Muscle diseases - just learn Duchenne's and myasthenia Flashcards

1
Q

Symptoms (3) /signs (5) of muscle disorders in general

A

Symptoms:

  • Dyspnoea (respiratory muscles not getting enough O2)
  • Dysphagia (–> aspiration)
  • Muscle cramp/pain/stiffness

Signs:

  • Motor weakness
  • Myoglobinuria (myoglobin in urine; usually present inside muscle but released into blood as a result of muscle destruction) –> dark brown urine
  • Muscle wasting OR hypertrophy
  • Normal/reduced tone + reflexes
  • Cardiomyopathy (diseases of skeletal muscle can affect cardiac muscle)
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2
Q

2 classifications of muscle diseases/disorders

A

Congenital/genetic

Acquired

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3
Q

Name 4 subcategories of congenital muscle diseases + give an example for each category

A

Structural: muscular dystrophies
Contractile: congenital myopathies
Coupling: channelopathies
Energy: enzyme defect or mitochondrial defect

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4
Q

Name 4 subcategories of acquired muscle diseases

A

Metabolic muscle diseases
Endocrine muscle diseases
Inflammatory muscle diseases
Iatrogenic muscle diseases - induced by medication

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5
Q

Investigation of muscle diseases (4)

A

History and examination

Bloods - Creatine Kinase (CK) (increased in muscle degeneration)

EMG (electromyography) - inserting a small needle into the muscle, to view and listen to the electrical activity generated within the muscle

Muscle biopsy

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6
Q

Name 2 types of muscular dystrophy (congenital muscle disease)

A

Duchenne’s

Becker’s

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7
Q

What is muscular dystrophy (congenital muscle disease)

A

PROGRESSIVE degeneration and re-generation of muscle fibres

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8
Q

Pathophysiology of muscular dystrophies

A

GENETIC CAUSE:

Gene deletion or mutation –> defective or specifically absent glycoproteins (e.g., dystrophin) in the muscle membrane

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9
Q

Clinical features of muscular dystrophies (6)

A
Muscle weakness
Muscle wasting
Decreased muscle tone (flaccidity)
Walking on toes/heel
Imbalance of lower limbs 
Respiratory difficulty
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10
Q

Clinical features of muscle disease in babies (3)

A

Failure to thrive
Poor feeding/latching
Floppy/weak

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11
Q

Describe channelopathies (congenital muscle disease)

A

Disorders of Ca, Na, K and Cl channels on the muscle membrane

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12
Q

Duchenne’s muscular dystrophy treatment (not curative)

A

Steroids

Supportive care - treating respiratory distress, swallowing issues, physio

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13
Q

Relationship of dystrophin and duchenne’s (congenital muscle disease)

A

Absent dystrophin which usually provides structural stability to cell membranes

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14
Q

Main treatment for muscle diseases

A

Supportive care/symptomatic care

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15
Q

Metabolic/energy deficient muscle diseases* pathophysiology

*congenital muscle disease

A

Enzyme defect or mitochondrial defect due to genetic mutations

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16
Q

Name the 2 types of *inflammatory muscle disease (myositis) + describe them

*acquired muscle disease

A

Polymyositis - uncommon inflammatory disease that causes muscle weakness affecting both sides

Dermatomyositis - additional rash + muscle weakness; rash appears before the onset of muscle symptoms

17
Q

Cause of polymyositis and dermatomyositis (inflammatory muscle disease)

A

Autoimmune - immune system starts attacking healthy tissue

18
Q

Clinical features of inflammatory muscle disease (4) + identify the sign suggestive of dermatomyositis

A

Muscle weakness
Muscle atrophy
Frequent falls
Fatigue after standing/walking

Rash specifically in dermatomyositis

19
Q

Definitive investigation for inflammatory muscle disease, e.g. polymyositis, dermatomyositis

A

Muscle biopsy

20
Q

Definitive investigation for duchenne’s

A

DNA testing for specific mutation

21
Q

What is myasthenia gravis

A

An AUTOIMMUNE disorder of the neuromuscular junction

22
Q

Pathophysiology of myasthenia gravis

-involves 1 main autoantibody + another in minority

A

End plate potential (EPP) fails to effectively activate the muscle fibre due to an AUTOIMMUNE reaction against acetylcholine receptors on the muscle

  • ACh RECEPTOR AUTOANTIBODY binds to acetylcholine receptor on the muscle membrane surface preventing ACh from binding to it so can’t activate muscle
  • minority of people produce MUSCLE SPECIFIC TYROSINE KINASE RECEPTOR AUTOANTIBODY (Anti MuSK ab) which attacks proteins INSIDE muscle cell rather on muscle membrane surface
23
Q

Symptoms/signs of myasthenia gravis (7)

+ does it affect sensation

A
Fatiguable weakness of skeletal muscle:
– Proximal limb weakness
– Ptosis
– Facial weakness - facial expression muscles + muscles of mastication
– Dysphagia
– Dysarthria 
– Diplopia
– Dyspnoea  (less common) --> resp failure if severe

NO SENSORY SIGNS

24
Q

List the 3 main clinical features of myasthenia gravis

A

Muscle weakness
Fatigue
Normal sensation

25
Q

Myasthenia gratis investigations (4)

A

AChR ab (ACh receptor auto- antibody)

Anti MuSK ab (muscle-specific tyrosine kinase auto-antibody)

Neurophysiology

  • Repetitive nerve stimulation (similar to nerve conduction study) - shows slowed response
  • Single fibre EMG - shows increased Jitter
26
Q

What result does an EMG show in myasthenia gravis

A

INCREASED Jitter

27
Q

Myasthenia gravis treatment
-1st line
+/- (3)

A

Acetylcholinesterase inhibitor - Pyridostigmine

+/- prednisolone/other immunosuppressant

+/- plasma exchange OR IV immunoglobulin

+/- thymectomy

28
Q

What tumour is myasthenia gravis associated with

A

Thymoma

-causes myasthenia to present as a paraneoplastic syndrome where thymoma secretes ACh receptor autoantibodies