Jennifer Michaels, DVM, DACVIM (Neurology)
Idiopathic head tremor syndrome (IHTS; also known as episodic head tremor syndrome or idiopathic head tremors) is a benign condition of unknown cause resulting in focal head tremors. Having knowledge of IHTS and being able to identify likely affected patients is important in order to allow for appropriate discussion with the owner about the nature of the condition and prognosis.
The condition has only been reported in dogs, although there are rare anecdotal reports of cats with clinical history and clinical signs consistent with IHTS. IHTS is classically reported in Bulldogs and Doberman Pinschers which comprise 37% and 8% of affected dogs, respectively. Any breed can be affected including mixed breed dogs. Most affected dogs (88%) have their first episode of head tremor before 4 years of age, although reported age of onset ranges from 3 months to 12 years. Bulldogs have a younger average age of onset at 2 years compared with other breeds at almost 3 years.
The classic clinical sign of IHTS is a focal head tremor in an otherwise normal and alert dog. Tremors most commonly begin when dogs are awake or dozing/resting. The head tremor most commonly occurs in either a vertical “yes” movement (Video 1: angell.org/VerticalIHTS) or, more commonly, a horizontal “no” movement (Video 2: angell.org/HorizontalIHTS), although up to 15% of cases have a rotational component. The frequency and amplitude of the tremor can vary in an individual dog from one episode to another or even within a single episode, although the direction remains consistent within a single episode.
In a majority of dogs, each tremor episode lasts less than 5 minutes, although up to 15% will have episodes lasting up to an hour. There are a few reports of dogs with continuous head tremors lasting more than 12 hours. A majority of dogs will have episodes frequently (ranging from a few times per week to many times per day), although they can be as infrequent as one episode every 3-4 months or less. A hallmark of IHTS and an important feature when considering differentials is that affected animals remain alert, mobile, and responsive during the episodes (although a small proportion of dogs are reported to be agitated, nervous, or lethargic during episodes). No autonomic signs (e.g. hypersalivation, urination, defecation) are noted during episodes.
One of the most unique clinical features of IHTS is that a majority of dogs (up to 87%) can be “distracted” out of an episode. Calling their name, providing a treat or toy, or otherwise drawing their attention elsewhere will result in at least temporary halting of the head tremor.
The primary differential for IHTS is focal seizure activity. The absence of autonomic signs, ability to stop the tremor by providing a distraction, and the lack of response to conventional anti-epileptic medications argue against seizure activity. Tremors have many possible causes including toxins (pyrethrins, metronidazole toxicity, mycotoxins) and movement disorders (essential tremor syndrome, cervical dystonia, paroxysmal dyskinesia); however, tremors associated with these conditions are not episodic nor is the head a commonly affected area, especially in isolation. An additional differential includes intention tremor caused by underlying cerebellar disease; however in this case the head tremor is elicited by a goal-oriented movement but does not occur at rest or in a neutral head position which is contradictory to what is seen with IHTS.
Unfortunately, there is no specific test to diagnose IHTS. It is typically a clinical diagnosis made by the presence of classic, focal head tremor and lack of other neurological symptoms. Neurologic examination in affected animals is normal. Full diagnostic evaluations performed in affected patients including laboratory evaluation (hematology, biochemistry, urinalysis, liver function analysis, thyroid hormone testing), cervical and brain MRI, cerebrospinal fluid analysis, neurometabolic screening of the urine and blood, PCR testing for canine distemper virus, and electrodiagnostic evaluation were normal.
The one feature of IHTS that is arguably the most useful when attempting to make this diagnosis is the ability to distract patients out of an episode. Typically before recommending any other diagnostic tests, especially advanced imaging, the owner is instructed to attempt to distract their pet out of an episode when one occurs. If there is a repeatable response (in support of consistent clinical history and examination findings) this is often sufficient to make a confident clinical diagnosis.
One proposed theory of the cause of IHTS is that it involves abnormalities in the stretch reflex mechanisms in the neck muscles resulting in alternating contraction and relaxation of the neck muscles thus yielding a tremor. This theory is based in the lack of abnormalities noted on advanced imaging, CSF analysis, or post-mortem analysis in affected patients as well as the frequency with which a change in head/neck position leads to at least momentary discontinuation of the tremor. Interestingly, pedigree analysis of a group of both affected and unaffected Doberman Pinscher dogs showed that all affected dogs in the population could be traced back to a single sire suggesting a genetic origin of IHTS in this breed.
Several treatments have been attempted for IHTS including anti-seizure medications (phenobarbital, potassium/sodium bromide, diazepam), clonazepam, calcium carbonate, corticosteroids, and supplements (fish oil, supplements). There is no evidence that any of the reportedly used treatments lead to a significant improvement in clinical signs.
Fortunately, dogs with IHTS are otherwise healthy and are not expected to develop any other neurological or other health problems directly related to IHTS. Two-thirds of dogs will experience a spontaneous improvement in the severity or frequency of episodes or complete resolution of episodes over time. Of the reported affected dogs, none died or were euthanized as a result of IHTS.
In summary, IHTS is a benign condition resulting in focal head tremors in dogs. Affected dogs do not typically respond to treatment nor is treatment indicated. The condition does not progress to include other neurological deficits and often will spontaneously regress over time. This is an important condition to be aware of as these patients are often presumed to have focal seizures which carries a much different diagnostic work-up recommendation, treatment recommendation, and prognosis.
- Guevar J, De Decker S, Van Ham LML, et al. Idiopathic head tremor in English Bulldogs. Movement Disorders. (2014). 2: 191 – 194.
- Lowrie M, Garosi L. Classification of involuntary movements in dogs: tremors and twitches. Vet J. (2016). 214: 109 – 116.
- Shell LG, Berezowski J, Rishniw M, et al. Clinical and breed characteristics of idiopathic head tremor syndrome in 291 dogs: a retrospective study. Vet Med Internat. (2015). dx.doi.org/10.1155/2015/1645463
- Wolf M, Bruehschwein A, Sauter-Louis C, et al. An inherited episodic head tremor syndrome in Doberman Pinscher dogs. Movement Disorders. (2011). 23: 2381 – 2386.