Headshaking: Part 2 Management
Is there a cure for headshaking?
Since the underlying cause of the trigeminal nerve hypersensitivity is unknown (Part 1), current treatments are neither specific nor curative so the majority of horses with TMHS are managed (which is not always possible) rather than cured. The aim is to reduce clinical signs to limit distress and allow the horse to be used for its intended purpose. This is not always possible and sometimes an alternative (lower grade) career for the horse is required to allow the horse to be symptom-free.
The high failure rate of many TMHS treatments is not surprising given that they have no effect on correcting the abnormal trigeminal firing. Additionally, there are few scientific trials of promoted TMHS treatments. The success of any given treatment can be very difficult to interpret due to the intermittent nature of clinical signs, which vary with exercise and environmental and seasonal triggers. It is advised to try and assess the horse under the same conditions each time. For example, if headshaking is initiated by 5 minutes trotting exercise on the lunge, use this test each time you evaluate a treatment. A considerable placebo effect (30%) has been shown in owner scored trials of headshaking treatments. This means that owners think the horse is better when the horse has received a placebo rather than the trial medication and so it is important to try and be as objective as possible.
What is the best treatment?
Management of TMHS can be divided into environmental, physical, pharmaceutical (drugs), surgical, and nerve stimulation strategies. Only one treatment should be tried at a time to ensure its usefulness can be accurately assessed. A summary of the available evidence is presented in Table 1 however, it should be remembered that, for the majority of these drugs, there are no data on how well the drug is absorbed or utilised by the horse, or any safety data regarding side effects.
Avoidance of known triggers is often helpful e.g. riding at night or in an indoor arena if light is a trigger. However, for most owners such management is impractical, particularly for competition horses.
1. Nose nets
Nose nets are one of the most successful therapies for TMHS and are now permitted at some equestrian competitions. Up to 75% of owners report some improvement in TMHS using either a full net (covering the muzzle and upper and lower lips) or a half net (covering only the nostrils and upper lip) although signs are reduced rather than resolved for most horses. Some TMHS horses that do not respond to a traditional nose net improve with the use of soft rope plaits, or a similar device, that attaches to the noseband or browband and therefore trying several different types of mask, e.g. a mesh half mask, a solid half mask and a full muzzle mask, is advised before deciding if they are useful or not.
2. Face masks
Face masks are similarly effective at reducing TMHS in >50% of horses. Horses with photic (bright light stimulated) TMHS benefit from fly masks with >90% ultraviolet blocking sunshade so ensure you are buying one with high protection. Tinted contact lenses do not appear useful in the majority of horses.
The mechanism by which nose nets and face masks improve TMHS is unknown. They might act to reduce aversive stimulation of the hypersensitive areas or, alternatively, the constant presence of the net might work to downregulate this hypersensitivity.
All medications listed have potential adverse effects and most are they prohibited by governing sport federations who should be consulted regarding regulations and withdrawal times in competing or show horses. The risks and benefits associated with all medications should be discussed with your horse’s veterinary surgeon before use.
Cyproheptadine (Pericatin®) is an old type of antihistamine with other additional effects which has been used to treat human vascular headaches. There is no data on how well the drug is absorbed in horses, however studies report 48-70% of horses with TMHS improved with its use although lethargy, drowsiness and anorexia were reported in half the horses which precluded riding in some.
Carbamazepine (Tegretol®) is an anticonvulsant which stabilises sodium channels and is the drug treatment of choice for human trigeminal neuralgia (HTN), a neuropathic pain condition with some similarities to TMHS. There are mixed reports regarding its use in treating TMHS and there is wide individual variability between horses in the absorption of this drug, which may explain treatment failure in some horses. Depression, lethargy, and drowsiness are frequent side effects.
Antihistamines may improve TMHS in low numbers of horses although response is highly variable. The choice of antihistamine does not appear to affect outcome although mild drowsiness is reported in many horses with treatment.
Phenobarbitone, an anti-seizure medication, has been used with some success to treat extremely severe TMHS where the horse is particularly distressed. Mild sedation is a common effect of the treatment and horses should not be ridden when using this drug.
Gabapentin is an anti-epileptic drug used for the treatment of HTN and other neuropathic pain conditions. Absorption following oral dosing of gabapentin is poor in the horse and success for the treatment of TMHS is variable. Other newer drugs for neuropathic pain such as pregabalin might also be useful but long-term treatment is costly and horses cannot usually be ridden.
Few horses with TMHS respond to corticosteroid anti-inflammatory treatment as there is no inflammation of the nerve or other tissues. Dexamethasone pulse therapy has been used for the treatment of human neuropathic pain but a prospective, blinded, placebo controlled clinical trial did not find it decreased TMHS in susceptible horses.
The hormone melatonin is released in response to changes in daylength and plays an important role in reproductive performance of seasonal breeders like the mare. Melatonin also modifies pain (including neuropathic pain) and acts on various anti-pain pathways. The use of melatonin has been reported to improve seasonal TMHS in some horses and is most successful when started before the onset of spring in horses with seasonal TMHS. As the horse’s brain is effectively tricked into thinking it is winter, approximately half of treated horses do not shed their winter coat and require clipping.
Very few nutritional supplements that claim to alleviate TMHS have been subject to scientific validation and therefore great scepticism should be used when reading marketing claims.
Magnesium increases the threshold for nerve firing, therefore appears to be a rational therapy given the reduced activation threshold of the trigeminal nerve in TMHS horses. However, optimal blood magnesium concentrations and whether this can be achieved via oral supplementation are unknown. Just under half of owners have previously reported improvement in TMHS following oral supplementation with 10-20 g of magnesium daily with absence of any side effects. Some forms of magnesium are more absorbable than others, with citrate and malate being best. Look for a product with as close to 100% magnesium as possible. Boron increases the availability of magnesium but is not available on the UK animal feed register.
Surgical therapies to resolve TMHS such as cutting a lower branch of the trigeminal nerve or compression of the nerve with coils have been discontinued due to the high post-surgical complication rates, some of which required euthanasia.
Electrical Nerve Stimulation
Electrical nerve stimulation is the therapeutic alteration of nerve activity by use of electricity and dates back thousands of years to when the ancient Greeks recommended treading on electric eels to control pain. The modulation of pain by electrotherapy is not completely understood but may be explained by gate control theory, the science that also underpins the common parental response of rubbing a sore area on a child.
The most common use of electrotherapy is probably transcutaneous nerve stimulation (TENS) to manage pain associated with contractions in labour where a electrically conductive pads are worn on the abdominal skin. Percutaneous nerve stimulation (PENS), where an electrical probe is inserted under the skin, is used for human neuropathic pain conditions and is also successful in management of horses with TMHS, with approximately 50% successfully returning to the previous level of activity. The procedure must be performed under sedation and a series of three initial treatments is required to determine if therapy has been successful. Mild swelling can occasionally occur at the site of needle insertion and transient worsening of TMHS is infrequently seen following treatment, but these do not appear to influence response to the procedure. A similar technique electroacupuncture has also been reported as successful in a small study for the management of TMHS and can be performed without sedation.
Table 1: Dosage guidelines, evidence of success and reported side effects of treatments used in trigeminal mediated headshaking (TMHS). N/A, not applicable; PO, per os (orally).
|Evidence of success
|Reported side effects
|Reduction of clinical signs in >70% of 36 seasonal TMHS horses1 and >50% of 110 TMHS horses2
|Irritated by mask, panic2
|Reduction of clinical signs in >50% of 83 TMHS horses2
|Spookiness, reduced vision2
|0.3 mg/kg PO twice daily
|70% of 61 headshakers improved moderately to greatly within 7days 3; no improvement in five horses4 and three horses (1 received only 0.2 mg/kg) 5
|Mild lethargy, drowsiness, anorexia2,6
|2-8 mg/kg PO 2-4 times daily
|Successful in 88% of cases but results were unpredictable at pre-defined dose rates 5; decreased TMHS in 2/9 horses 2
|Obtundation, lethargy and drowsiness2
|Cyproheptadine + carbamazepine
|80–100% improvement in 80% of 12 cases within 3-4 days 5
|Hydroxyzine: 0.8 mg/kg PO twice daily
|General antihistamine use decreased TMHS in 12/36 horses 2
|50 mg IM, repeat every 1-4 months
|Improvement in 7/16 horses 7
|Extrapyramidal effects 8
|3-6 mg/kg PO twice daily
|Improvement in reducing distress of severely affected horses 9
|Mild sedation common
|5-20 mg/kg PO once or twice daily
|Anecdotal variable success; no response in one severely TMHS horse 9
|17/31 2 and 3/20 3 horses improved; dexamethasone pulse therapy had no effect in 20 horses 10
|Sodium cromoglycate eye drops
|1 drop/eye four times daily
|Successful in three (atypical) seasonal headshakers 11
|15-18 mg PO once daily at 17.00 h
|Improvement 2/7 3 and 8/17 2 horses
|May not shed winter coat 2
|10-20 mg PO once daily
|Improvement in 25/58 horses 2
|1/28 horses improved, 4/28 slight improvement 3; partial improvement 4/50 horses 1
|6/93 horses improved, 29/93 partial improvement 1
|4/25 horses improved, 6/25 slight improvement 3
|18/109 2 and 3/109 horses improved 3
|Percutaenous electrical nerve stimulation (PENS)
|3 sessions recommended initially
|5/712 and 72/13613 in remission
|Transient increase in TMHS, mild swelling at insertion site (9%)12,13
|Reduction in signs in 6/6 horses14
References: 1 Mills et al. (2003); 2 Pickles et al., (2014); 3 Madigan and Bell (2001); 4Mair (1999); 5Newton et al. (2000); 6 Madigan et al. (1995); 7 P.Smith and J. Madigan unpublished observations; 8 Baird et al. (2006); 9 Aleman et al. 2014; 10 Nolen-Walston et al. (2014); 11 Stalin et al. (2008); 12 Roberts et al. (2016); 13Roberts et al. in press; 14Devereux (2017)
Part 3 of this series will cover Diet and Anecdotal Management of TMHS. Available on Mon 1st Jan 2024
READ Part 1 – Why does my horse headshake? – Headshaking: Part 1 Clinical Signs and Diagnosis. Click here.
We have a lot more information on Head Shaking, follow the links below to find out more:
- Article – Headshaking, Head Flicking, Head Tossing, Trigeminal Mediated Headshaking
- Latest Headshaking Research Presented at BEVA Congress 2023
- Webinar – Headshaking – Dr Kirstie Pickles
- Webinar – Headshaking Q&A – Kirstie Pickles
- Podcast – Equine Head Shaking – Dr David Marlin
- Boron Supplementation for head shakers (Trigeminal Mediated Headshaking)