Tremors in people are classified/described in many different ways: neuroanatomical location, rate, amplitude, rhythmicity,
relationship to rest and movement, posture, performance of specific tasks, medical and family history. Other classification
schemes are based on underlying pathological change and etiology.
Currently there is no formal classification scheme in dogs/cats. Veterinary neurologists often use the signalment and history,
neurological examination findings, and the conditions under which the tremor is activated to develop list of likely causes.
Description of Tremors or Other "Shakes"
A sudden, rapid, involuntary muscle movement of short duration caused by muscle contractions and pauses in muscle activity.
It may be regular or irregular. Usually involves an entire muscle or groups of muscles. In dogs, myoclonus is commonly associated
with distemper encephalomyelitis. Limb or jaw muscles are commonly affected and the myoclonus persists during sleep. It may
be seen during the acute phase of infection, but more commonly occurs in the chronic phase. It may persist for years after
recovery from other signs of distemper.
Spontaneous contractions of muscle fibers within a motor unit causing a subtle or fine "rippling" or "twitching" of muscles.
Fasciculations arise from ectopic electrical activity of the distal axon. They are most often associated with neuromuscular
These rhythmical oscillatory movements may be focal (just affecting one limb or the head) or generalized. They are only recognized
when animals are awake; true tremors abate during sleep.
These tremors become more obvious as the animal approaches a target with its body or head. They are associated with cerebellar
Action tremor is a tremor that occurs during voluntary contraction of skeletal muscle, and is further classified as postural,
kinetic, isometric, or task specific. Kinetic tremor is an oscillation during directed voluntary movement.
Tremors are associated with motor unit disease, brain disease (particularly cerebellar disease), diffuse CNS disease,
and central myelination disorders
Motor Unit Diseases
Myopathy, junctionopathy or neuropathy may have tremors secondary to weakness. The neuroanatomical localization of neuromuscular
disease is made on the basis of the history (weakness with exercise) and finding generalized lower motor neuron deficits and/or
Animals with cerebellar disease typically have inability to judge distance (dysmetria), which usually results in overstepping
(hypermetria). An intention tremor and truncal swaying are also common. Vestibular dysfunction can also occur. Pure cerebellar
disease does not cause weakness or changes in mentation.
Diffuse Central Nervous System
The most common myelination problems are seen in younger animals with myelin deficiencies.
Specific Diseases or Recognized Syndromes Causing Tremors
Head Tremors/Movement Disorders/Head Bobs
Breeds affected: Boxer, English Bulldog, French Bulldog, American Bulldog, Labrador retriever, Bassett hound, Great Dane,
Malamute, Doberman pinscher.
Pathophysiology: This is a clinical sign and/or syndrome of unknown etiology that results in intermittent head tremors. There
are no retrospective or prospective publications and much of what is known about the disorder has been passed down from neurologist
to neurologist. There have been at least 3 theories as to the origin of the tremors.
1. They may represent a focal seizure discharge.
2. They are a form of dyskinesia or a spontaneous discharge from the basal nuclei, which are involved in patterned motor activity.
3. They may be an abnormality of the stretch mechanism and the proprioceptive pathway of the head (trigeminal nerve). This
theory may explain why the head tremors temporarily stop when the animal's attention is diverted i.e. when they are directed
to look at you or something else, the neck muscles tense which may release the stretch mechanism that is provoking the tremors.
Description: Episodes of head tremors that last seconds to a few minutes. Affected dogs appear to be fully alert during these
episodes. Tremors consist of either up and down (like shaking the head "yes") or back and forth (like shaking the head "no")
motions. They seem to start and stop spontaneously, but dogs may be momentarily distracted during the episode by directing
their attention to something. Thus far, patients have not developed any other neurological deficits and generalized seizure
activity has not been reported.
Diagnosis: Diagnostic evaluations, including MRI and spinal fluid analysis results, are typically normal.
Prognosis: Good as signs are not life-threatening and are intermittent.
Treatment: The response to anticonvulsants is somewhat controversial with some neurologists indicating no response and others
indicating that some dogs seem to respond.