Generalized Tremors: Identifying a White Shaker Dog
Tremors are involuntary, repetitive, rhythmic, oscillating contractions of antagonistic muscle groups. They are generally characterized by rate, rhythm and movement type and may be localized to one area or involve the entire body (generalized). They are often difficult to characterize because tremors are a relatively common neurologic abnormality in many diseases of the central nervous system (CNS) and peripheral nervous system (PNS).
Accurate characterization and diagnosis of generalized tremors is difficult. Discovering the cause requires a thorough history as well as a complete physical, neurologic and orthopedic exam. Other diagnostics that may be helpful include a CBC and chemistry panel to evaluate body systems and look for metabolic dysfunctions. An EMG and/or muscle and nerve biopsies may be warranted if a primary muscle disorder is suspected. Collection and analysis of cerebrospinal fluid is useful when looking for inflammation or an infectious cause. Tremors have been associated with congenital disease, chemical/plant intoxication, secondary to drug therapies, and bacterial or viral diseases.
Congenital diseases that cause generalized tremors in dogs, such as insufficient myelination of the CNS, often manifest themselves early in life. The tremors involved with these abnormalities are primarily noticed during goal-oriented activity (intention tremors), seem to lessen with rest and resolve with sleep. The pathogenesis of these tremors is unclear, but spontaneous discharge of the unmyelinated axons, and loss of coordinated muscle control may be the cause.
Ingestion of toxins such as organophosphates, hexachlorophene and bromethalin may result in generalized tremors. Suspected pathogenesis centers around altered nerve impulse conduction brought on by intramyelin edema. Mycotoxins have commonly been associated with tremors in dogs. Tremors resulting from drug therapies have also been documented. The pathogenesis is not always known, but discontinuing drug therapy will also stop the tremors.
Bacterial and viral encephalitis are also rule-outs when presented with a tremoring dog. The agents most frequently associated with tremors are canine distemper virus, adenovirus, parvovirus, herpes virus and tick-borne diseases. An extensive evaluation of cerebrospinal fluid is necessary for diagnosis. This includes cytologic evaluation of, as well as laboratory titers for, the suspected agent in the fluid.
Finally, there are those dogs that are referred to as little white shaker dogs. The syndrome was given this name because it was historically recognized in small breed white dogs such as the Maltese, West Highland White Terrier and poodle, although dogs with all coat colors are susceptible. They are generally young adults, initially showing signs at less than two years of age, and are small to medium sized dogs (<15 kg/33lbs). The generalized head and body tremors can range in severity from mild to incapacitating and tend to worsen with exercise, stress or excitement and lessen or resolve with sleep. Other neurologic signs (deficit in menace, nystagmus) may be present, but are not always noticed. Screening for infectious agents is generally negative. Cerebrospinal fluid analysis often reveals a mild lymphocytic pleocytosis, but may also be normal. Histologic exam of the CNS tissue of affected animals varies. A mild, non-suppurative meningoencephalitis with mild perivascular cuffing, most evident in the cerebellum, may be identified, but normal CNS tissue can also be found. The underlying disease process is still unknown. Some speculate that the tremors are due to an immune reaction targeted against the tyrosine producing cells. Tyrosine is important in the production of melanin as well as the neurotransmitters dopamine and norepinephrine. An imbalance of these neurotransmitters may lead to the clinical signs observed; however, variation in the pathologic changes identified in tissues suggests that inflammation is not the only mechanism involved in the pathogenesis. Diagnosing a white shaker dog is one of exclusion and response to treatment. These dogs respond to an immunosuppressive dose of cortico-steroids (prednisone). Tremors will generally resolve the first week or two after instituting therapy. The dose of steroids can then be tapered to the minimum effective dose, or discontinued completely. If the tremors return, reinstitution of the initial immunosuppressive dose may be necessary. Because of the response to steroids and the recognition of this syndrome in breeds other than those with a white hair coat, this disease is also referred to as steroid responsive tremor syndrome.
-by Kelly Smith, Class of 2004
-edited by Dr. Leon Thacker, ADDL Director
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