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Neurologic Causes of Weakness and Paralysis¶

Chapter 26 | Part 2 – Cardinal Manifestations and Presentation of Diseases · Part 2 – Cardinal Manifestations & Presentation · Chapter 26

Key Clinical Points¶

Weakness is a reduction in muscle power; paralysis (plegia) indicates severe weakness where a muscle cannot be contracted at all; paresis refers to less severe weakness.
Upper motor neuron (UMN) weakness typically affects extensors/abductors of the upper limb and flexors of the lower limb, characterized by spasticity and hyperreflexia.
Lower motor neuron (LMN) weakness depends on the level of lesion (anterior horn, nerve root, plexus, or peripheral nerve) and presents with atrophy, fasciculations, and hyporeflexia.
Myopathic weakness is generally most marked in proximal muscles; neuromuscular junction (NMJ) disorders often present as fatigable weakness.
Psychogenic weakness is variable, inconsistent, and lacks a clear neuroanatomic basis; antagonists may contract during attempted agonist activation.
Spasticity is velocity-dependent with a 'clasp-knife' phenomenon; rigidity is constant throughout the range of motion (lead pipe or cogwheel).
Distal weakness suggests LMN or peripheral nerve disease; proximal weakness suggests myopathy or NMJ disorders.
α motor neurons drive muscle force and cause atrophy when lost; γ motor neurons maintain tension on muscle spindles to regulate tone and stretch reflexes.
Diagnostic logic utilizes clinical signs (UMN vs. LMN) to determine the primary imaging modality (Brain vs. Spinal MRI) or electrodiagnostic testing (EMG/NCS).
Table 26-1 provides a definitive comparison of UMN, LMN, Myopathic, and Psychogenic weakness based on atrophy, fasciculations, tone, distribution, reflexes, and Babinski sign.

1. DEFINITION & OVERVIEW¶

• Weakness: A reduction in the power that can be exerted by one or muscle. • Paralysis (plegia): Weakness so severe that a muscle cannot be contracted at all. • Paresis: Less severe weakness. • Terminology: 'hemi-' (one-half), 'para-' (both legs), 'quadri-' (all four limbs). • Tone: The resistance of a muscle to passive stretch. • Types of Increased Tone: ◦ Spasticity: Associated with UMN disease; velocity-dependent; features the 'clasp-knife' phenomenon; predominantly affects antigravity muscles (upper-limb flexors, lower-limb extensors). ◦ Rigidity: Hypertonia present throughout the range of motion ('lead pipe' or 'plastic' stiffness); affects flexors and extensors equally; may have a 'cogwheel' quality. ◦ Paratonia (gegenhalten): Increased tone varying irregularly with degree of relaxation; present throughout range of motion; typically results from disease of the frontal lobes. • Flaccidity: Weakness with decreased tone or normal tone, occurring in disorders of motor units. • Atrophy: ◦ UMN: Muscle bulk generally not affected (except mild disuse atrophy). ◦ LMN: Often conspicuous when a lower motor neuron lesion is responsible. ◦ Myopathy: Can be prominent in advanced stages.

1.1 Tone and Reflexes¶

• Muscle Stretch (Tendon) Reflexes: ◦ UMN lesions: Usually increased (hyperreflexia); may be absent briefly after acute injury. ◦ LMN lesions: Depressed or absent due to involvement of specific reflex arcs. ◦ Myopathic: Generally preserved, except in advanced stages where they may be attenuated. ◦ NMJ disorders: May be affected by preceding voluntary activity of the muscle. • Babinski Sign: ◦ UMN: Present. ◦ LMN/Myopathy/Psychogenic: Absent.

2. ETIOLOGY & PATHOPHYSIOLOGY¶

• Upper Motor Neuron (UMN) Weakness: ◦ Mechanism: Decreased activation of lower motor neurons. ◦ Distribution: Distal muscle groups affected more severely than proximal; axial movements spared unless lesion is severe and bilateral. ◦ Clinical Features: Spasticity, slowed/coarse rapid movements, hyperreflexia. ◦ Corticobulbar Involvement: Weakness in lower face and tongue; extraocular, upper facial, pharyngeal, and jaw muscles typically spared. ◦ Pseudobulbar Palsy: Bilateral corticobulbar lesions → dysarthria, dysphagia, dysphonia, emotional lability, and brisk jaw jerk. • Lower Motor Neuron (LMN) Weakness: ◦ Mechanism: Decreased number of muscle fibers activated due to loss of α motor neurons or disruption of their connections to muscle. ◦ α Motor Neurons: Large; numerous; innervate extrafusal muscle fibers. Loss leads to weakness and atrophy. ◦ γ Motor Neurons: Smaller; less numerous; innervate intrafusal fibers (muscle spindles). Loss does not cause weakness but decreases tension on muscle spindles, decreasing tone and attenuating stretch reflexes. ◦ Fasciculations: Spontaneous discharge of motor units, common in anterior horn cell diseases. • Neuromuscular Junction (NMJ) Weakness: ◦ Mechanism: Variable number of muscle fibers activated over time; dependent on state of rest. ◦ Clinical Feature: Fatigable weakness (e.g., Myasthenia Gravis); strength declines with sustained/repeated contraction. • Myopathic Weakness: ◦ Mechanism: Decreased number or contractile force of muscle fibers within motor units. ◦ EMG Findings: Reduced size of each motor unit action potential; rapid recruitment required to produce power. ◦ Specificity: Some myopathies involve loss of contractile force or selective involvement of type II (fast) fibers. • Psychogenic Weakness: ◦ Characteristics: No organic basis; variable/inconsistent; distribution not explained by neuroanatomy. ◦ Testing: Antagonists may contract during attempted agonist activation; severity out of keeping with daily activities.

2.1 Pathogenesis of Weakness Types¶

• Corticospinal System: ◦ Origin: Layer V of primary motor cortex (precentral gyrus, Brodmann area 4), premotor and supplemental motor cortex (area 6). ◦ Pathway: Subcortical white matter → posterior limb of internal capsule → cerebral peduncle → basis pontis → medullary pyramids. ◦ Decussation: At cervicomedullary junction, most decussate to contralateral lateral spinal cord; 10–30% remain ipsilateral in anterior spinal cord. ◦ Function: Innervate hand muscles; involved in learned, fine movements. • Bulbospinal Pathways: ◦ Ventromedial (Reticulospinal, Vestibulospinal, Tectospinal): Influence axial and proximal muscles; maintain posture/balance. ◦ Ventrolateral (Rubrospinal): Facilitate distal limb muscles. ◦ Note: Referred to as the extrapyramidal upper motor neuron system.

3. CLINICAL FEATURES¶

• General Observations: ◦ UMN: Muscle bulk not affected; hyperreflexia often accompanied by loss of cutaneous reflexes and Babinski sign. ◦ LMN: Atrophy is conspicuous; reflex arcs are depressed. ◦ Myopathic: Distal weakness likely neuropathic; symmetric proximal weakness myopathy.

3.1 Distribution of Weakness¶

• Hemiparesis: ◦ Clinical Context: Result of focal structural lesions (vascular, rapidly expanding, or inflammatory). ◦ Investigation: Start with brain CT; if normal/subacute/chronic, proceed to MRI of brain and/or cervical cord. • Paraparesis: ◦ Clinical Context: Often involves spinal cord or brain pathology. • Quadriparesis: ◦ Clinical Context: Can result from central nervous system (CNS) or muscle disorders. • Monoparesis: ◦ Clinical Context: Usually LMN; UMN only if distal and nonantigravity muscles are involved. • Distal vs. Proximal: ◦ Distal weakness → likely neuropathic/LMN. ◦ Symmetric proximal weakness → likely myopathic.

4. INVESTIGATIONS & DIAGNOSIS¶

• Table 26-1: Signs That Distinguish the Origin of Weakness ◦ UMN: Atrophy (None), Fasciculations (None), Tone (Spastic), Distribution (Pyramidal/regional), Reflexes (Hyperactive), Babinski (Present). ◦ LMN: Atrophy (Severe), Fasciculations (Common), Tone (Decreased), Distribution (Distal/segmental), Reflexes (Hypoactive/absent), Babinski (Absent). ◦ Myopathic: Atrophy (Mild), Fasciculations (None), Tone (Normal/decreased), Distribution (Proximal), Reflexes (Normal/hypoactive), Babinski (Absent). ◦ Psychogenic: Atrophy (None), Fasciculations (None), Tone (Variable/paratonia), Distribution (Variable/inconsistent with daily activities), Reflexes (Normal), Babinski (Absent).

• Table 26-2: Causes of Episodic Generalized Weakness ◦ 1. Electrolyte disturbances (hypokalemia, hyperkalemia, hypercalcemia, hypernatremia, hyponatremia, hypophosphatemia, hypermagnesemia). ◦ 2. Muscle disorders: ◦ a. Channelopathies (periodic paralyses). ◦ b. Metabolic defects of muscle (impaired carbohydrate or fatty acid utilization; abnormal mitochondrial function). ◦ 3. Neuromuscular junction disorders: ◦ a. Myasthenia gravis. ◦ b. Lambert-Eaton myasthenic syndrome. ◦ 4. Central nervous system disorders: ◦ a. Transient ischemic attacks of the brainstem. ◦ b. Transient global cerebral ischemia. ◦ c. Multiple sclerosis. ◦ 5. Lack of voluntary effort: ◦ a. Anxiety. ◦ b. Pain or discomfort. ◦ c. Somatization disorder.

• Figure 26-3: Algorithm for Initial Workup of a Patient with Weakness 1. Identify Distribution of Weakness (Hemiparesis, Paraparesis, Quadriparesis, Monoparesis, Distal, Proximal, or Restricted). 2. Assess for UMN signs or LMN signs (Note: LMN signs considered only if no signs of myopathy). 3. Decision Pathway based on Distribution: a. Hemiparesis/Paraparesis/Monoparesis Path: i. If UMN signs present → Check "Cerebral signs". - If Yes → Brain CT or MRI. - If No → Check LMN signs → If present → Spinal MRI. ii. If LMN signs present → Spinal MRI. b. Distal/Proximal/Restricted Path:* i. Proceed directly to EMG and NCS. ii. Interpret results as: - UMN pattern. - LMN pattern (further categorized into Anterior horn, root, or peripheral nerve disease). - Myopathic pattern (further categorized into myopathy or neuromuscular junction disease).

5. MANAGEMENT & TREATMENT¶

• Supportive Care: 1. Management of underlying cause (e.g., correcting electrolyte imbalances, treating infections, surgical decompression for spinal cord compression). 2. Physical therapy to maintain muscle strength and range of motion. 3. Respiratory support if pharyngeal/bulbar muscles are involved.

6. PROGNOSIS & COMPLICATIONS¶

• Clinical Monitoring: 1. Monitor for respiratory failure in patients with bulbar involvement. 2. Assess for progression of weakness in inflammatory or neoplastic processes.

7. SPECIAL CONSIDERATIONS¶

• Fatigability: Suspect NMJ disorder (e.g., Myasthenia Gravis) if strength declines with sustained/repeated contraction. ◦ Acute Monoparesis: If distal + UMN type + no sensory loss → suspect focal cortical ischemia.

8. KEY PEARLS & CLINICAL TRAPS¶

• Distal vs. Proximal: Distal weakness → likely neuropathic; symmetric proximal weakness → likely myopathic. ◦ Spasticity vs. Rigidity: Spasticity is velocity-dependent (UMN); Rigidity is constant/lead-pipe (Extrapyramidal). ◦ α vs. γ: α loss = weakness/atrophy; γ involvement affects tone and stretch reflex. ◦ Corticobulbar Rule: In corticobulbar lesions, extraocular, upper facial, pharyngeal, and jaw muscles are typically spared.

Reference Tables¶

TABLE 26-1 Signs That Distinguish the Origin of Weakness SIGN Atrophy Fasciculations Tone Distribution of weakness…¶

Harrison's 22e, p.168

SIGN	UPPER MOTOR NEURON	LOWER MOTOR NEURON	MYOPATHIC	PSYCHOGENIC
Atrophy	None	Severe	Mild	None
	None	Common	None
Tone	Spastic	Decreased	Normal/decreased	Variable/paratonia
	Pyramidal/regional	Distal/segmental	Proximal
Muscle stretch reflexes	Hyperactive	Hypoactive/absent	Normal/hypoactive	Normal
	Present	Absent	Absent

TABLE 26-2 Causes of Episodic Generalized Weakness 1. Electrolyte disturbances, e.g., hypokalemia, hyperkalemia…¶

Harrison's 22e, p.171

1. Electrolyte disturbances, e.g., hypokalemia, hyperkalemia, hypercalcemia,
  hypernatremia, hyponatremia, hypophosphatemia, hypermagnesemia
  2. Muscle disorders
  a. Channelopathies (periodic paralyses)
  b. Metabolic defects of muscle (impaired carbohydrate or fatty acid
  utilization; abnormal mitochondrial function)
  3. Neuromuscular junction disorders
  a. Myasthenia gravis
  b. Lambert-Eaton myasthenic syndrome
  4. Central nervous system disorders
  a. Transient ischemic attacks of the brainstem
  b. Transient global cerebral ischemia
  c. Multiple sclerosis
  5. Lack of voluntary effort
  a. Anxiety
  b. Pain or discomfort
  c. Somatization disorder