Weakness overview
Distribution: reflects the site of lesion (e.g. proximal, distal, bilateral, unilateral, etc.)
Type: reflects the element of the nervous system that's impaired |
To understand nature of pathology & aetiology of muscle weakness, what 4 things need to be considered?
1. Type of weakness
2. Distribution of weakness
3. Time course of onset
4. Mode of onset |
Weakness vs. fatigue:
- Weakness: is regional
- Fatigue: is generalised |
Pt records:
- Sense of clumsiness, tightness, instability
- Uncoordinated movement
- Numbness |
Physical manifestations:
- Clumsiness
- Uncoordinated movement
- Loss of fine motor skills
- Imbalance |
4 considerations of weakness:
- Type of weakness
- Distribution of weakness
- Time course of onset
- Mode of onset |
Etiology
Introduction: Causes of weakness are categorised by lesion location. Some disorders have characteristic of lesions in more than one location. E.g. Amyotrophic lateral sclerosis (ALS) may have findings of both UMN & LMN dysfunction. Disorders of the SC may affect tracts from UMN, LMN (anterior horn cells), or both |
Common causes of focal weakness
- Stroke: unilateral weakness
- Neuropathies: including trauma or entrapment caused (e.g. carpal tunnel syndrome) & that are immune-mediated (e.g. Bell palsy) |
Temporary focal weakness:
- TIA (transient ischemic attack)
- Hypoglycaemia: with treatment hypoglycaemia & resulting weakness resolve |
Generalised weakness:
- Disuse atrophy: resulting from illness or frailty (specifically in older pts)
- Generalised muscle wasting: due to prolonged immobilisation in an intensive care unit (ICU) - aka ICU myopathy
- Critical illness polyneuropathy - aka ICU neuropathy
- Common myopathies: e.g. alcoholic myopathy, hypokalaemia, corticosteroid myopathy
- Use of paralytic drugs in critical care pts |
Fatigue
Introduction: Many pts report weakness when they have fatigue. This can prevent maximal effort & muscle performance during strength testing |
Common causes:
- Acute severe illness of almost any cause, cancers, chronic infections (e.g.HIV, hepatitis, endocarditis, mononucleosis), endocrine disorders, renal failure, hepatic failure, heart failure, & anaemia
- Multiple sclerosis can cause daily fatigue that increases with exposure to heat & humidity
- Pts with fibromyalgia, depression or chronic fatigue syndrome may report weakness or fatigue but have no defined objective abnormalities |
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Pathophysiology
Introduction: Voluntary movement initiated in cerebral cortex in the posterior aspect of the frontal lobe. Neurons involved - UMN or corticospinal tract nuerons - synapse with LMN in the spinal cord. LMN transmit impulses to the neuromuscular junction to initiate muscle contraction |
Common mechanism of weakness include dysfunction of:
- UMN: corticospinal & corticobulbar tract lesions
- LMN: due to peripheral poly neuropathies or anterior horn cell lesions
- Neuromuscular junction
- Muscle: due to myopathies |
Location correlates with physical findings:
- UMNL: disinhibits LMN → causing ↑ muscle tone (spacticity) & ↑ muscle stretch reflexes (hyperreflexia); Babinski reflex is specific for corticospinal tract dysfunction; UMNL can also ↓ tone & reflexes if motor paralysis is sudden & severe (e.g spinal cord transections → tone 1st ↓, then ↑ gradually over days/weeks) or if lesion damages the motor cortex of the precentral gyrus & not nearby motor associations
- LMNL: disrupts reflex arcs → causing hyporefelxia & ↑ muscle tone (flaccidity), & may cause fasciculations; with time, muscle atrophy
- Peripheral polyneuropathies: most noticeable in the longest nerves (i.e. weakness is prominent in the distal limb than the proximal & in legs more than arms) & produce signs of LMNL (e.g. ↓ reflexes & muscle tone)
- Neuromuscular junction - myasthenia gravis: (most common disorder) fluctuating weakness that worsens with activity & lessens with rest
- Diffuse muscle dysfunction: (myopathies) most noticeable in the largest muscle groups (proximal muscles) |
Evaluation
Temporal pattern: |
Anatomic pattern: |
Physical examination: |
Additional findings: |
Key points
Distinguish loss of muscle strength from a feeling of fatigue |
⏺ If fatigue has no anatomic or temporal pattern of weakness in pts with a normal physical examination, suspect chronic fatigue syndrome, an as-yet undiscovered systemic illness (e.g. severe anaemia, hypothyroidism, Addison disease), a psychologic problem (e.g. depression), or an adverse drug effect |
⏺ If pts have true muscle weakness, first focus on determining whether weakness is caused by dysfunction of the brain, spinal cord, plexuses, peripheral nerves, neuromuscular junction, or muscles |
⏺ If pts have hyperreflexia & increased muscle tone (spasticity), particularly if Babinski reflex is present, suspect an UMN (corticospinal tract) lesion in the brain or spinal: MRI is usually required |
⏺ If pts have hyporeflexia, ↓ tone, atrophy, & fasciculations, suspect a LMNL |
⏺ If pts have difficulty climbing stairs, combing hair, & standing up with predominantly proximal muscle weakness & intact sensation, suspect myopathy |
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