Motor Neuron Signs

Upper Motor Neuron:

Spasticity

Hyperreflexia

Pronator drift

Babinski

Lower Motor Neuron:

Flaccidity

Hyporeflexia

Fasciculation

Atrophy

Causes of Weakness

Weakness Syndromes

Unilateral weakness, ipsilateral face

Lesion: Contralateral cortex, internal capsule

Causes: Stroke (sudden onset), demyelination/mass (gradual onset)

Symptoms: Neglect, visual field cut, aphasia

Findings: UMN signs

Key features: Association with headache suggests hemorrhage or mass

Unilateral weakness, contralateral face

Lesion: Brainstem

Causes: Vertebrobasilar insufficiency, demyelination

Symptoms: Dysphagia, dysarthria, diplopia, vertigo, nausea/vomiting

Findings: CN involvement, cerebellar abnormalities

Unilateral weakness, no facial involvement

Lesion: Contralateral medial cerebral cortex, discrete internal capsule

Causes: Stroke

Rare Cause: Brown-Sequard if contralateral hemibody pain and temperature sensory disturbance

Unilateral weakness single limb (monoparesis/plegia)

Lesion: Spinal cord, peripheral nerve, NMJ

UMN signs: Brown-Sequard if contralateral pain and temperature sensory disturbance

LMN signs: Radiculopathy if associated sensory disturbance

Normal reflexes, normal sensation: Consider NMJ disorder

Bilateral weakness of lower extremities (paraparesis/plegia)

Lesion: Spinal cord, peripheral nerve

UMN signs: Anterior cord syndrome (compression, ischemia, demyelination) if contralateral pain and temperature sensory disturbance

Cauda equina: Loss of perianal sensation, loss of rectal tone, or urinary retention

GBS: If no signs of cauda equina and sensory disturbances paralleling ascending weakness (with hyporeflexia)

Bilateral weakness of upper extremities

Lesion: Central cord syndrome

Causes: Syringomyelia, hyperextension injury

Findings: Pain and temperature sensory disturbances in upper extremities (intact proprioception)

Bilateral weakness of all four extremities (quadriparesis/plegia)

Lesion: Cervical spinal cord

Findings: UMN signs below level of injury, strength/sensory testing identifies level

Bilateral weakness, proximal groups

Lesion: Muscle

Causes: Rhabdomyolysis, polymyositis, dermatomyositis, myopathies

Findings: Muscle tenderness to palpation, no UMN signs, no sensory disturbances

Facial weakness, upper and lower face

Lesion: CNVII

Causes: Bell’s palsy, mastoiditis, parotitis

Other CN involvement suggests brainstem lesion, multiple cranial neuropathies, or NMJ

Review of Spinal Cord Anatomy

• Dorsal Column – Medial Lemniscus (fine touch, proprioception)
  1. Afferent sensory fibers with cell body in DRG
  2. Ascend in ipsilateral posterior column
  3. Synapse in medulla, decussate, ascend in contralateral medial lemniscus
  4. Synapse in thalamus (VPL)
  5. Synapse in sensory strip of post-central gyrus
• Spinothalamic Tract (pain, temperature)
  1. Afferent sensory fibers with cell body in DRG
  2. Ascends 1-2 levels
  3. Synapse in ipsilateral spinal cord, decussate, ascend in contralateral lateral spinothalamic tract
  4. Synapse in thalamus (VPL)
  5. Synapse in sensory strip of post-central gyrus
• Lateral Corticospinal Tract (motor)
  1. Efferent cell body in motor strip of pre-central gyrus
  2. Descends through internal capsule
  3. Decussates in pyramid of medulla, descends in contralateral lateral corticospinal tract
  4. Synapse in anterior horn, lower motor neuron to muscle fiber

References

1. Morchi, R. (2013). Weakness. In Rosen’s Emergency Medicine – Concepts and Clinical Practice (8th ed., Vol. 1, pp. 124-128). Elsevier Health Sciences.