Weakness and Paralysis

Weakness and Paralysis is a topic covered in the Harrison's Manual of Medicine.

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Approach to the patient

Weakness is a reduction of power in one or more muscles. Paralysis indicates weakness that is so severe that the muscle cannot be contracted at all, whereas paresis refers to less severe weakness. The prefix hemi- refers to one half of the body, para- to both legs, and quadri- to all four limbs. The suffix -plegia signifies severe weakness or paralysis.

Increased fatigability or limitation in function due to pain or articular stiffness is often confused with weakness by pts. Increased time is sometimes required for full power to be exerted, and this bradykinesia may be misinterpreted as weakness. Severe proprioceptive sensory loss may also lead to complaints of weakness because adequate feedback information about the direction and power of movements is lacking. Finally, apraxia, a disorder of planning and initiating a skilled or learned movement, is sometimes mistaken for weakness.

The history should focus on the tempo of development of weakness, presence of sensory and other neurologic symptoms, medication history, predisposing medical conditions, and family history.

Weakness or paralysis is typically accompanied by other neurologic abnormalities that help to indicate the site of the responsible lesion (Table 53-1). It is important to distinguish weakness arising from disorders of upper motor neurons (i.e., motor neurons in the cerebral cortex and their axons that descend through the subcortical white matter, internal capsule, brainstem, and spinal cord) from disorders of the motor unit (i.e., lower motor neurons in the ventral horn of the spinal cord and their axons in the spinal roots and peripheral nerves, neuromuscular junction, and skeletal muscle).

SignUpper Motor NeuronLower Motor NeuronMyopathicPsychogenic
Distribution of weaknessPyramidal/regionalDistal/segmentalProximalVariable/inconsistent with daily activities
Muscle stretch reflexesHyperactiveHypoactive/absentNormal/hypoactiveNormal
Babinski signPresentAbsentAbsentAbsent

Table 53-2 lists common causes of weakness by the primary site of pathology. Table 53-3 summarizes patterns with lesions of different parts of the nervous system.

Upper Motor Neuron
Cortex: Ischemia; hemorrhage; intrinsic mass lesion (primary or metastatic cancer, abscess); extrinsic mass lesion (subdural hematoma); degenerative (amyotrophic lateral sclerosis)
Subcortical white matter/internal capsule: Ischemia; hemorrhage; intrinsic mass lesion (primary or metastatic cancer, abscess); immunologic (multiple sclerosis); infectious (progressive multifocal leukoencephalopathy)
Brainstem: Ischemia, immunologic (multiple sclerosis)
Spinal cord: Extrinsic compression (cervical spondylosis, metastatic cancer, epidural abscess); immunologic (multiple sclerosis, transverse myelitis); infectious (AIDS-associated myelopathy, HTLV-I–associated myelopathy, tabes dorsalis); nutritional deficiency (subacute combined degeneration)
Motor Unit
Spinal motor neuron: Degenerative (amyotrophic lateral sclerosis), infectious (poliomyelitis)
Spinal root: Compressive (degenerative disk disease); immunologic (Guillain-Barré syndrome); infectious (AIDS-associated polyradiculopathy, Lyme disease)
Peripheral nerve: Metabolic (diabetes mellitus, uremia, porphyria); toxic (ethanol, heavy metals, many drugs, diphtheria); nutritional (B12 deficiency); inflammatory (polyarteritis nodosa); hereditary (Charcot-Marie-Tooth); immunologic (paraneoplastic, paraproteinemia); infectious (AIDS-associated polyneuropathies and mononeuritis multiplex); compressive (entrapment)
Neuromuscular junction: Immunologic (myasthenia gravis); toxic (botulism, aminoglycosides)
Muscle: Inflammatory (polymyositis, inclusion body myositis); degenerative (muscular dystrophy); toxic (glucocorticoids, ethanol, AZT); infectious (trichinosis); metabolic (hypothyroid, periodic paralyses); congenital (central core disease)
Abbreviation: AZT, azidothymidine.
Location of LesionPattern of WeaknessAssociated Signs
Upper motor neuron
Cerebral cortexHemiparesis (face and arm predominantly, or leg predominantly)Hemisensory loss, seizures, homonymous hemianopia or quadrantanopia, aphasia, apraxias, gaze preference
Internal capsuleHemiparesis (face, arm, leg may be equally affected)Hemisensory deficit; homonymous hemianopia or quadrantanopia
BrainstemHemiparesis (arm and leg; face may not be involved at all)Vertigo, nausea and vomiting, ataxia and dysarthria, eye movement abnormalities, cranial nerve dysfunction, altered level of consciousness, Horner’s syndrome
Spinal cordQuadriparesis if midcervical or aboveSensory level; bowel and bladder dysfunction
Paraparesis if low cervical or thoracic
Hemiparesis below level of lesion (Brown-Séquard)Contralateral pain/temperature loss below level of lesion
Motor unit
Spinal motor neuronDiffuse weakness, may involve control of speech and swallowingMuscle fasciculations and atrophy; no sensory loss
Spinal rootRadicular pattern of weaknessDermatomal sensory loss; radicular pain common with compressive lesions
Peripheral nerve
 PolyneuropathyDistal weakness, usually feet more than hands; usually symmetricDistal sensory loss, usually feet more than hands
 MononeuropathyWeakness in distribution of single nerveSensory loss in distribution of single nerve
Neuromuscular junctionFatigable weakness, usually with ocular involvement producing diplopia and ptosisNo sensory loss; no reflex changes
MuscleProximal weaknessNo sensory loss; diminished reflexes only when severe; may have muscle tenderness

An algorithm for the initial workup of weakness is shown in Fig. 53-1.


An algorithm for the initial workup of a pt with weakness. EMG, electromyography; LMN, lower motor neuron; NCS, nerve conduction studies; UMN, upper motor neuron.

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