Myopathies Flashcards
Types of myopathy (rhabdomyolysis)?
Exertional myopathy:
- Sporadic exertional rhabdomyolysis (SER) due to dietary imbalance or over-exertion = not had an episode before
- Recurrent exertional rhabdomyolysis (RER) = episodic episodes
- Polysacharide storage myopathy (PSSM) type 1 or 2
- Myofibrillar myopathy (MFM)
(Malignant hyperthermia (MH)) - more common in Quarter horses
Non-exertional myopathy
Sporadic exertional rhabdomyolysis (tying up) - causes?
Classically horses that do a large exercise session with inadequate training
Worsened if recent viral disease, or if hot or humid
Also more symptomatic if inappropriate diet e.g. deficient in certain electrolytes, high starch content meal, imbalanced vitamins and minerals
Recurrent exertional rhabdomyolysis (RER) - which horses? Typical history? How to manage?
TB and SB racehorses mostly
Also reported in WBs, Arabs, QHs
More common in females
Especially with nervous temperament
More likely if fed >2.5kg concentrates/day
Most common when horse held back to jog or paced gallop (so more often clinical signs during training, rather than competition)
Might be able to avoid by allowing more free movement to start with and then once warmed up can do more paced work
Find triggers - e.g. might happen more if worked at certain time of day
Familial lines affected, so genetic predisposition suspected, but no specific gene identified
Associated with dysfunction in intramuscular calcium regulation
No specific diagnostic test
PSSM - what are the 2 types? Which horses are affected?
Type 1:
- Associated with glycogen synthase (GYS) genetic mutation –> enhanced glycogen storage
- Most common in European draft breeds (beneficial when used in battle for long slow speed movement)
- Low prevalence in light breeds e.g. Arabs, TBs, SBs (extremely unlikely)
Type 2:
- Changes on biopsy, no genetic association identified yet
- 35% of total PSSM cases in UK are type 2
- >80% of WBs with PSSM are type 2
Both have the same biopsy result: amylase resistant polysaccharide storage
Myofibrillar myopathy (MFM) - what is it? Which horses? Diagnosis?
Potential subset of PSSM2
Arabs and WBs mostly:
- WBs causes poor performance, may not have elevated CK so can be easily missed
- Arabs causes acute tying up episodes (usually severe), mildly elevated CK
Diagnose with biopsy:
- Similar to PSSM2
- Requires Desmin staining (Desmin aggregation)
What types of non exertional myopathies are there?
Atypical myopathy
Post anaesthetic myopathy
Compartmental syndrome/trauma (e.g. trapped in gate)
Immune or infection associated myopathies (e.g. Strep equi equi or Clostridial myositis)
Less common:
(Malignant hyperthermia) - most commonly Quarter horses in America
(Hyperkalaemic periodic paralysis) - often quarter horses
(Toxic ionophore) - more common in places using ionophore fertilisers (banned in UK)
Atypical myopathy - Cause? Weather conditions? Which horses?
Hypoglycin A in sycamore seeds/seedlings (seedlings have helicopter appearance and red stem) - metabolised in liver, resulting in multiple acyl CoA dehydrogenase deficiency
Primarily affects type 1 oxidative muscles - muscles become starved of energy
Pasture contaminated Acer spps
Typically Autumn from seeds
Also Spring from seedlings
Young animals without supplementary feeding - see outbreaks after wet and windy conditions
Post anaesthetic myopathy - presentation, diagnosis, when more likely?
Can be local e.g. triceps or generalised
Typically occur within 1hr of entering recovery (may not be able to stand, or if focal may become acutely lame once in stable and sweating over affected site)
May be associated with neuropathy
Dx with CK 4hrs post GA - mild elevation is normal, but dramatic suggests myopathy
More likely if:
- MAP <70mmHg
- Long surgery time
- Underlying myopathy e.g. PSSM, RER
- Poor position or padding
Compartmental syndrome - when seen? Which muscles?
Due to increased pressure in a tight space
Occasionally seen post trauma or post focal myoapthy e.g. post anaesthetic
More common in muscles with dense fascia:
- Triceps
- Deltoid
- Masseter
- HL extensors
- Gluteals
Can be very painful and difficult to manage
Occasionally indication to cut fascia to release pressure
What immune or infection related myopathies are there?
Clostridial myonecrosis (more common in warm weather places):
- Most commonly associated with IM injection (also penetrating injury)
- Get dramatic necrosis, starting with crackling of the skin and then rapidly sloughs
- Needs aggressive treatment and supportive care
- Becomes systemically unwell and can result in death <48hrs
Strep equi equi:
- Acute rhabdomyolysis (dramatic myopathy)
- Infarctive purpura haemorhhagica (well delineated oedema of limbs)
- Both of above require anti-inflamms and elimination of Strangles bacteria
- Occurs more commonly in horses vaccinated with high titres
Viral associated myopathies:
- Rare but possible with EI, EHV
What are the acute effects of myopathies? Secondary complications?
Marked pain, distress
Often rapidly become dehydrated and have electrolyte (e.g. K released into bloodstream from lysed cells) and acid base derangements (often tachypnoeic due to pain, and high metabolic demands)
Inappropriate energy usage, leading to energy depletion despite enough fuel
Further muscle damage due to strain in one area leading to muscle damage elsewhere
Secondary complications:
- AKI - myoglobin filtered by kidney (can cope with this well if well hydrated and euvolaemic, but can run into trouble if hypovolaemic - care if also giving NSAIDs)
- Colic - more common with atypical myopathy
- Dysphagia - more common with atypical myopathy
- Head oedema - more common with atypical myopathy with head down for long period
What to do if exertional myoapthy at competition site?
Best thing is transport over short distance to stabilise somewhere
Shouldn’t travel long distances for at least first couple days until doing better
Triage and treatment for acute myopathy?
Delay transport for 24-48hrs if possible (one exception is atypical myopathy - need to be in hospital, transport to nearest hospital ASAP)
Acepromazine - anxiolytic and muscle relaxation (give IV to avoid further IM damage)
+/- A2 agonists - good analgesia but care in atypical myopathy (causes further glucose dysregulation, but may need to use to stop box walking, or if very painful, or to pass urinary catheter - use cautiously)
NSAIDs - ideally after rehydration (probably fine just with enteral fluids in mild exertional case, others IV fluids)
Dantrolene? Good for RER and post anaesthetic. Reduces Ca release and to reduce further muscles being recruited.
Additional analgesia - CRI lidocaine can be very helpful, esp atypical myopathies. Methadone over morphine in theory (NMDA action).
Fluid and electrolyte supplementation - correct acid-base balance, hypoCa and hyperK
Nutritional and nursing support - elevate head if horse has sore neck with head down for long periods (some won’t tolerate this), put food right in front of them if needed
What complications can occur with acute myopathies?
Atypical myopathy:
- Cardiac and respiratory dysfunction
- Colic and dysphagia
- Bladder dysfunction - can become distended and require catheterisation
- Hyperlipaemia
- All above can be fatal if left unmanaged
All acute myopathies:
- AKI if hypovolaemic
- Recumbency - avoid as will likely become increasingly difficult to get back on feet and get further worsening (not so bad if small pony that can lift back up)
Continued treatment of acute myopathy after stabilisation/triage?
Box rest vs turnout:
- Depends on cause but with exertional myopathies, rest for 48hrs, then get out quite quickly in small paddock with gradual T/O if now relatively comfortable in stable - gentle frequent movement is useful, use sedation at first
- Atypical myopathy different - will be hospitalised for a while
Diet:
- Need to give fuel they can actually use
- PSSM: if normal or underweight give them fat as energy provision, if overweight then cut calories down initially (<10% NSC)
- Atypical myopathies: can’t use fat, need to provide forms of glucose (often on glucose infusion if hyperlipaemic)
Exercise:
- RER: literature states to gradually start once CK <3000
- PSSM: very gradual, increments of 2 mins at a time (including the walk from the stable to the menage) once diet is well under control
Diagnostic approach for muscle pain/suspected myopathy?
