Disorders of MM tone & exertional myopathies Flashcards
Spastic paresis is seen in what age calves? And manifests as:
Calves: 2 months-7 months
— decreased ability to flex the hock because of continuous tension on the gastrocnemius & straight angle to hock & stifle
What is a similar condition to spastic paresis in calves seen in horses?
Shivers
Shivers is seen in horses of what breed/size typically
Draft breeds, warmbloods and warmblood crosses older than 1 yr of age
**usually taller than 16.3 hands
Define myotonia
Prolonged contraction of mm contraction
Clinical signs of shivers:
Primarily affects hindlimbs
-periodic, involuntary spaspsm of muscles in the pevlic reigon, pelvic limbs and tail
**exacerbated by backing and up or picking up hind libs
** tailhead usually elevates concurrently/trembles
-hindlimb is suddenly raised, semi-flexed and abducted with hoof held in air for several seconds or minutes
Myotonic muscle disorders share the feature of what?
Delayed relaxation of muscle after mechanical stimulation or voluntary contraction
**abnormal muscle membrane excitability
Appearance of myotonia congenita in horses
Mild to moderate pelvic limb stiffness
**bilateral bulging of thick and rump muscles
Pronounced when exercise begins & diminishes as exercise continues
Is there progression of myotonia congenita in horses?
Not beyond 6 to 12 months of age
In goats what is the inheritance of myotonia congenita?
Autosomal dominant mutation in skeletal muscle chloride channel (CLCN1) that is incoplete penetrance
Myotonia dystrophica is separate from myotonia congenita in horses. What are the differences?
Severe clinical signs of myotnoia that progress to amarke dmm atrophy & involve a variety of organ systems
—> Quarterhorses, Appaloosa, Italian-bred foals
What is the definitive diagnosis of myotonia in horses?
Based on electromyographic examination
Examination of muscle biopsies from foals with myotonia congenita show:
Normal or demonstrate extremely variable muscle fiber dimensions up to twice those of normal age matched controls
+/- Type 1 fiber hypertrophy or hypotrophy
Changes in muscle biopsy seen with myotonic dystrophy
Ringed fibers
Alterations in shape & position of myonuclei, sarcoplasmic masses & inc in ednomysial & perimysial connective tissue
Fiber type grouping & atrophy of both type 1 & type II muscle fibers may be present
What are the treatment recommendations of myotonia?
No treatment
Myotonia prognosis
Variable & dependent on clinical signs
**regression of C/S unknown
** euthanasia often warranted
Is myotonia congenital?
Unknown
**warn owners of possibility of this disease is heritable
Equine hyperkalemic periodic paralysis (HYPP) is caused by:
Inherited defect in the skeletal mm sodium channel
Equine HYPP manifests as
Abnormal skeletal mm membrane excitability leading to episodes of myotonia or sustained mm contraction and paralysis
How is HYPP inherited in horses?
Autosomal dominant trait in Quarterhorses, American Paint Horses, Appaloosas & QH horse crossbreeds
In HYPP, intermittent clinical signs start at what age?
2 to 3 years of age with no apparent abnormalities between episodes
What are examples of diets high in potassium (>1.1%)?
Alfalfa hay
Molasses
Electrolyte ysupplements
Kelp-based supplements
**sudden dietary changes
What can precipitate clinical signs of HYPP?
**unpredictable
Fasting
Anesthesia or heavy sedation
Trailer rides
Stress
Exposure to cold
Fasting
Pregnancy
Concurrent dz
Rest following exercise
Clinical episodes of HYPP start as
— brief period of myotonia
Prolapse of third eyelid
Sweating & muscular fasciculations
**muscular weakness is a common characteristic of HYPP
Clinical signs of severe HYPP attacks
Apparent weakness with swaying
Staggering
Dog sitting
Recumbency w/in a few minutes
INC HR and RR
How long do HYPP episodes last?
15 to 60 minutes
Respiratory distress can result in HYPP episodes due to:
Paralysis of upper respiratory muscles
** may require tracehostomy
What is the concern for horses homozygous for HYPP?
Dysphagia/respiratory distress
However horses with HYPP may be normal between attacks, does electromyography show any abnormalities?
Yes
* abnormal fibirllation potentials
Complex repetitive discharges with occasional myotonic potentials & trains of doublets between episodes
What is the exact mutation that leads to HYPP horses?
Point mutation that causes a phenylalanine/leucine substitution in voltage- dependent skeletal muscle sodium channel alpha subunit
The result of the mutation in HYPP causes:
Resting membrane potential is closer to firing than in normal horses
- subpopulation of sodium channels inactivate when serum potassium concentrations are increased
—> excessive inward flux of sodium and outward flux of potassium ensues
—> results in persistent depolarization of muscle cells and temporary weakness
Does serum concentration of potassium increase in HYPP episodes?
6 to 9 mEq/L increase during episode
** serum potassium concentration return to normal following abatement of C/S
What are differentials for hyperkalemia?
Delay before sample centrifugation
Hemolysis
Acidosis
Renal failure
Severe rhabdomyolysis
High-intensity exercise
Treatment options for HYPP episodes
Feeding grain/corn syrup to stimualte insulin-mediated movement of potassium across cell membranes
Epinephrine 0.006 mg/kg/500 kg IM
Acetazolamide: 3 mg/kg PO every 8 to 12 hours
**most recover from episodes of paralysis and appear normal by time a veterinarian arrives
In severe cases of HYPP intravenous treatment with what medications can be used to enhance intracellular movement of potassium?
Calcium gluconate: 0.2-0.4 ml/kg of 23% solution in 1 L of 5% dextrose or combinated with sodium bicarbonate (1 to 2 mEq/kg)
How does calcium gluconate assist in treatment of HYPP?
Increase in extracellular calcium concentration raises muscle membrane threshold potential— decreasing membrane hyperexcitability
For horses with recurrent episodes of muscle fasciculations with HYPP?
Acetazolamide: 2-3 mg/kg PO every 8 to 12 hours
Hydrochlorothiazide: 0.5 to 1 mg/kg PO, every 12 hours
Chronic fibrotic myopathy stride
Short anterior phase with characteristic hoof-slapping gait
Clinical signs of exertional rhabdomyolysis
Develop a stiff, stilted gait, with excessive sweating & high respiratory rate during or after exercise
- seen 15 to 30 minutes after light exercise
- firm, painful muscles (back and hind limb mm)
- myoglobinuria
Sporadic exertional rhabdomyolysis is seen in what population of horses?
Any age, breed or sex involved in a wide variety of athletic disciplines
Diagnosis of sporadic exertional rhabdomyolysis
History
C/S
Elevations of serum muscle enzymes
What are causes of sporadic exertional rhabdomyolysis?
Overexertion
Exhaustion
Dietary imbalances
What are clinical signs of heat exhaustion?
Weakness
Ataxia
Rapid breathing
Muscle fasciculations
Sweating
Severe cases of collapse
Body temp: 105 to 108
INC Ck activity
Myoglobinuria
Sporadic forms of exertional rhabdomyolysis due to
an extrinsic event or recurring extrinsic events that induce muscle damage with exercise
Causes of sporadic exertional rhabdomyolysis include
focal or generalized trauma to muscle
exercise performed beyond any training adaptation or performed to the point of exhaustion
dietary imbalances that affect muscle fasciculation
Resolution of sporadic exertional rhabdomyolysis occurs after:
- period of rest
- provision of balanced diet
- gradual introduction of a training program matched with performance demands
dietary imbalances that can trigger exertional rhabdomyolysis episdoes
high nonstructural carbohydrates (NSC) & low forage content
diets deficient in electrolytes
+/- exacerbation inadequate selenium/vit E
Horses competing in hot, humid weather what electrolytes require higher concentrations?
sodium chloride
- 30 to 50 g/day combined with 15 to 25 g of “lite” salt containing KCL
- ideal Ca:Phos ratio of 2:1
Reason for administration of dantrium sodium in exertional rhabdoymyolysis?
in severely affected horses may decrease muscle contracture sand possible prevent furthe rmuscle necrosis
Overdoing dantrium sodium can lead to?
muscle weakness
What are causes of chronic exertional myopathies in horses?
Polysaccharide storage myopathy T1
Polysaccharide storage myopathy T2
Malignant hyperthermia
Recurrent exertional rhabdomyolysis
Idiopathic exertional rhabdomyolysis
Mutation in what gene has shown to be highly associated with the presence of amylase resistant polysaccharide in skeletal muscle from Quarter Horses with PSSM?
GYS1 gene
**glycogen synthetase gene
Do all horses with PSSM have gene mutations in GYS1 gene?
No–
PSSM type 1- GYS1 gene mutation
PSSM type 2– unknown origin
what is the gene mutation at GYS1?
single base pair mutation in GYS1 gene resulting in arginine to histidine substitution at codon 309