Myopathy

By Holly Craig, Catherine Roberts and Hayley Almond (4th year students)

MYOPATHY = DISEASE OF MUSCLES
(NORMAL INNERVATION AND NMJ FUNCTION)

History

  • Age at onset is very variable.
PC/HPC:
  • Muscle weakness may impair ADLs.
  • Symptoms may help identify involved muscle groups:
    • Proximal muscle weakness: Difficulty rising from chairs, getting out of the bathtub, climbing stairs, and/or shaving or combing the hair, etc.
    • Weakness of distal muscles: Weak grasp, handwriting problems, and walking difficulties etc.
  • Rate of onset may help with aetiology:
    • Acute (progression over hours): ?toxic aetiology or ?periodic paralysis (acute weakness due to potassium shifts, leading to muscle dysfunction)
    • Subacute (progression over days): ?acute inflammatory myopathy or ?rhabdomyolysis
    • Chronic (progression over weeks): ?inflammatory myopathy, ?drug-induced myopathy or ?endocrine-induced myopathy
    • Chronic (progression over years): congenital myopathy
  • Most myopathy gets worse as day progresses (though marked fatiguability suggests a defect in neuromuscular junction transmission).
  • Rarely, muscle pain and tenderness (Significant muscle pain and tenderness without weakness should prompt consideration of other causes).
  • Also, consider the following:
    • Any fever (polymyositis)?
    • Malaise and fatigue?
    • Skin changes (e.g. Gottron’s papules in dermatomyositis)?
    • Dark-coloured urine (myoglobinuria)?
    • Absence of sensory complaints or paraesthesias?
PMH:
  • Any autoimmune disease?
  • Any endocrine disease (e.g.hyper/hypothyroidism)?
  • Any CKD (uraemic myopathy)?
  • Alcoholism (chronic use or binge drinking)?
  • Any previous episodes of severe weakness (e.g. post-exercise, post-cold exposure, post-high-carbohydrate meal – possibly one of the periodic paralyses)?
DH:
  • Steroids?
  • Lipid-lowering agents?
  • Alcohol?
  • Retrovirals?
  • Heroin?
FH:
  • Any periodic paralysis?
  • Any muscular dystrophy?
SH:
  • Take an occupational history (any risk of chemical inhalation, e.g. barium chloride or barium carbonate in the ceramics industry)?

Examination

INSPECTION:
  • normal muscle mass (atrophy is later finding – early presence usually implicates neuropathies)
  • skin changes (e.g. Gottron’s papules, heliotrope rash in dermatomyositis)
File.128, 05/03/2001, 11:34 AM, 8C, 874×1299 (285+344), 25%, Default Settin, 1/120 s, R33.0, G54.0, B90.0
TONE:
  • normal tone
POWER:
  • reduced power
  • look for waddling gait (pelvic girdle weakness)
  • difficulty rising from a chair with the arms crossed = proximal weakness
  • Gower’s sign can be looked for (if the floor is clean and the patient willing!)
duchenne-muscular-dystrophy-11-638

REFLEXES:
  • usually preserved (hyporeflexia is late finding – early presence usually implicates neuropathies)
SENSORY:
  • normal sensation
Here is a great example of how muscle disease typically presents

Top tips

MOST BASIC ASSESSMENT OF MUSCLE WEAKNESS (if pushed for time):
  • tip toe/heel walking – difficult with distal weakness.
  • squatting (or standing from sitting without using arms) – difficult with proximal weakness.
SPECIFIC MUSCLE GROUP – Head Drop:

neck extensor weakness

There are only a handful of differential diagnoses for a neck weakness:

ACUTE HEAD DROP

(days-weeks)

CHRONIC HEAD DROP
GBS MND
Polymyositis Polymyositis
Myasthenia Gravis Dermatomyositis

Investigation

  • baseline bloods – FBC, U&Es, LFTs, calcium and magnesium, TFTs
  • inflammatory markers – CRP and ESR
  • CK
  • autoantibodies – e.g. ANA, ds-DNA
  • urinalysis – casts, myoglobin
  • genetic testing
  • ECG – hypokalaemic changes (T wave flattening, ST depression, T wave inversion, increased P wave, prolonged PR interval, U wave)
  • EMG and NCS
  • muscle biopsy

 

Diagnosis-IBM_biopsy
muscle biopsy features in inclusion body myositis
inclusion body mysositis
and the little inclusion body itself (blue arrow)

Aetiology

There are a huge number of possible causes of myopathies which include:

  • immune-mediated myopathy (e.g. sarcoidosis, polymyositis, dermatomyositis)
  • CTD-associated idiopathic myopathy (e.g. SLE, RA, polyarteritis nodosa)
  • alcoholic myopathy (acute and chronic component)
  • infectious myopathy (e.g. toxoplasmosis, HIV, influenza, lyme, Staphylococcus aureus)
  • endocrine-induced myopathy (Addison’s, Cushing’s, hypo/hyperthyroidism, hyperparathyroidism)
  • drug-induced or toxic myopathy (e.g. steroids, statins, cocaine, colchicine, amiodarone and others that inhibit CYP3A4 when combined with simvastatin)
  • acute periodic paralysis
  • inherited myopathies (e.g. muscular dystrophies)

Management

I would advocate that the right answer to any question on management is best answered with the following statement:
"I would start with patient and carer education and a multi-disciplinary approach..."
You can then follow this up with medication, surgery etc. etc.

Treatment is dependent on aetiology, but broadly can be separated into either SUPPORTIVE therapy or TARGETED therapy.

SUPPORTIVE

Supportive therapy can include maintaining a patent airway and minimising risk of aspiration when profound weakness is present, or giving fluids and diuretics to maintain renal function and minimise risk of acute tubular necrosis following precipitation of myoglobin in the renal tubules in alcoholic myopathy.

Always make sure you have a good idea of the respiratory and bulbar function. FVC is a better test than oxygen sats, respiratory rate (please do not do peak flow!). A normal blood gas can mislead you, especially if the respiratory rate is high (patients should have a low pCO2 in this setting; if they don't then they are getting tired.

If you are really stuck, go with breath count - ask the patient to take a deep breath in and slowly count as far as they can in one breath. This is the baseline and, if it is dropping steadily, you have every right to start to worry...

TARGETED

Targeted therapy is treatment of the underlying pathology, e.g. normalising thyroid function by giving thyroxine for hypothyroidism.

Prognosis

Prognosis is also very dependent of aetiology. E.g. congenital myopathies are chronic and progress over years, whereas other myopathies are more acute and can quickly lead to respiratory failure and other complications. Some myopathies (e.g. drug-induced myopathy, or endocrine-induced myopathy) are completely reversible.

Revision cards

Myotonic dystrophy

myotonic dystrophy

Muscular dystrophy

muscular dystrophy

Polymyositis

polymyositis

Dermatomyositis

dermatomyositis

Inclusion body myositis

inclusion body myositis

Drug-induced myositis

drug induced myositis

References:

  1. Handy emedicine article on muscle disease
  2. Genetics home reference article on myotonic dystrophy
  3. Patient website article on muscular dystrophy
  4. Patient’s journey article on Polymyositis
  5. Patient website article on myositis
  6. Practical Neurology article showing the clinical features of Inclusion Body Myositis.
  7. Emedicine article on Inclusion Body Myositis.
  8. Practical Neurology article on drug-induced muscle disease (and the article itself – mastaglia_practical neurology_2006_Drug induced myositis).
  9. Practial Neurology article covers the whole topic in an easy to digest fashion merrison_prac neurol_2009_bare essentials muscle disease.
  10. And another frankly brilliant one on mimics and chameleons walters_prac neurol_2014_muscle disease.

 

 

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