Patients who present with weakness may be left with marked disability unless a diagnosis is made quickly. The motor divisions of the nervous system are responsible for every movement a person makes. Any injury to the motor parts will have a direct consequence on one’s ability to perform meaningful motions. Motor disorders can cause a variety of weakness patterns; therefore, it is important for the reader to recognize that the disorders mentioned in this chapter are not restricted to a specific pattern and have tremendous overlap. In this chapter, readers will review patterns of weakness in specific motor disorders of the nervous system.

The motor parts of the nervous system are responsible for every movement a person makes. Any injury to the motor division will have a direct consequence on one’s ability to perform meaningful motions. Therefore, it is vital to identify sources of motor injury in order to assist patients in regaining the ability to move. The purpose of this chapter is to identify patterns of weakness in specific motor disorders of the nervous system (Table 28-1). Motor disorders can cause a variety of weakness patterns; therefore, it is important for the reader to recognize that the disorders mentioned in this chapter are not restricted to a specific pattern and have tremendous overlap. Further descriptions of each disorder can be found in subsequent chapters in much more detail and are beyond the scope of this chapter.

Several objective assessments can be performed to ascertain a pattern of weakness in order to better localize the neurologic dysfunction involved in a patient’s presentation of weakness. Common motor assessments include power, muscle bulk, tone, and muscle stretch reflexes (MSRs). Motor examination findings differentiating upper from lower motor neuron injury can be found in Table 28-2.

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  Power: Muscle power (strength) can be objectively tested using the Medical Research Council motor grading system.¹

  
  
  
  
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    Grade 5: Strength normal against resistance.

    
    
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    Grade 4: Reduced strength but can still move joints against resistance.

    
    
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    Grade 3: Movements against gravity but not against resistance.

    
    
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    Grade 2: Movements only with the elimination of gravity.

    
    
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    Grade 1: Only fasciculations are noticed, and no movement is observed.

    
    
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    Grade 0: No muscle contractions.

  
  
  
  
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  Bulk: Bulk can be assessed by the inspection of muscles. The main objective is to observe symmetry of muscles on each side. Atrophy of muscles is the result of denervation from neural injury (commonly with peripheral nerve injuries). Confounding factors may include disuse or systemic illness and must be distinguished from neural injury.

  
  
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  Tone: Muscle tone can be normal, decreased, or increased. The simplest method to assess tone is by passively moving a patient’s muscles. Spasticity and rigidity are the commonest forms of increased tone. Spasticity is referred to as the “clasp-knife” form due to upper motor neuron injury. Rigidity is the “lead pipe” form and can be associated with Parkinson disease or other extrapyramidal disorders. Decreased tone is the result of lower motor neuron injury.

  
  
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  MSRs: Reflexes can be normal, reduced, or increased. Reduced MSRs are the result of lower motor neuron lesions, and increased MSRs are the result of upper motor neuron lesions. The presence of the Babinski sign can suggest an upper motor neuron lesion as well (first toe extends to stimulus).^1,2

Motor signals originate in the primary motor cortex (precentral gyrus) in the cerebral cortex and are somatotopically organized; control of the leg is in the medial portion of the cortex followed by arm and then face most laterally. Most axons from the primary motor cortex follow the lateral corticospinal tract, although there are other less clinically relevant motor pathways. The lateral corticospinal tract descends from the primary motor cortex to the posterior limb of the internal capsule. From there the tract enters the cerebral peduncle of the midbrain, through the basis pontis, and into the medullary pyramid where tracts cross to the opposite side. After leaving the brainstem, the corticospinal tract continues within the lateral spinal cord and synapses onto various motor neurons within the ventral horn of the spinal cord. Upon exiting the spinal cord, nerve roots join to ultimately form peripheral nerves to innervate muscles.²

Stroke

Stroke is a clinically ubiquitous cause of weakness due to lesions within the brain and brainstem that can result in monoparesis, hemiparesis, paraparesis, or quadriparesis. The localization of stroke can be traced to the cerebrovascular anatomy. While weakness may be the result of other conditions that do follow cerebrovascular pathways, these disease processes may be found within the regions of the neuroaxis supplied by respective cerebrovascular territories.

Injury to the primary motor strip or the basal ganglia is the most common sites of stroke that result in weakness. These areas are primarily supplied by the anterior and middle cerebral arteries. If an occlusion occurs in the parent artery, cortical signs accompany weakness including gaze deviation toward the hemisphere injured, aphasia with injury to the dominant hemisphere, and neglect with injury to the nondominant hemisphere. Visual fields can also be affected. Middle cerebral artery territory infarcts commonly cause hemiparesis and can also, at times, result in monoparesis of the upper extremity. The anterior cerebral artery supplies the motor cortex involved in lower extremity control; therefore, occlusion of the anterior cerebral artery often results in lower extremity monoparesis. Deep gray nuclei found within the basal ganglia are supplied by lenticulostriate vessels, which are terminal branches off of the anterior and middle cerebral arteries and are not associated with cortical signs when injured (Figure 28-1).

Isolated infarcts in the midbrain are uncommon and usually involve structures in other parts of the brain such as the pons, cerebellum, or thalamus, and are typically the result of small vessel disease in the posterior cerebral artery distribution. Midbrain infarcts often produce contralateral hemiparesis with upper cranial nerve involvement (usually the oculomotor nerve).

Pontine infarctions can be caused by atherosclerosis, stenosis, or occlusion of the basilar artery, resulting in damage to pontine perforating arteries. Lacunar stroke syndromes are common causes of pontine ischemia, of which there are 4 types: pure sensory, pure motor, ataxic-hemiparesis, and dysarthria-clumsy hand.

Medullary infarction is one type of ischemic brainstem stroke; lateral medullary infarctions are more common than medial infarctions. Vertebral artery occlusions often result in this type of ischemia. This can be due to vertebral artery atherosclerosis or dissection and is evidenced by medullary symptoms and neck pain. In medial medullary syndrome (Dejerine syndrome), contralateral hemiparesis, contralateral vibration/proprioception loss, ipsilateral tongue weakness, and dysarthria are seen. In lateral medullary syndrome (Wallenberg syndrome), ipsilateral palatal and vocal paralysis, Horner syndrome, ipsilateral facial sensory loss to pain and temperature, contralateral sensory loss of pain and temperature, and contralateral ataxia are seen; there is no weakness observed in lateral medullary syndrome. In hemimedullary lesions, lateral and medial medullary symptoms occur simultaneously.

Intracerebral hemorrhages (ICH) can manifest clinically with weakness. In younger patients, hemorrhages are often caused by ruptured arteriovenous malformations (AVMs) and have a more optimistic prognosis than their adult hypertensive counterparts. Common locations for hypertensive ICH include the basal ganglia, thalamus, pons, and cerebellum.

When associated with monoparesis or hemiparesis, weakness from cranial nerve (CN) damage can assist in localization. Damage to CNs and nuclei of CN III, IV, V, VI, VII, XI, and XII can cause weakness of the muscles that they innervate (Table 28-3). Apart from infarction, other disorders can cause CNs damage and precipitate weakness, such as meningitis, neoplasms, vascular disorders, bone disorders, and trauma.

Imaging is widely used to evaluate stroke. A computed tomography (CT) scan of the head without contrast is usually the first-line test performed in assessing for stroke to determine the presence of hemorrhage. CT angiography (CTA) is the preferred technique in assessing the cerebrovasculature. MRI is employed in the setting of ischemic strokes. These imaging modalities are all useful in narrowing the differential diagnosis, which includes but is not limited to multiple sclerosis (MS), cerebral abscess, brain neoplasms, acute disseminated encephalomyelitis, myelinolysis, and diffuse axonal injury (Figure 28-2). The reader should also be aware of stroke mimics that can present as hemiparesis or monoparesis including seizures with post-ictal Todd’s paresis, hypoglycemia, and migraines.^2–4

Multiple sclerosis

MS is a CNS disorder in which white matter of the brain and/or spinal cord becomes damaged by the immune system, thereby interrupting the motor pathway. Multiple inflammatory lesions then become sclerotic, or scarred, giving the disease its name. While its etiology remains unclear, it is postulated that MS is an autoimmune disorder of both environmental and genetic causes.

MS affects 250,000–350,000 patients in the United States. Clinically, patients with MS present with a variable spectrum of signs and symptoms, including weakness, and every patient will have a unique clinical course. Although some MS episodes are asymptomatic, others cause symptoms and may progressively worsen in severity with time. MS is often diagnosed between the ages of 20 and 45 years, and women are affected twice as often as men. Many patients experience weakness or paralysis that can interfere with their activities of daily living. However, the individual’s definition of “weakness” is highly varied and must be further elucidated. Weakness is often due to upper motor neuron tract damage. Weakness in MS patients can also be due to medication side effects of antispasmodics or corticosteroids, and this must also be taken into consideration.

The diagnosis of MS is made clinically, and rarely histological confirmation is acquired at a time other than autopsy. There is no definitive diagnostic test, but neuroimaging with MRI is often the most sensitive imaging test used (Figure 28-3). With MRI, the neurologic lesions must exhibit two characteristic properties: the lesions must be located in different parts of the brain or spinal cord, and the lesions must occur independently of one another at different points of time. Although no one radiologic finding can confirm or refute the diagnosis, MRI has been used in conjunction with the McDonald criteria to arrive at the diagnosis of MS.⁶ Cerebrospinal fluid (CSF) analysis also assists in the diagnosis.

The overarching goal of MS treatment is a reduction in disease severity and progression; there is no curative treatment. Therapies for MS are targeted toward acute exacerbations and maintenance. An acute episode of MS is defined as a neurologic episode that lasts for more than 24 hours. In the treatment of acute MS symptoms, the approach is targeted toward inflammation. Principally, high-dose corticosteroids such as dexamethasone and methylprednisolone are used. In the chronic management of MS, the goal is to slow disease progression and lessen relapses by aiming to inhibit demyelination. Current evidence supports the use of treatment modalities including, but not limited to, corticosteroids and various other immunosuppressants, beta interferon, glatiramer acetate, and monoclonal antibodies.

The prognosis for MS with treatment is relatively good. Periods of remission and favorable quality of life are possible, so long as the appropriate diagnosis is made in a timely fashion and the immune-targeted therapies described above are explored and pursued.^5,6

Neuromyelitis optica

Neuromyelitis optica (NMO) is an inflammatory disease of the CNS characterized by recurrent myelitis and/or optic neuritis. With the discovery of serum immunoglobulin autoantibody NMO-IgG, which targets astrocyte water channel aquaporin-4 (AQP4), it has become clear that NMO is an autoimmune condition distinct from MS, in spite of the symptomatic overlap between the two distinct diseases.

Clinically NMO presents with relapsing attacks of myelitis and/or optic neuritis. Weakness is a principle manifestation, and patients are often greatly incapacitated in terms of ambulation. Impaired vision and blindness also occur, and NMO mortality is often attributed to respiratory failure secondary to progressive brainstem injury or ascending cervical myelitis.

Diagnostically, AQP4-antibody testing is an extremely sensitive and specific modality by which to pinpoint NMO. A definitive diagnosis of NMO also requires that at least 2 of the following criteria be met: NMO-IgG seropositivity, brain MRI inconsistent with MS, and spine MRI showing a contiguous spinal lesion encompassing at least 3 vertebral segments.

There are no curative treatment modalities for NMO. Instead, treatment focuses on reducing symptom relapse and severity through longitudinal immunosuppression as prophylaxis. Methylprednisolone, intravenous immunoglobulin (IVIG), or cyclophosphamide therapies are often used to treat relapses. For chronic immunosuppression, corticosteroids, azathioprine, and rituximab are commonly employed. Additionally, monoclonal antibody treatment is currently under investigation as a potential new avenue of medical therapy.

Without curative therapy, NMO is a progressive and severely debilitating disease. Therefore, it is imperative that a diagnosis is made quickly and immunosuppressive therapy is started as soon as possible.⁷

Transverse myelitis

Transverse myelitis is characterized by inflammation in the spinal cord that can cause moderate-to-severe weakness (paraparesis or quadriparesis depending on the level of spinal cord involved) and disability. The symptoms of transverse myelitis develop over a period of hours to days and are typically exacerbated over the course of subsequent weeks. Most cases develop acutely, although subacute cases have been documented. A typical presentation consists of weakness, sensory deficit, and autonomic dysfunction, depending on the exact location of the spinal lesion. Weakness most often targets leg flexors and arm extensors in a pyramidal distribution. Although weakness most often presents bilaterally due to the transverse nature of the injury, it is important to note that this is not always the case. The term “transverse” refers to an area of abnormal sensation at the dermatome corresponding to the cord lesion.

History and physical examination are the most important diagnostic tools regarding transverse myelitis. For instance, in Brown-Séquard syndrome, a patient presents with characteristic motor weakness due to a unilateral lesion of the spinal cord resulting in dorsal column dysfunction on the ipsilateral side and spinothalamic dysfunction on the contralateral side of the lesion. Conversely, a central lesion can present with spinothalamic deficits, autonomic dysfunction, and pyramidal weakness inferior to the lesion level; meanwhile, in an anterior spinal cord syndrome, acute flaccid weakness may be observed with preserved dorsal column function and spinothalamic dysfunction.

Once myelopathy is suspected, MRI is the chosen imaging modality (Figure 28-4), with CT myelogram serving as an acceptable alternative. A lumbar puncture (LP) is often performed to determine whether inflammatory markers are present within the CSF. If imaging and CSF findings indicate CNS inflammation, then the differential must include demyelination, infection, and autoimmune etiologies. More favorable outcomes tend to arise from cases that are most rapidly diagnosed and treated.

If CSF analysis shows a bacterial or viral etiology, then the patient should be promptly started on an appropriate antimicrobial regimen. For bacterial meningitis, dexamethasone can be used prior to or along with the first course of antibiotics. If an autoimmune, neoplastic, or inflammatory process is suspected as the cause for cord compression, intravenous methylprednisolone may be started. In refractory cases, plasma exchange (PLEX) can be considered.

If the appropriate treatment course is followed, then the prognosis of transverse myelitis is fair. Factors indicating poorer prognoses include rapidly progressing symptoms such as spinal shock and back pain. Factors portending more optimistic outcomes include rapid initiation of treatment for the underlying cause of the myelitis; chronic causes tend to have worse long-term outcomes than its acute counterparts.⁸

Ruptured intervertebral disk

A ruptured (herniated, prolapsed, slipped) disk (nucleus pulposus) occurs when part or the entirety of an intervertebral disk erupts through a weak portion of the disk. Pressure is placed on the surrounding spinal nerves and cord, causing a clinical presentation of weakness.

Weakness, numbness, and pain are the primary clinical symptoms of a ruptured intervertebral disk. The most common location for this pathology is the lumbar spine, and it most commonly occurs in older men who engage in strenuous physical activity. Weakness is often localized to only one side of the body and the corresponding unilateral extremities. Exacerbation of pain may occur with sneezing, coughing, bending, sitting, or standing.

History and physical examination findings of numbness, hyporeflexia, abnormal posture, unilateral weakness, and pain elicited by joint maneuvering are diagnostic of ruptured intervertebral disk. In addition to the history and physical examination, an electromyogram (EMG) can pinpoint the exact nerve root affected, CT myelogram can elucidate the location and extent of herniation, and nerve conduction velocity (NCV) testing may be used to evaluate the magnitude of injury. Imaging modalities such as spine CT and MRI (Figure 28-5) are used to confirm compression of the spinal cord, and spine radiography may be considered to rule out other etiologies.