Weakness

Definition:

Subjective cause of weakness:

• Anaemia, Infection, Malignancy, Cachexia, Depression
• De-conditioning
• Chronic Cardiac or Respiratory disease

Objective causes of weakness

Generalized weakness

• Myasthenia Gravis (worse on exertion)
• Metabolic:
  • Severe hypokalaemia (Familial Periodic Paralysis Syndrome)
  • Glycogen disorders (McArdle’s syndrome)
• Paralytic Shellfish poisoning [Reference]
• Heavy metal poisoning e.g. lead

Localized weakness

Symetrical

Asymetrical

UMNX (Upper Motor Neuron Lesion)

• Generally produces hypertonic or spastic paralysis:
• Stroke: Cerebral cortex, subcortical or brainstem stroke
• Demyelination
• Space-occupying lesion such as tumour, abscess or haematoma
• Spinal cord (above level of L1) – trauma, infarct, transverse myelitis, spinal epidural or subdural collection (abscess, haematoma), central disc herniation, tumour or demyelination

LMNX (Lower Motor Neuron Lesions)

• Generally produces flaccid paralysis:
• Spinal nerve root compression from disc herniation.
• Ascending polyneuropathy from Guillain-Barré syndrome.
• Peripheral neuropathy from diabetes, carcinoma, alcohol, B1 or B12 deficiency, drugs such as isoniazid, nitrofurantoin and phenytoin.
• Neuromuscular junction (NMJ) disorders (e.g. myasthenia gravis, organophosphate poisoning and botulism).
• Myopathies (e.g. polymyositis, alcohol-induced, steroid-related, muscular dystrophy, hypokalaemia and thyroid disease).

Progressive

• Proximal to distal (descending paralysis)
• Distal to Proximal (ascending paralysis)

Examination findings:

• Mixed UMN and LMN
  • Consider motor neuron disease, B12 deficiency, Friedrich’s ataxia.
• Anatomically disparate
  • Consider multiple sclerosis or mononeuritis multiplex.
• Bilateral LMN
  • Proximal LMN weakness with normal sensation is likely myopathy.
  • Distal LMN weakness with decreased reflexes and some sensory involvement is likely polyneuropathy.
  • Diffuse LMN weakness (including oculomotor and facial muscles) with normal reflexes and normal sensation is likely NMJ disorder.
• Unilateral LMN lesion
  • Radiculopathy (dermatomal or myotomal distribution).
  • Plexopathy (involvement of more than one spinal or peripheral nerve), usually lumbar or brachial.
  • Peripheral neuropathy.
• Bilateral UMN lesion
  • Large bilateral cerebral or brainstem lesion if altered mental status.
  • Spinal cord lesion if normal mental status. Sensory level and level of weakness determine site of lesion.
• Unilateral UMN lesion is likely to be contralateral cortical, subcortical or brainstem lesion, rarely a spinal cord hemisection (Brown-Séquard syndrome). Cortical lesions have ipsilateral limb and facial signs.
  • Anterior cerebral artery stroke syndrome—leg greater than arm weakness, personality change, oculomotor palsy, urinary incontinence.
  • Middle cerebral artery stroke syndrome—arm greater than leg weakness, dysphasia, dyspraxias, agnosias, neglect, poor two-point discrimination, dysgraphaesthesia.
  • Posterior cerebral artery stroke syndrome—visual disturbance.
  • Subcortical lacunar stroke syndrome—pure motor strokes, dense ipsilateral hemiplegia and hemi-anaesthesia.
  • Brainstem stroke syndrome—crossed abnormalities in facial and limb distributions.