Page 1270 - Cote clinical veterinary advisor dogs and cats 4th

638 MDR1 Mutation

○ Immunohistochemical staining for • Relapse occurs rapidly if treatment is tapered necrosis leads to fibrosis of the muscles of
antibody deposits found in 85% of cases too quickly. mastication and inability to open the jaw.
VetBooks.ir ○ CT is superior for imaging bones and Nutrition/Diet used to assess the number of healthy versus
Muscle biopsy in the chronic phase can be
• Advanced imaging (CT or MRI) (p. 1132)
fibrotic muscle fibers. If no viable muscle
joints.
• A range of food consistencies should be offered
○ MRI is more sensitive for identifying muscle
to find one that is best for an individual dog.
inflammation than serum CK or CT. • Feeding tubes may be needed temporarily fibers remain, prognosis is worse.
○ Both show changes in the size of the in the acute phase or permanently in the PEARLS & CONSIDERATIONS
muscle and in attenuation (CT) or inten- chronic phase to maintain body condition
sity (MRI) associated with inflammation and prevent starvation (pp. 1106 and 1109). Comments
and provide ideal locations for biopsy. • Trismus can be severe enough to inhibit
○ It is proposed that MRI be used to assess Behavior/Exercise tracheal intubation. Care should be taken
response to therapy. Physical therapy exercises for the jaw can be with anesthesia or sedation, and supplies
considered, such as offering rawhides or favorite should be in place to perform temporary
TREATMENT toys to play with. emergency tracheostomy if needed.
• After initiation of treatment in the acute
Treatment Overview Possible Complications phase, muscle atrophy alone should not
Successful treatment depends on prompt Additional immunosuppressive medications be considered evidence of treatment failure
recognition of the disease and initiation of such as azathioprine and cyclosporine have been because steroid therapy causes muscle atrophy.
immunosuppressive glucocorticoid therapy. used as adjuncts to glucocorticoids when side
Delays in treatment can result in perma- effects are unacceptable or clinical signs are Technician Tips
nent disability secondary to fibrosis of the resistant to monotherapy. • If feeding tubes are placed, knowledge of
muscles of mastication and inability to open appropriate feeding protocols is required,
the jaw. Recommended Monitoring with sufficient familiarity to teach owners.
First month: • Monitor for tongue or pharyngeal swelling in
Acute General Treatment • Animals with difficulty eating should be patients under sedation or anesthesia because
• Prednisone/prednisolone: 1-2 mg/kg PO q weighed weekly to ensure proper nutritional this can potentially occlude the airway during
12h until resolution of clinical signs and return intake. the procedure or recovery.
of serum 2M titers and CK to a normal range. • Hydration status should be monitored by the
In the acute stage, this can last for 2-4 weeks. owner via urination habits, quantification of Client Education
○ Expect muscle atrophy to occur during water consumption, and daily weighing. • Because pets can have pain in the acute
treatment and as a side effect of steroid • The owners can assess jaw mobility and stages, owners with small children should
therapy; do not assume this is a treatment comfort at home by watching the patient be warned that painful dogs may be more
failure. consume food, water, and treats. In the prone to defensive biting if inappropriately
• The chronic form of disease involves less hospital, this can be quantified by physical handled by the children.
inflammation and more fibrotic changes, and exam and measurements. • If feeding tubes are required, owners should
a lower dose of glucocorticoids 0.5-1mg/ Chronic monitoring: be provided instruction on appropriate
kg/day for a shorter duration (1 month) is • Serum 2M antibody titers can be monitored feeding and maintenance of the tube.
recommended to determine if further steroid to assess response to therapy every 3-6
therapy will be effective. months initially and before glucocorticoid SUGGESTED READING
• DO NOT ATTEMPT TO FORCE THE JAW OPEN. dose reductions or discontinuation. Melmed C, et al: Masticatory muscle myositis: patho-
It does not affect the clinical outcome and genesis, diagnosis, and treatment. Compendium
can result in subluxation of the temporo- PROGNOSIS & OUTCOME 26:590-604, 2004.
mandibular joint or mandibular fracture.
• The acute phase offers a good prognosis RELATED CLIENT EDUCATION
Chronic Treatment when diagnosed and treated promptly with
• After the initial immunosuppressive treat- immunosuppressive doses of glucocorticoids, SHEETS
ment for the acute stage, a slow, tapering although treatment may be required lifelong. Consent to Perform Computed Tomography
dose of glucocorticoid is administered until ○ Relapse occurs quickly with a premature (CT Scan)
the lowest effective daily or every other day reduction in dosage. How to Syringe-Feed, Tube-Feed, or Bottle-Feed
dosage is found. • Inadequate treatment of the acute phase or a Pet
• Tapering occurs over 4-6 months, with no more presentation in the chronic phase offers only
than a 50% reduction in dose each month. a guarded to fair prognosis for return of AUTHOR: Michaela J. Beasley, DVM, MS, DACVIM
Some dogs can be withdrawn from therapy. normal jaw function. If left untreated, muscle EDITOR: Karen R. Muñana, DVM, MS, DACVIM

MDR1 Mutation Client Education
Sheet

Epidemiology
BASIC INFORMATION extremely susceptible to adverse drug reactions
at doses of drugs well tolerated by normal dogs. SPECIES, AGE, SEX
Definition Dogs; similar mutation recently identified in cats
The MDR1 (multidrug resistance 1) gene Synonyms
mutation is a common problem in many Ivermectin or avermectin sensitivity; ABCB1 GENETICS, BREED PREDISPOSITION
herding-breed dogs. It causes enhanced sensitiv- gene, ABCB1-1Δ gene mutation Herding breeds (collie, Australian shepherd,
ity to a number of drugs, and affected dogs are Shetland sheepdog, Old English sheepdog,
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