Trigeminal Nerve Root Demyelination Not Seen in Six Horses Diagnosed with Trigeminal-Mediated Headshaking

Effect of repetitive transcranial magnetic stimulation on trigeminal-mediated headshaking in 17 horses

BACKGROUND

Trigeminal-mediated headshaking is a neuropathic facial pain condition in horses. No treatment has been entirely successful. Repetitive transcranial magnetic stimulation (rTMS) is used in human medicine as a treatment for various neuropathic pain conditions, and good results have been achieved in cases of trigeminal neuralgia.

OBJECTIVES

Apply rTMS to horses with trigeminal-mediated headshaking (TMHS) and to evaluate tolerability, application of the setting, and success rate.

ANIMALS

Seventeen horses with nonseasonal signs of TMHS.

METHODS

Other underlying causes of headshaking were ruled out. The rTMS was performed under standing sedation on 5 consecutive days applying 3 sets of 500 stimulations each, with a stimulation strength of 5 Hz. Horses were evaluated on Day 1 (t0) and Day 5 (t1) of the treatment and 2 (t2) and 4 weeks (t3) afterwards using a special scoring system.

RESULTS

The rTMS was well tolerated. Headshaking signs during exercise were decreased by 70% (Day 5; t1). Four weeks after rTMS, signs were still decreased (mean reduction of 50%) during exercise. Improvement of mean resting and exercise scores was significant (P < .05) and effect sizes between pretreatment and all time points after treatment (t1, t2, t3) were large (>±0.8).

CONCLUSIONS AND CLINICAL IMPORTANCE

Repetitive transcranial magnetic stimulation may be a promising treatment for neuropathic pain and headshaking in affected horses. Pain-free periods after treatment differ individually, and repeated treatment may be necessary. More studies should be performed to determine ideal settings for horses.

An anatomical study of the subarachnoid space surrounding the trigeminal ganglion in horses-in preparation for a controlled glycerol rhizotomy in equids

Introduction

Equine trigeminal-mediated headshaking is a painful neuropathic disorder comparable to trigeminal neuralgia in humans. The selective destruction of pain fibers within the trigeminal ganglion, called rhizotomy, is the surgical treatment of choice for idiopathic trigeminal neuralgia refractory to medical treatment in humans. The human trigeminal ganglion is enclosed by a dural recess called the Meckel's or trigeminal cave, in which the ganglion is surrounded by a cerebrospinal fluid (CSF)-filled subarachnoid space. During glycerol rhizotomy, glycerol is percutaneously injected in this CSF-filled space. Until now, information about the anatomy of the dural recess and the subarachnoid space surrounding the trigeminal ganglion is lacking in horses. The aim of this study was to explore if a CSF-filled subarachnoid space around the trigeminal ganglion exists in horses.

Materials and methods

Six equine cadaver heads were investigated for CSF accumulation around the ganglion with a 3 Tesla MRI. After anatomical dissection to expose the trigeminal root, a polymer-based radiopaque contrast agent was injected through the porus trigeminus into the subarachnoid space (cisternography). The exact delineation and the volume of the contrast agent accumulation were determined on subsequent micro-computed tomographic scans and segmentation. Finally, the distribution of the contrast agent within the subarachnoid space was examined histologically in three specimens.

Results

In all 12 specimens included in this study, the trigeminal ganglion was surrounded by a subarachnoid space forming a trigeminal cistern. The mean volume of the trigeminal cave in this study was 0.31 mL (±SD: 0.11 mL). Distribution of the contrast agent along the peripheral nerves (i.e., ophthalmic, maxillary and/or mandibular nerve) was observed in 7 out of 12 specimens.

Discussion/conclusion

A subarachnoid space surrounding the trigeminal ganglion exists in the horse and could be targeted for glycerol rhizotomy in horses suffering from trigeminal-mediated headshaking. However, the clinical relevance of contrast agent distribution along the peripheral nerves remains to be assessed.

Sensory nerve conduction stimulus threshold measurements of the infraorbital nerve and its applicability as a diagnostic tool in horses with trigeminal-mediated headshaking

BACKGROUND

To determine whether sensory nerve conduction stimulus threshold measurements of the infraorbital nerve are able to differentiate horses with idiopathic trigeminal-mediated headshaking (i-TMHS) from healthy horses and from horses with secondary trigeminal-mediated headshaking (s-TMHS). In a prospective trial, headshaking horses were examined using a standardized diagnostic protocol, including advanced diagnostics such as computed tomography and 3-Tesla-magnetic resonance imaging (MRI), to differentiate s-TMHS from i-TMHS. Clinically healthy horses served as controls. Within this process, patients underwent general anesthesia, and the minimal sensory nerve conduction stimulus threshold (SNCT) of the infraorbital nerve was measured using a bipolar concentric needle electrode. Sensory nerve action potentials (SNAP) were assessed in 2.5-5 mA intervals. Minimal SNCT as well as additional measurements were calculated.

RESULTS

In 60 horses, SNAP could be recorded, of which 43 horses had i-TMHS, six had suspected s-TMHS, three horses had non-facial headshaking, and eight healthy horses served as controls. Controls had a minimal SNCT ≥ 15 mA, whereas 14/43 horses with i-TMHS and 2/6 horses with s-TMHS showed a minimal SNCT ≤ 10 mA. Minimal SNCT ≤ 10 mA showed 100% specificity to distinguish TMHS from controls, but the sensitivity was only 41%.

CONCLUSION

A minimal SNCT of the infraorbital nerve ≤ 10 mA was able to differentiate healthy horses from horses with TMHS. Nevertheless, a higher minimal SNCT did not exclude i-TMHS or s-TMHS and minimal SNCT does not distinguish s-TMHS from i-TMHS.

Expression of Cannabinoid Receptors in the Trigeminal Ganglion of the Horse

Cannabinoid receptors are expressed in human and animal trigeminal sensory neurons; however, the expression in the equine trigeminal ganglion is unknown. Ten trigeminal ganglia from five horses were collected post-mortem from an abattoir. The expression of cannabinoid receptors type 1 (CB1R) and type 2 (CB2R), and the cannabinoid-related receptors like transient receptor potential vanilloid type 1 (TRPV1), peroxisome proliferator-activated receptor gamma (PPARɣ), and G protein-related receptor 55 (GPR55) in the trigeminal ganglia (TG) of the horse were studied, using immunofluorescence on cryosections and formalin-fixed paraffin-embedded (FFPE) sections. Neurons and glial cells were identified using fluorescent Nissl staining NeuroTrace® and an antibody directed against the glial marker glial fibrillary acidic protein (GFAP), respectively. Macrophages were identified by means of an antibody directed against the macrophages/microglia marker ionized calcium-binding adapter molecule 1 (IBA1). The protein expression of CB1R, CB2R, TRPV1, and PPARɣ was found in the majority of TG neurons in both cryosections and FFPE sections. The expression of GPR55 immunoreactivity was mainly detectable in FFPE sections, with expression in the majority of sensory neurons. Some receptors were also observed in glial cells (CB2R, TRPV1, PPARγ, and GPR55) and inflammatory cells (PPARγ and GPR55). These results support further investigation of such receptors in disorders of equine trigeminal neuronal excitability.

Overview of the Current Situation in a Sample of Headshakers and Owner Assessment of Effective Therapeutic Measures Used in Germany

Equine headshaking syndrome is a problematic behavior that has been described in literature for more than 100 years. The signs of headshaking syndrome appear frequently and violently so that riding the horse can be dangerous. The aim of this research was to gain an overview of the underlying causes of equine headshaking syndrome to identify effective treatment options, reduce the distress of horses and, in a second step, potentially improve therapeutic possibilities for horse owners and veterinarians. Most studies on prevalence originate from Anglo-American countries, so this research was to provide an overview of German horses; therefore, an online survey was designed by experts in the field (researchers and veterinarians) and answered by German horse owners. Questionnaires were pretested by a small group of horse owners (n = 5) and redesigned accordingly. The final questionnaire consisted of 27 questions, which were divided into seven open questions with an associated text field and 20 multiple-choice questions (Supplementary Material 1). For some of the multiple-choice questions, multiple answers were permitted. In total, 163 completed questionnaires were obtained from owners of headshaking horses (n_total = 163). Gender distribution of the horses resulted in 64.4% geldings, 33.7% mares, and 1.8% stallions. Most horses were German warmbloods (55.4%). The average age was 12.7 years (with a wide range of 5 to 34 years). The vertical movement of the head was the most frequently mentioned symptom in 75.5% of the evaluated horses. In 18.4% of the cases, the horse owners identified stress as the main trigger for headshaking. Furthermore, 18.4% claimed that their horses suffered from additional stereotypical behavior. The percentages do not refer to the same horses here. 11.0% of the participants in this study tried scientifically evaluated therapies such as surgical therapy (1.2%) or medical treatment (9.8%). In addition, 54.0% of the owners utilized a nose cover to reduce the symptoms of the headshaking syndrome. A high percentage of 84.0% used alternative therapies such as physiotherapy (31.9%), change of equipment (22.7%), or change of riding style (29.4%). Overall, this study provides a useful overview of causes and effective therapies.

The safety and efficacy of neuromodulation using percutaneous electrical nerve stimulation for the management of trigeminal-mediated headshaking in 168 horses

Background

Early results from the use of neuromodulation by percutaneous electrical nerve stimulation for the management of trigeminal‐mediated headshaking in horses were promising but lacked sufficient case numbers and long‐term follow‐up. The neuromodulatory procedure has since been established as EquiPENS™.

Objectives

The aim of this study was to report long‐term results from a larger number of cases and to investigate for predictors of outcome.

Study design

Prospective case series using international, multi‐centre data.

Methods

Eligible cases were horses with a veterinary diagnosis of trigeminal‐mediated headshaking, which received EquiPENS™ neuromodulation at trained centres between August 2013 and November 2017. The standard protocol was an initial three‐procedure course, with additional procedures should a horse go into remission but then relapse. Data collected included signalment, history, diagnostic tests performed, details of any complications, whether horses had gone into remission and the length of remission.

Results

Results were obtained from 168 horses, with 530 procedures. The complication rate was 8.8% of procedures. In all but one case, complications were mild and transient, without self‐trauma. Remission of headshaking following the initial course occurred in 53% (72/136) of horses. Median length of time recorded in remission was 9.5 weeks (range 2 days to 156 weeks ongoing). Where signs recurred, most horses went back into remission following additional procedures, usually for longer than from the previous procedure. No predictors for outcome were determined.

Main limitations

No placebo or control group, owner‐assessed results.

Conclusions

EquiPENS™ neuromodulation can be an effective and safe treatment for the management of trigeminal‐mediated headshaking in some horses. An increased understanding of neuromodulation could help optimise the technique. Advances in treatment for trigeminal‐mediated headshaking will remain limited until there is a greater understanding of the aetiopathogenesis of the condition.

Is Electrical Nerve Stimulation the Answer for Management of Equine Headshaking?

Horses with trigeminal mediated headshaking (TMHS) have a decreased activation threshold of the trigeminal nerve and clinical signs are suspected to be a manifestation of trigeminal neuralgia. Electrical nerve stimulation (ENS) is used for management of neuralgia in humans and appears to work via gate control theory. Use of an equine specific percutaneous ENS program in over 130 TMHS horses has resulted in approximately 50% success return to previous work. Electroacupuncture may also be useful in the management TMHS. Optimization of ENS procedures for TMHS is likely to require a greater understanding of the etiopathogenesis of the aberrant neurophysiology.

Intravenous infusion of magnesium sulfate and its effect on horses with trigeminal-mediated headshaking

Background

Trigeminal‐mediated headshaking results from low‐threshold firing of the trigeminal nerve resulting in apparent facial pain. Magnesium may have neuroprotective effects on nerve firing that potentially dampen signs of neuropathic pain. This hypothesis has not been investigated in horses with trigeminal‐mediated headshaking.

Objective

To investigate head‐shaking behavior in affected horses after IV magnesium sulfate infusion.

Animals

Six geldings with trigeminal‐mediated headshaking.

Methods

Prospective randomized crossover study. Horses were controlled for diet and infused IV with 5% dextrose solution (DS; control solution at 2 mL/kg body weight [BW]) and MgSO₄ 50% solution (MSS at 40 mg/kg BW). Head‐shaking behavior was recorded at times T0 (baseline, before infusion) and T15, T30, T60, and T120 minutes post‐infusion. Venous blood variables such as pH, HCO₃⁻, standard base excess (SBE), Na⁺, Cl⁻, K⁺, Ca²⁺, Mg²⁺, total magnesium (tMg), glucose, and lactate were measured; strong ion difference (SID) and anion gap (AG) were calculated for each time point.

Results

Blood variables including pH, Na⁺, Cl⁻, K⁺, SID, AG, lactate, Ca²⁺, tMg, and Mg²⁺ had significant changes with MSS as compared to DS treatment. Glucose, SBE, and HCO₃⁻ did not have significant changes. A 29% reduction in head‐shaking rate occurred after MSS treatment but no change occurred after DS treatment.

Conclusions and Clinical Importance

Administration of MSS IV increased plasma total and ionized magnesium concentrations and significantly decreased head‐shaking behavior in horses with trigeminal‐mediated headshaking.

Trigeminal-mediated headshaking in horses: prevalence, impact, and management strategies

Trigeminal-mediated headshaking is a little-understood neuropathic facial pain condition of the horse. The condition may affect around 1% of the equine population to a degree of severity sufficient to require veterinary attention. As a pain condition, this represents a significant welfare issue. Horses are usually more severely affected at exercise which can leave them unable to perform, or even dangerous to ride and handle. With little known about the condition and variable response to treatments, severely affected horses are often euthanized. This review article considers the literature on trigeminal-mediated headshaking, focusing on what is known, and indeed not known, about the prevalence of the condition and its impact. The current published management options are considered, along with their challenges and limitations.