reference paper: Ultrasound-guided cervical centesis to obtain cerebrospinal fluid in the standing horse Anthony Pease, Ashley Behan, George Bohart Vet Radiol Ultrasound 2012 53(1) 92-5
In some cases a cerebrospinal fluid analysis is used to support a presumptive diagnosis of equine protozoal myeloencephalitis. The presence of antibodies against S. neurona is considered, by some, as evidence that parasites are in the central nervous system. Research done so far on on oocyst challenge infections fails to support this view. We believe that the inflammatory component of the EPM syndrome should not be ignored and it is unlikely that CSF fluid analysis will alter a treatment decision in a horse suspected of EPM.
We found that ataxic horses with serum antibodies against S. neurona respond best to specific treatment of the protozoa and the inflammation associated with the infection. The positive treatment response in these EPM-suspect horses is as high as 91%. We found that ataxic horses that don’t have serum antibodies to S. neurona don’t respond as well to antiprotozoal treatments, closer to 65% positive treatment response. When ataxic horses are identified as idiopathic (IE, the cause of the ataxia is unknown), based on absence of serum antibodies against SAG 1, 5, 6, the benefit of treatment is 87%. The treatment of these IE horses does not involve an anti-protozoal agent. These results indicate serum antibodies are highly useful in the selection of cases to treat with anti-protozoals, when to treat for inflammation, and more important, when to look for another cause of disease.
Defining the treatment population (of ataxic horses) using serum antibodies increases the selection of cases with an expected positive outcome. It has not been shown that CSF analysis will change the decision to treat or predict the outcome of treatment in these suspect EPM cases. In fact, one placebo controlled, blinded, challenge study examined the benefit of CSF analysis and we found this test did not (statistically) enhance the diagnosis.
There is a need to define cases that are associated with S. neurona and those that are unassociated with protozoa; for some clinicians a CSF tap may be the defining test. Testing CSF fluid will help us obtain the data to make an assessment of the utility of CSF analysis in field selection of treatable cases of EPM in a statistically meaningful study. We don’t charge for antibody determination on CSF fluid when it is submitted with a serum sample for SAG 1, 5, 6 testing. The veterinarian-signed paperwork must accompany the submissions for the no charge CSF testing.
Resistance to field collection of CSF are justified. The lumbosacral space is used for CSF collection in the standing horse. The limits of this procedure are technical expertise, blood contamination, and the distance from the cranium, the proposed site of the lesion. In the field the risk of trauma to the veterinarian is great due to the close proximity of the clinician to the rear limbs.
Alternatively, a sample can be obtained from the atlanto-occipital space under general anesthesia. Experience with obtaining an AO tap is a plus with this technique. The risk of injury during recovery from general anesthesia in an ataxic horse is high. And transporting an ataxic animal is not advised.
The ultrasound-guided cervical centesis was used in normal horses in the above referenced paper and may be a viable alternative for field veterinarians that want a sample from an ataxic horse. A brief survey of veterinarians indicates that this was not a procedure that is in common use for acquiring a CSF sample from an ataxic horse, therefore we are reviewing the procedure here. The hypothesis is that obtaining a tap at C1-2, with assistance from ultrasound, may provide an alternative to collection of CSF in a standing horse. The horses used in the Pease study were in stocks. The horses were sedated with detomidine hydrochloride followed by morphine. The area is aseptically prepared prior to the sample collection. A nose twitch is used when the needle is placed.
A 10-4 MHz, microconvex curvilinear transducer (oriented dorsoventrally) is placed at the level of C1-2. An 18g, 3.5 inch needle with stylet is advanced to the dura mater, in a dorsomedial direction approximately 4 cm through the dura mater (with the stylet) and into the subarachnoid space. The CSF sample is then collected with gentle suction (changing syringes after the initial 5 ml decreases blood contamination). Post collection observations are conducted for 48 hours. We have a demonstration video of this procedure available on line. Call for information.