WORKSHOP: MOTOR EVOKED POTENTIAL TEST: TECHNIQUE AND

CLINICAL APPLICATIONS

How does a MMEP-test work?

Transcranial magnetic stimulation (TMS) and recording of magnetic motor evoked potentials

(MMEP) in ponies was already reported by Mayhew andWashbourne [1] but the technique

was standardized and reviewed by Nollet et al. [2] and Nollet et al. [3]:

In short, the principle of transcranial magnetic stimulation is as follows: A brief current pulse

passes through an insulated coil and induces a brief electromagnetic pulse. When the coil is

placed on the scalp, the electromagnetic pulse is capable of inducing a current within the

brain that leads to excitation of the descending motor tracts. The evoked responses in the

muscles (magnetic motor evoked potentials, MMEP) can be used to assess the functional

integrity of spinal cord motor pathways. On these MMEP, onset latency (= time between

stimulation of the brain and onset of the muscle response) and peak-to-peak amplitude (=

difference between the 2 biggest peaks of opposite polarity) can be measured. In recumbent

or ataxic horses with spinal cord disease, onset latencies are delayed and peak-to-peak

amplitudes are smaller, while the parameters remain normal in cases with orthopaedic issues

[4-6]. In this way, the MMEP test can help differentiating between neurological or orthopaedic

gait abnormalities or to confirm motor deficits in horses suspected of neurological issues.

Why would I use a TMS-MMEP-test?

Spinal ataxia is a common problem in European horses and most frequently caused by

cervical vertebral malformation (CVM). CVM causes static or dynamic compression of the

spinal cord and is commonly seen in warmblood horses and young, rapidly growing

thoroughbreds. However, the diagnosis remains challenging.

The clinical neurological examination is critical in the diagnostic work up, but remains

subjective. Even experienced observers might have a different interpretation of the clinical

symptoms of horses [7]. Furthermore, many studies highlight the controversy, difficulties and

limitations of cervical radiography, myeolography, computed tomography (CT), magnetic

resonance imaging (MRI) and myeloscopy to diagnose spinal cord disease in horses.

Sensitivities (47-50%) and specificities (70-78%) of cervical radiographs and sagittal ratio

calculations are too low for adequate diagnosis of spinal cord compression [8, 9] and

variation between observers is high [10]. Myelography also has a low sensitivity (43-85%)

and additionally requires general anaesthesia and intrathecal contrast injection [9, 11]. Most

CT and MRI scanners can only image the cranial cervical spinal cord because of the limited

diameter of the CT and MRI gantry. This is an important limitation since 37-54% of CVM

lesions occur in the caudal (C5-C7) cervical vertebral column [9, 11]. Furthermore, no flexion

or extension of the neck is possible in CT or MRI scanning [12-14]. Cervical vertebral canal

endoscopy is not routinely performed. There is a high risk of complications associated with

entering the spinal canal or due to neck movement during the procedure. The visual

assessment of subarachnoid space narrowing may not be reliable in cases with mild to

moderate stenosis. So, the very high sensitive diagnostic TMS-MMEP test could provide

interesting information about the functionality of the spinal cord.

Several studies have demonstrated the usefulness of the test in cases with confirmed spinal

cord disease already, but only the functionality of the motor tracts is evaluated. No

information concerning the sensitive function can be gathered. To test the sensitive tracts of

the spinal cord, somatosensory evoked potentials should be measured. With this test, an

electrical stimulus is given at the level of the limb and the provoked response is measured at

the level of the brain. However, the technique is not (yet) common practice in equine

medicine. In experimental circumstances, somatosensory evoked potentials are used to

evaluate the efficacy of analgesic drugs [15, 16] or to evaluate the involvement of the

trigeminal nerve in headshaking horses [17, 18], but the technique to evaluate ataxia as such

is not described.

An alternative to the transcranial magnetic stimulation to evoke MEP could be transcranial

electrical stimulation (TES). TES is thought to be a more robust technique with a reduced

onset latency compared to TMS and less sensitive to the depressive effect of sedatives and

anaesthetics. The technique is developed more recently [19] and normative data are

available [20]. As the electrical stimulation is more painful than the magnetic stimulation, the

horses need to receive a higher dose of sedation and a local subcutaneous ring block

anaesthesia but the results are similar to the magnetic stimulation.

How do I perform a MMEP test?

Equipment necessary:

Sedation: combination of detomidine and butorphanol (both at 10µg/kg)

Magnetic stimulator (output capacity of at least 4 Tesla) + connection cable to EMG

machine

Round 70 mm coil

Standard EMG equipment

Amplifier (with at least 2 channels is preferred for simultaneous measurements)

Electrodes (needle or surface)

o Monopolar, coated, disposable needle electrodes (25-37mm) + leads or

o Adhesive surface electrodes (for example those used for ECG recording in

horses) + leads

Technique:

For practical issues and to minimize discomfort for the horses and examiners, the horses are

sedated with a commonly used combination of detomidine (dose 10µg/kg) and butorphanol

(dose 10µg/kg). Nollet et al. [21] found that there were no significant differences in latency

and amplitude measurements made before and 10 or 30 minutes after sedation with this

combination. Other sedatives or anaesthetics can have a depressive influence on MMEP so

they should be avoided.

Figure 1: Surface electrode placement on the extensor carpi radialis (left) and tibialis

cranialis muscle (right).

For EMG recording, intramuscular needle electrodes or adhesive surface electrodes can be

used. Needle and surface electrodes result in similar latency times, but the surface

electrodes are better tolerated by the horses [22].Two electrodes are placed per limb. Both

are placed at the tibialis cranialis muscle in the pelvic limbs an at the extensor carpi radialis

muscle in the front limbs (figure 1). The ground electrode is attached in the groin or elbow

region respectively. The skin does not need to be clipped. If needle electrodes are used, the

skin is disinfected with alcohol before placement. If surface electrodes are used, the skin has

to be dry for optimal attachment of the electrodes. Depending on the number of channels

available, the connection leads can be put at the electrodes of 1, 2 (left and right side) or 4

limbs. The more limbs tested at the same time, the less magnetic stimulations are required.

Figure 2: Optimal coil positions for MMEP recording at the extensor carpi radialis and

tibialis cranialis muscle (from [3]).

To register the MMEP with the shortest latency time and the biggest amplitude, at least 4

stimulations are necessary. The most influencing factor to gather the best MMEP, is the coil

position. In general, the optimal stimulation site on the equine forehead is median, just

underneath the horizontal line between the base of the ears (figure 2) [3]. Stimulation output

is always set at 100%, being 4 Tesla. If lower intensities are used, latency times are longer

and amplitudes of MMEP are smaller. The direction of the current flow in the coil, the age or

the sex of the horses, or left or right side recordings do not have an influence on the results

in horses. Latency times do alter with height and weight of the horses and are significantly

different in pelvic and thoracic limbs [3, 23]. In human medicine, a lot of attention is paid to

the facilitation principle or the effect of voluntary contraction. It is known that slight voluntary

contraction of the target muscles shortens the onset latency, lowers the threshold and

increases amplitude of MMEP. In horses, it is impossible to standardize this pre-stimulation

muscle tension exactly, but it is tried by ensuring that the horses are standing square

(=weight baring equally divided on 4 feet) the moment of magnetic stimulation.

On the registered MMEP, onset latency and

amplitude values can be determined (figure

3). Onset latency is defined as the shortest

distance between the moment of stimulation

and the first deflection from the baseline

(representing the muscle contraction).

Latency is a valuable parameter with a low

variability. In normal horses, mean ±

standarddeviation of latency, using surface

electrodes, is 21.2 ± 1.4 and 39.2 ± 3.8 ms

for the extensor carpi radialis and tibialis

cranialis muscle, respectively [22]. Using intramuscular needle electrodes, Nollet et al. [23]

found 19.32 ± 2.50 and 30.54 ± 5.28 ms for latency time and 9.54 ± 3.73 and 6.62 ± 3.62 mV

for amplitude, respectively. In horses with mild to severe spinal cord disease, latency time

can be strongly prolonged to 27-100 (or even more) ms in the extensor carpi radialis muscle

and 53-150 ms in the pelvic limbs [5, 6]. Amplitude is measured between the two biggest

peaks of opposite polarity. The parameter is clinically less valuable than latency time and has

a larger variation. Mean ± s.d. of peak-to-peak amplitude in normal horses, using surface

electrodes, is 7.2 ± 4.7 and 3.8 ± 2.4 mV for the extensor carpi radialis and tibialis cranialis

muscle, respectively [22]. Horses with spinal cord disease will often have low amplitudes

(0.2-1.5 mV), in some cases even too low to be measurable [5]. If the thoracic limbs produce

normal MMEP but the pelvic limb MMEP are abnormal, a thoracic or lumbal lesion is

suspected. If MMEP are abnormal at all 4 legs, a cervical spinal cord lesion is suspected and

medical imaging can be used to confirm this.

Figure 3: Example of 4 magnetic motor evoked potentials recorded from the extensor carpi radialis muscle in a horse with indication of onstet latency and peak-to-peak amplitude (from [5])

In conclusion, a MMEP test is easy to perform, sensitive and well tolerated by the horses and

it provides interesting information about the motor function of the spinal cord. Therefore, it

can be considered as a valuable diagnostic tool for neurological examination in horses.

REFERENCES

[1] Mayhew IG ,JR Washbourne, Magnetic motor evoked potentials in ponies . J Vet Intern Med, 1996. 10; 5: 326-9.[2] Nollet H, L Van Ham, P Deprez, and G Vanderstraeten, Transcranial magnetic stimulation: review of the technique, basic principles and applications. Vet J, 2003. 166; 1: 28-42.[3] Nollet H, L Van Ham, J Dewulf, G Vanderstraeten, and P Deprez, Standardization of transcranial magnetic stimulation in the horse. Vet J, 2003. 166; 3: 244-50.[4] Nollet H, K Vanschandevijl, L Van Ham, G Vanderstraeten, and P Deprez, Role of transcranial magnetic stimulation in differentiating motor nervous tract disorders from other causes of recumbency in four horses and one donkey. Vet Rec, 2005. 157; 21: 656-8.[5] Nollet H, P Deprez, L Van Ham, F Verschooten, and G Vanderstraeten, The use of magnetic motor evoked potentials in horses with cervical spinal cord disease. Equine Vet J, 2002. 34; 2: 156-63.[6] Nollet H, L Van Ham, F Verschooten, G Vanderstraeten, and P Deprez, Use of magnetic motor-evoked potentials in horses with bilateral hind limb ataxia. Am J Vet Res, 2003. 64; 11: 1382-6.[7] Saville WJA, SM Reed, JP Dubey, DE Granstrom, PS Morley, KW Hinchcliff, et al., Interobserver Variation in the Diagnosis of Neurologic Abnormalities in the Horse. J Vet Intern Med, 2017. 31; 6: 1871-1876.[8] Levine JM, E Adam, RJ MacKay, MA Walker, JD Frederick, and ND Cohen, Confirmed and presumptive cervical vertebral compressive myelopathy in older horses: A retrospective study (1992-2004). J Vet Intern Med, 2007. 21; 4: 812-819.[9] Levine JM, PV Scrivani, TJ Divers, M Furr, IJ Mayhew, S Reed, et al., Multicenter case-control study of signalment, diagnostic features, and outcome associated with cervical vertebral malformation-malarticulation in horses. J Am Vet Med A, 2010. 237; 7: 812-22.[10] Hughes KJ, EH Laidlaw, SM Reed, J Keen, JB Abbott, T Trevail, et al., Repeatability and intra- and inter-observer agreement of cervical vertebral sagittal diameter ratios in horses with neurological disease. J Vet Intern Med, 2014. 28; 6: 1860-70.[11] van Biervliet J, PV Scrivani, TJ Divers, HN Erb, A de Lahunta, and A Nixon, Evaluation of decision criteria for detection of spinal cord compression based on cervical myelography in horses: 38 cases (1981-2001). Equine Vet J, 2004. 36; 1: 14-20.[12] Yamada K, F Sato, T Hada, N Horiuchi, H Ikeda, K Nishihara, et al., Quantitative evaluation of cervical cord compression by computed tomographic myelography in Thoroughbred foals . J Equine Sci, 2016. 27; 4: 143-148.[13] Mitchell CW, SG Nykamp, R Foster, R Cruz, and G Montieth, The use of magnetic resonance imaging in evaluating horses with spinal ataxia. Vet Radiol Ultrasound, 2012. 53; 6: 613-20.[14] Janes JG, KS Garrett, KJ McQuerry, AP Pease, NM Williams, SM Reed, et al., Comparison of magnetic resonance imaging with standing cervical radiographs for evaluation of vertebral canal stenosis in equine cervical stenotic myelopathy. Equine Vet J, 2014. 46; 6: 681-6.[15] van Loon JPAM, PJ Stienen, A Doornenbal, and LJ Hellebrekers, Use of epidurally derived evoked potentials for quantification of caudal nociception in ponies. Am J Vet Res, 2009. 70; 7: 813-819.[16] van Loon JPAM, H van Oostrom, A Doornenbal, and LJ Hellebrekers, Lumbosacral spinal cord somatosensory evoked potentials for quantification of nociception in horses. Equine Vet J, 2010. 42; 3: 255-260.[17] Pickles KJ, TJ Gibson, CB Johnson, V Walsh, JC Murrell, and JE Madigan, Preliminary investigation of somatosensory evoked potentials in equine headshaking. Veterinary Record, 2011. 168; 19.[18] Aleman M, DC Williams, RJ Brosnan, JE Nieto, KJ Pickles, J Berger, et al., Sensory Nerve Conduction and Somatosensory Evoked Potentials of the Trigeminal Nerve in Horses with Idiopathic Headshaking. J Vet Intern Med, 2013. 27; 6: 1571-1580.[19] Journee SL, HL Journee, CM de Bruijn, and CJG Delesalle, Design and Optimization of a Novel Method for Assessment of the Motor Function of the Spinal Cord by Multipulse Transcranial Electrical Stimulation. in Horses. Journal of Equine Veterinary Science, 2015. 35; 10: 793-800.

[20] Journee SL, HL Journee, CM de Bruijn, and CJG Delesalle, Multipulse transcranial electrical stimulation (TES): normative data for motor evoked potentials in healthy horses . BMC veterinary research, 2018. 14.[21] Nollet H, L Van Ham, F Gasthuys, J Dewulf, G Vanderstraeten, and P Deprez, Influence of detomidine and buprenorphine on motor-evoked potentials in horses. Vet Rec, 2003. 152; 17: 534-7.[22] Rijckaert J, B Pardon, LV Ham, G van Loon, and P Deprez, Magnetic Motor Evoked Potential Recording in Horses Using Intramuscular Needle Electrodes and Surface Electrodes. Journal of Equine Veterinary Science, 2018. 68: 101-107.[23] Nollet H, R Deprez, L Van Ham, J Dewulf, A Decleir, and G Vanderstraeten, Transcranial magnetic stimulation: normal values of magnetic motor evoked potentials in 84 normal horses and influence of height, weight, age and sex. Equine Vet J, 2004. 36; 1: 51-57.