Weakness

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Approach to weakness.

This article outlines a systematic approach to weakness. The examination of the motor system is covered elsewhere (See Motor System Examination)

Overview

When evaluating weakness the following distinction is made:

  1. Upper motor neuron lesions (lesion in cerebral cortex, brainstem, or descending motor pathway of spinal cord). Also called pyramidal tract disease, long tract signs, or central weakness. Causes increased tone, increased reflexes, pyramidal pattern of weakness (weak extensors in the arm, weak flexors in the leg)
  2. Lower motor neuron lesions (lesion in peripheral nerve and anterior horn cells of the spinal cord). Also called denervation disease or peripheral weakness. Causes wasting, fasciculation, decreased tone, and absent reflexes
  3. Muscle disease: Causes wasting, decreased tone, impaired or absent reflexes
  4. Neuromuscular junction disorders: Causes fatigable weakness, normal or decreased tone, normal reflexes
  5. Functional weakness: Causes normal tone, normal reflexes without wasting with erratic power.

The first two are the most common. Each of them is associated with distinct physical signs, neuroanatomy, and causes. Table 1 outlines some characteristic findings, however the findings are only help if present, not if absent. Also full lower or upper motor neuron syndromes are often incomplete. For example up to a quarter of patients with upper motor neuron weakness lack hyperreflexia. Also in many lower motor neuron weakness conditions there is no clinical reflex (e.g. median or ulnar neuropathy).

Table 1. Differential Diagnosis of Weakness
MOTOR EXAMINATION
Location of Lesion Muscle Tone Atrophy or Fasiculations Sensory Findings Muscle Stretch Reflexes Other Findings
Upper motor neuron Spasticity No Sometimes Increased Babinski or Hoffman Sign
Lower motor neuron Hypotonia Yes Usually‡ Decreased/absent Depends on lesion, e.g. Tinel's
Neuromuscular junction Normal or hypotonia No No Normal/decreased Ptosis, diplopia
Muscle Normal No (except if late) No Normal/decreased Myotonia
Functional Normal No Non-anatomical Normal Multiple, e.g. Hoover's sign
‡ in the distribution of the spinal segment, plexus, or peripheral nerve.

Upper Motor Neuron Weakness

Upper motor neuron lesion: weak and flaccid (generally distal) muscles that eventually become spastic, hypertonic, and hyperreflexive. There is an association with pathologic reflexes (such as Babinski's and Hoffmann) and induced clonus of the ankle or wrist. Spasticity is prominent in the antigravity muscles (flexors of the upper extremities and extensors of the lower extremities). Spasticity is associated with clasp-knife finding due to a variable degree of resistance to passive motion.

Upper motor neurons cross over to the contralateral side at the decussation of the pyramids (between the brainstem and spinal cord). Therefore upper motor neuron type weakness can result from:

  • Ipsilateral spinal cord lesion: In spinal cord lesions there may be both upper and lower motor neuron type weakness. Lower motor neuron type weakness occurs at the level of the lesion, and upper motor neuron type weakness occurs below the level of the lesion.
  • Contralateral brainstem lesion
  • Contralateral cerebral hemisphere lesion:

Common aetiologies are cerebrovascular disease, multiple sclerosis, and brain tumours.

The upper motor neuron pathway extends from the cerebral cortex down through the spinal cord. It travels in tight quarters with central neurons that innervate other structures. Therefore there are localising lesions in this pathway to pinpoint its location:

Table 2. Localising Signs in Upper Motor Neuron Weakness
Anatomic Location Associated Finding
Cerebral hemisphere Seizures
Hemianopia
Aphasia (right hemiparesis)
Inattention to left body, apraxia (left hemiparesis)
Cortical sensory loss
Hyperactive jaw jerk
Brainstem Crossed motor findings
—Contralateral third nerve palsy (midbrain)
—Contralateral sixth nerve palsy (pons)
Spinal cord Sensory level
Pain and temperature sensory loss on contralateral arm and leg
No sensory or motor findings in face
Additional lower motor neuron findings (atrophy, fasciculations)

Lower Motor Neuron Weakness

Lower motor neuron lesion: weakness and paralysis of (generally distal) affected muscles. Flaccidity, hypotonia, diminished or absent stretch reflexes, and eventually atrophy. Fasciculations occur which are visible twitches of small groups of muscle fibres. There are no pathologic reflexes.

Common aetiologies are polyneuropathy (diabetes, alcohol use disorder), disc herniation, peripheral nerve entrapment, and trauma.

In monoparesis of lower motor neuron type, first determine if the affected muscles are supplied by a single spinal segment (radiculopathy), peripheral nerve (peripheral neuropathy), or a combination (plexopathy). The lesion is always ipsilateral to the side of the weakness. Some discussion on how differentiation can be done is provided in the pages on cervical radicular pain and lumbar radicular pain. The myotomal pattern of innervation is different to the dermatomal pattern of innervation. By referencing the complete myotomes chart, one can differentiate between a proximal lesion (spinal nerve or root) versus a lesion in a peripheral nerve (radial, ulnar, median, peroneal nerve, etc). For example, patients with C6 radiculopathy may have scapular winging in addition to distal weakness. A peripheral nerve lesion is not going to cause proximal motor weakness.

Combined Upper and Lower Motor Neuron Weakness

Seen in myelopathy and amyotrophic lateral sclerosis

Differentiating Upper from Lower Motor Neuron Weakness

Both cause a weakness that is typically confined to the distal muscles and both can be either symmetric or asymmetric.

Hemiparesis is a feature of upper motor neuron disease. Other than hemiparesis, location of weakness doesn't distinguish upper from lower motor neuron disease.

What does distinguish them is associated findings especially muscle tone, reflexes, fasciculations and atrophy. Lower motor neuron disease weakens or paralyses muscles, while upper motor neuron disease impairs their movements (see Table 1).

Neuromuscular Disease

Neuromuscular disease: Consider in patients where the weakness varies during the day, or in those with diplopia or ptosis. Neuromuscular disease can be excluded if there are abnormalities of sensation, tone, or reflexes. Fatiguability is a unique hallmark. Resting improves strength. Neuromuscular disease symptoms are also very proximal, hence they affect the facial muscles with ptosis, diplopia, difficulty chewing and swallowing, slurred speech, and facial weakness. There is no sensory loss.

Myaesthenia gravis is the cardinal clinical disease here.

Muscle Disease

Muscle disease: consider if a patient has symmetrical weakness of the proximal muscles of the arms and legs. This can be associated with muscle pain, dysphagia, and weakness of the neck muscles.

Aetiologies include drug-induced myopathy, thyroid disease, polymyositis, and rhabdomyolysis.

Weakness Syndromes

Part or all of this article or section is derived from Weakness by ddxof, used under CC-BY-SA

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

See Also