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Neurology > Muscle weakness > Flashcards

Muscle weakness Flashcards

1
Q

Describe the potential lesion locations causing muscle weakness from spinal cord to muscle.

A

Anterior horn cell –> Spinal roots –> Ganglion –> Plexus –> Peripheral nerve (polyneuropathy) –> Peripheral nerve (mononeuropathy) –> NMJ –> Muscle

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2
Q

What can cause damage to the anterior horn cell/motor neuron to produce weakness?

A
  • Spinal muscular atrophy
  • Infections (Polio, West Nile virus, enteroviruses)
  • AML
  • Progressive bulbar palsy
  • Paraneoplastic syndrome
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3
Q

What can cause damage to the spinal roots to produce weakness?

A
  • Neurofibromatosis
  • Herniated disc
  • Infections
  • Neoplastic
  • Spinal foraminal stenosis
  • Trauma
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4
Q

What can cause damage to a nerve plexus to produce weakness?

A
  • Acute brachial neuritis
  • Autoimmune disorders
  • Diabetes mellitus
  • Haematoma
  • Neoplasm
  • Neurofibromatosis
  • Traction during birth
  • Trauma
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5
Q

What can cause damage to peripheral nerves to produce weakness?

A
  • Entrapment
  • Hereditary (Charcot-Marie-Tooth disease)
  • Infections (Hep C, HSV, HIV, Lyme disease etc)
  • Inflammatory (CIDP, polyradiculoneuropathy, GBS)
  • Ischaemia
  • Diabetes
  • Alcohol
  • Vitamin B deficiency
  • Amyloidosis
  • Renal insufficiency
  • Poisoning - lead, arsenic
  • Porphyria
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6
Q

What can cause damage to the NMJ to produce weakness?

A
  • Myasthenia gravis
  • Lambert-Eaton syndrome
  • Congenital myasthenia (rare)
  • Botulism in infants
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7
Q

What can cause damage to the muscle fibres to produce weakness?

A
  • Dystrophies (DMD, Becker’s, limb-girdle, fascioscapulohumeral)
  • Myotonic dystrophy
  • Congenital causes
  • Endocrine (acromegaly, Cushing’s, hypothyroidism)
  • Inflammatory (polymyositis, dermatomyositis)
  • Metabolic
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8
Q

What is the aetiology and pathophysiology of botulism?

A

An uncommon but lethal disease which is caused by exotoxins of the anaerobe Clostridium Botulinum –> toxin is carried to the NMJ where it binds irreversibly

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9
Q

When do features of botulism begin post contamination?

A

6 hours to 8 days

- earlier onset associated with more severe features

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10
Q

What are the clinical features of botulism

A
  • Progressive, descending flaccid weakness
  • GI disturbance
  • Sore throat
  • Fatigue
  • Dizziness
  • Paraesthesia
  • Potential cranial nerve involvement
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11
Q

How is botulism diagnosed?

A
  • Isolating Clostridium from the stool, gastric aspirate, or wound
  • Mouse bioassay –> survival of immunised mice and death of non-immunised mice when infected serum is injected
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12
Q

What is the management of botulism?

A
  • Respiratory care (regular FVCs)
  • Toxin removal (sorbitol, avoid magnesium)
  • Antitoxin (prevent further paralysis)
  • Wounds: surgical debridement and benzylpenicillin + metronidazole
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13
Q

What is the prevalence and incidence of myasthenia gravis?

A

Prevalence: 15 per 100,000 population

Bimodal peak in incidence: 20-30s and 60-70s

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14
Q

Describe the pathophysiology of myasthenia gravis.

A
  • Acetylcholine receptor antibodies (AChR-Abs) bind to ACh-Rs on the motor endplate of skeletal muscles
  • Binding of AChs blocked
  • Once a threshold of motor endplates is lost, the endplate potentials are reduced and fail to trigger a muscle action potential
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15
Q

What are the causes of myasthenia gravis?

A
  • Most cases are autoimmune (80-90%)

- Thymus abnormalities (thymic hyperplasia or thymoma)

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16
Q

Which autoimmune conditions are associated with myasthenia gravis?

A
  • Thyroiditis
  • Graves’ disease
  • Rheumatoid arthritis
  • SLE
  • Pernicious anaemia
  • Addison’s disease
  • Vitiligo
  • NMO
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17
Q

What are the clinical subtypes of myasthenia gravis?

A
  • Generalised MG: affects multiple muscle groups

- Ocular MG: weakness limited to eyelids and extraocular muscles

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18
Q

What are the antibody subtypes of myasthenia gravis?

A
  • MG with AChR-Abs (80-90%)
  • MG with Anti-MuSK (4%)
  • MG with Anti-LRP4 (2%)
  • Seronegative MG
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19
Q

What is the hallmark feature of myasthenia gravis?

A

Fluctuating, fatiguable weakness

  • Symptoms worse at end of the day or following exercise
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20
Q

What are the ocular features of MG?

A
  • Diplopia
  • Ptosis (enhanced on prolonged upward gaze)
  • Weak eye movements (unusual pattern of weakness that does not correlate to a single nerve/muscle)
  • Pupillary sparing
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21
Q

What are the ‘generalised’ features of MG?

A
  • Bulbar muscles: fatiguable chewing, dysarthria, dysphagia
  • Facial muscles: expressionless face, poor smile/myasthenic sneer
  • Neck muscles: ‘dropped-head syndrome, unable to withstand resistance against forehead
  • Limbs: mainly proximal, and arms affected more than legs
  • Respiratory muscles: can be serious and lead to life-threatening respiratory failure
22
Q

What investigations should be performed for MG?

A
  • Serological (AChR Ab, MuSK, LRP4)
  • EMG
  • Examination of thymus
  • TFTs
  • Ice- pack test
  • Rheumatological screen (ANA,ENA, CCP, RF)
  • Edrophonium (tensilon) test –> no longer done in clinical practice
23
Q

Describe the ice pack test in the diagnosis of myasthenia gravis.

A
  • Relies on the principal that NMJ transmission is better at lower temperatures
  • Apply an ice pack a closed eyelid
  • In patients with MG there should be an improvement in ptosis
24
Q

Describe the edrophonium (tensilon) test in the diagnosis of myasthenia gravis.

A
  • Infusion of acetylcholine esterase inhibitor to look for a brief improvement in symptoms
25
Q

Describe the appearance of MG on EMG.

A

Repeated nerve stimulation will result in a progressive decline in waveform amplitude, suggesting fatiguability

26
Q

What is the differential diagnoses of generalised MG?

A
  • Lambert-Eaton syndrome
  • Botulism
  • Drug-induced myasthenia (penicillamine)
  • Congenital myasthenic syndrome
  • Inflammatory myopathies
  • MND (bulbar onset)
27
Q

What is the differential diagnoses of ocular MG?

A
  • Disinsertion syndrome
  • Thyroid ophthalmopathy
  • Mitochondrial disease
  • Intracranial mass lesion
  • Wernicke’s encephalopathy
  • Oculopharyngeal muscular dystrophy
28
Q

What is the management of MG?

A
  • Pyridostigmine (15mg QDS for 2-4 days then titrate up to 300mg/day)
  • If no response: prednisolone (10mg in generalised MG and 5mg in ocular MG)
  • Thymectomy if suitable
29
Q

What is the definition of myasthenic crisis?

A

Worsening of weakness/myasthenia symptoms that required respiratory support

30
Q

What are the precipitants of myasthenic crisis?

A
  • Warm weather
  • Stress
  • Surgery, infections, intercurrent illness, comorbidities
  • Pregnancy
  • Medication
31
Q

What is the pharmacological management of a myasthenic crisis?

A
  • IVIG
  • Plasma exchange
  • Corticosteroids
  • Avoid pyridostigmine (respiratory secretions and risk of aspiration)
32
Q

What are the causes of Lambert-Eaton syndrome?

A
  • Small cell lung cancer (60%)

- Autoimmune (40%)

33
Q

Describe the pathophysiology of Lambert-Eaton syndrome.

A
  • Antibodies are produced against voltage-gated calcium channels
  • Voltage-gated calcium channels in the presynaptic terminals of the NMJ are damaged
  • Therefore less acetylcholine can be released into the synapse to stimulate muscle contraction
34
Q

What are the clinical features of Lambert-Eaton syndrome?

A
  • Proximal weakness around the pelvic girdle and weakness around shoulder girdle
  • Weakness improves with sustained/repeated exercise
  • Reflexes reappear after exercise
  • Cranial nerve involvement occurs in 30% (dysphagia, dysarthria, ptosis, diplopia)
  • Autonomic symptoms: dry mouth, blurred vision, impotence, dizziness
35
Q

What investigations can be carried out for Lambert-Eaton syndrome?

A
  • EMG
  • Single fibre studies
  • Imaging to detect malignancy
  • Serology for autoimmune cause
  • Edrophonium test (result may be +ve, but not to the same extent as in MG)
36
Q

What is the appearance of Lambert-Eaton syndrome on EMG?

A
  • Reduced amplitude of the compound muscle action potential (CMAP) after single supramaximal stimulus
  • Increase in amplitude after exercise (post-exercise facilitation)
  • Decremental response to repetitive stimulation at 3Hz and incremental response at 30Hz
37
Q

What is the management of Lambert-Eaton syndrome?

A
  • Diagnosis and management of malignancy
  • Amifampridine
  • Immunosuppression
  • IVIG
  • PLEX
38
Q

What is the MOA of Amifampridine?

A
  • Blocks voltage-gates potassium channels in the presynaptic channels
  • Prolongs the depolarisation of the presynaptic membrane
  • Allows calcium channels to release more acetylcholine into the synapse
39
Q

What is the aetiology + pathophysiology of polymyositis?

A
  • Inflammatory disorder causing symmetrical, proximal muscle weakness
  • T-cell mediated cytotoxic process directed against muscle fibres
  • May be idiopathic r associated with connective tissue disorders
  • Associated with malignancy
40
Q

What are the features of polymyositis?

A
  • Proximal muscle weakness +/- tenderness
  • Raynaud’s
  • Respiratory muscle weakness
  • Interstitial lung disease (fibrosing alveolitis, pneumonia)
  • Dysphagia, dysphonia
41
Q

What investigations should be done for polymyositis?

A
  • Creatine kinase (elevated)
  • Muscle enzymes (LDH, aldolase, AST, ALT)
  • EMG
  • Muscle biopsy
  • Anti-synthetase antibodies (Anti-Jo1 antibodies seen in diseases associated with lung involvement, Raynaud’s, and fever)
42
Q

What is the management of polymyositis?

A

Corticosteroids and immunosuppressants

43
Q

What is the aetiology + pathophysiology of dermatomyositis?

A
  • May be idiopathic or associated with connective tissue disorders or underlying malignancy
  • Unknown factor activates C3 –> formation of membrane attack complex –> accumulation in capillaries –> destruction of capillary wall –> microinfarctions
44
Q

What are the clinical features of dermatomyositis?

A
  • Progressive proximal muscle weakness
  • Heliotrope rash (purplish eyelids + potential periorbital oedema)
  • Gottron papules (scaling erythema of knuckles, elbows, knees)
  • Macular rash over back and shoulders
  • Photosensitivity
  • Raynaud’s
  • Interstitial lung disease
  • Dysphagia, dysphonia
45
Q

What investigations should be done for dermatomyositis?

A
  • CT if malignancy suspected
  • Creatine kinase (elevated)
  • LDH, aldolase, AST, ALT
  • ANA, Anti-Mi2
  • Muscle biopsy
  • EMG
46
Q

What is the management of dermatomyositis?

A
  • Corticosteroids
  • Immunosuppressive agents
  • IVIG
47
Q

Describe the pathophysiology of inclusion body myositis.

A

Uknown factor causes myofibres to present MHCI –> CD8+ T cells gather, recognise MHCI, bind –> express perforin –> pores form on myofibre membranes –> cell degeneration

48
Q

What are the clinical features of inclusion body myositis?

A
  • Slowly progressive muscle weakness, sometimes asymmetric
  • Muscle atrophy
  • Reduced tendon reflexes
49
Q

What investigations should be performed for inclusion body myositis?

A
  • CK (Mild elevation/may be normal)
  • Muscle biopsy
  • EMG (polyphasic motor unit action potentials with small amplitude, short duration)
50
Q

What is the management of inclusion body myositis?

A
  • Immunosuppressive therapy

- Physical + speech therapy

Neurology (46 decks)

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