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<< Weakness: A Systematic Approach To Acute, Non-traumatic, Neurologic And Neuromuscular Causes

Emergency Department Evaluation

General Approach

Most patients complaining of weakness will require a thorough history and physical examination to sort out this nonspecific complaint. The history should include a description of the distribution, time course, and clinical features associated with the weakness. (See Table 2.) Begin by categorizing the patient’s weakness as either unilateral or bilateral. Once this is established, follow the Clinical Pathways (for unilateral weakness) and (for bilateral weakness) to establish the anatomic lesion. Importantly, sometimes a psychiatric state or  nonclassifiable cause of weakness will be suspected. In those instances, as long as potentially rapidly deteriorating conditions are excluded (see Table 1), the patient can be referred to a specialist or primary  are provider for further evaluation.



Stabilizing Management

The immediate life threats from acute neuromuscular weakness include inability to protect or maintain the airway, respiratory failure from thoracic and diaphragmatic muscle weakness, and circulatory collapse from autonomic instability. Clinical evaluation begins with assessment of the airway and respiratory function. To establish adequacy of the airway and its protective reflexes, assess phonation and the ability to handle secretions. The most important clinical findings associated with neuromuscular respiratory failure are rapid, shallow breathing, the recruitment of accessory muscles, and paradoxical movement of the abdomen during the respiratory cycle (“belly breathing”). The easiest parameter to recognize is tachypnea. Patients with progressive generalized neuromuscular weakness commonly begin to lose tidal volume before upper airway weakness develops, resulting in an increased respiratory rate.8 Because of the intact ventilatory drive and increased rate of breathing, the pCO2 may remain normal or low until the tidal volume becomes dangerously low. Equally insensitive may be a relatively normal pulse oximetry reading, for much the same reason. Therefore, for patients suspected of having GBS, MG, or some other potentially rapidly deteriorating  cause of acute weakness, selected pulmonary function tests are often needed to aid in intubation decisions. Specifically, obtain a forced vital capacity (FVC), negative inspiratory force (NIF), or peak expiratory force. Either an FVC of less than 10-12 cc/kg or an NIF of less than 20 cmH2O may indicate the need for intubation.8 Importantly, an experienced respiratory technician needs to verify that the FVC and NIF are not artificially low due to inability to form a tight seal around the mouthpiece of the spirometer from weakness of the orbicularis oris muscle.

If intubation is considered, some authors suggest a non-depolarizing paralytic agent (such as atracurium or rocuronium) as opposed to a depolarizing agent (such as succinylcholine) to avoid hyperkalemia. However, the risk of severe hyperkalemia associated with succinylcholine administration and neurologic injury is largely a case report phenomenon.9 While neurologic injury may increase potassium release associated with succinylcholine, 10 the risk of severe hyperkalemia appears to begin several days after the inciting event. In acute spinal cord injury, administration of succinylcholine in the ED is not harmful, because the period during which severe hyperkalemia occurs begins at 21 days.11-15 Processes that are progressive, such as a polyneuropathy like GBS, seem to put the patient at risk of hyperkalemia after seven days.16 Based on all of the available literature, a conservative estimate would put the initial threat of markedly increased potassium release at three days after complete denervation or seven days after partial denervation (Class II-III).

Autonomic dysfunction can accompany some neuromuscular disorders—most notably GBS (see Table 1). Autonomic instability typically manifests as a hyper-sympathetic state and is heralded by a sinus tachycardia.8 Subsequent abnormalities in heart rate or fluctuating blood pressure can occur. Bradycardia is rare but may require temporary pacing.8 Importantly, autonomic failure and pulmonary embolism are the major causes of mortality in GBS, although specific referencing data are not available.8

History

After determining that a patient’s weakness is either unilateral or bilateral, a few key questions will begin to localize the anatomical area of a patient’s problem. Key components of the history for someone with unilateral weakness include the following:
  • Are cortical signs present, such as aphasia, agnosia, apraxia, or neglect?
  • Is the face involved?
  • Is there a dermatomal or myotomal pattern to the description of weakness distribution?
  • Is the description of weakness consistent with involvement of a particular peripheral nerve? 
  • Is there bowel or bladder involvement? As a general rule, unilateral facial weakness implies a lesion above the spinal cord, either in the brainstem or cortex (or, in the case of Bell’s palsy, a peripheral nerve).
For more specific localization between these two, see “Clinical Pathway: Diagnostic Algorithm For Acute Nontraumatic Unilateral Weakness” In the case of isolated extremity weakness, knowledge of common radicular and peripheral nerve entrapment syndromes is essential. A localized process (e.g., weakness associated with numbness and tingling of the ring and small finger of one hand) strongly suggests peripheral nerve entrapment (see “Clinical Pathway: Diagnostic Algorithm For Acute Nontraumatic Unilateral Weakness” for notable examples), although in some cases distinguishing spinal from peripheral nerve entrapment can be challenging. Similarly, familiarity with key cervical and lumbosacral dermatomes and myotomes (see Table 3) will foster recognition of compression of those spinal nerve roots and may help to differentiate spinal root compression from a more peripheral site of compression.



With regard to bilateral weakness, the answers to the following historical questions will define the likely
site of weakness:
  • Is the mental status impaired?
  • Which limbs are involved?
  • Is a sensory level deficit suggested, or is there any sensory involvement?
  • Is there bladder involvement?
  • Does the weakness tend to involve primarily proximal or distal muscles?
  • Is there a fluctuating pattern to the weakness?
  • Are there associated bulbar signs?

A central nervous system (CNS) lesion causing bilateral weakness will usually have accompanying diminished mental status, unless the pathology resides in the spinal cord. Presence of a sensory deficit at or below a sensory level points to a myelopathy, as does any abnormality in bladder function.

The pattern and distribution of weakness will help to narrow the etiology. The proximal motor weakness typically found early in the course of a myopathy may be suggested by difficulty walking up stairs or getting out of a chair if the lower limbs are involved, or difficulty combing or brushing hair with arm involvement. Patients who describe being “weak everywhere” or who have so called patternless weakness are most likely to be experiencing malaise stemming from either an associated medical illness or psychogenic cause. A fatiguing pattern to the weakness, suggested by worsening with repeated activity such as chewing, suggests a neuromuscular junction process. Alternatively, acute attacks of weakness lasting a few hours and then spontaneously resolving should prompt exploration for periodic paralysis due to imbalances in potassium regulation. Visual symptoms, particularly ptosis or diplopia, and bulbar signs are important to identify, since they are invariably associated with an NMJ process. Bulbar muscle weakness may manifest as nasal speech, coughing, dysphagia, or dysarthria and will call for careful cranial nerve testing.

Age extremes should prompt consideration of certain processes, such as occult infection or infantile botulism in the very young. In the elderly, consider occult infection, metabolic disorders, a CNS event, or a medication-related process.17 While various medications can classically lead to certain categories of weakness, such as myopathies from steroids or lipid-lowering agents, carefully scrutinize the impact of all medications in the geriatric population. In one study of patients with a chief complaint of weakness or dizziness, 20% of those over age 60 had symptoms attributed to prescription medications.18

The time course of symptom onset and progression can help categorize the cause of weakness. When it is abrupt in onset, a stroke or other vascular etiology is suggested. Most other categories of weakness develop over hours to days to weeks.

A few somewhat unusual historical points may provide clues. The complaint of “cold reversal” (cold stimuli being felt as painful and hot) is virtually pathognomonic for ciguatera poisoning.19,20 Consider multiple sclerosis when a patient describes an electrical or tingling sensation that travels down the spine or into the extremities. This is referred to as Lhermitte’s sign (transient sensory symptoms usually precipitated by neck flexion). Although commonly found in multiple sclerosis, a similar complaint can accompany radiculopathies, or more rare entities such as vitamin B12 deficiency and Chiari malformation.1 Another feature suggesting multiple sclerosis includes weakness precipitated by a hot bath. Finally, in a parenteral heroin abuser, consider wound botulism due to contaminated heroin.21-23

Physical Examination

The overall physical examination is most helpful for excluding an illness causing malaise, which the patient interprets as weakness. Start with the vital signs. Because occult infection may present as weakness, consider a rectal temperature, since variables such as tachypnea or mouth breathing can yield a falsely low oral temperature.24-26 Examine the skin for the bronze discoloration of Addison’s disease or the heliotropic eyelid rash (a red-purple or violaceous, edematous, macular rash) characteristic of dermatomyositis.27 If the history suggests a possibility of tick exposure, carefully search for a tick, since removal of the tick is rapidly curative in tick paralysis.28 Conjunctival pallor suggests anemia, while oral thrush implies immune compromise. Carefully assess the neck for signs of a thyroid abnormality, since thyroid disorders frequently cause weakness.29,30

Neurologic Examination

The motor examination is especially helpful in the weak patient. Components of a detailed examination include evaluation of strength, reflexes, fatigability, tone, and fasciculations. Importantly, there is no place for nebulous or uninterpretable documentation of the neurologic examination (e.g., “grossly intact”). For patients with histories highly consistent with spinal or peripheral nerve compression syndromes, an exhaustive neurologic examination is not required.

One critical aspect of localizing a weakness syndrome involves identifying whether upper motor or lower motor neuron signs are present. (See Table 4.) UMN disease signs include spasticity, hyperreflexia, and an extensor plantar response (the Babinski reflex) and are lacking in pure peripheral disease, which begins with the spinal nerve root. However, recognize that hyporeflexia and a flaccid paralysis may occur with acute central lesions, with hyperreflexia and spasticity developing later. A common example is the patient with an acute spinal cord injury from trauma. While muscle atrophy and fasciculations also classically help to distinguish LMN from UMN processes, these will rarely be present in patients presenting with acute or subacute weakness, since these typically develop over the long term. It is worthwhile to note that amyotrophic lateral sclerosis presents with both the UMN and LMN features of fasciculations and hyperreflexia.



If weakness is bilateral and signs of LMN disease are present, the major disorders to differentiate are neuropathies, myopathies, and neuromuscular junction disorders. (See Table 5.) The key characteristic that distinguishes neuropathies such as GBS is sensory involvement. While myopathies can have associated myalgias, other sensory symptoms should be lacking from these processes. Additionally, neuropathies tend to involve distal muscle groups over proximal muscles, while myopathies are more proximally distributed. Deep tendon reflexes will be decreased in neuropathies but are an unreliable distinguishing feature for myopathies, since they may be present, decreased, or absent. The most distinguishing feature of neuromuscular junction processes is early involvement of the ocular musculature, most often in the form of ptosis and diplopia.



Assessing Strength

The central component of the motor examination in a weak patient will be strength testing. To facilitate interpretation of the examination, employ the standardized motor grading scale agreed on by the Medical Research Council.31 (See Table 6.) Based on historical features, determine if there is a pattern to the weakness. Hemiparesis suggests a hemispheric lesion, while paraparesis is consistent with a spinal cord lesion. If the history suggests a proximal pattern of weakness, such as difficulty walking up stairs or getting out of a chair, seek to distinguish proximal from distal muscle weakness (which will point to a myopathic process vs. a neuropathy). Functional tests include the ability to rise from a squat or out of a chair, or stepping up onto a stool or chair. For isolated extremity weakness suggesting an isolated spinal nerve or peripheral nerve process, rely on testing specific muscle groups. For example, to differentiate an L5 from an S1 radiculopathy due to a herniated disc, test dorsi and plantar flexion of the foot; if an L5 lesion is present, the patient may not be able to walk on his or her heels, while if an S1 lesion is present, the patient will have trouble walking on his or her toes. If a neuromuscular junction process is suspected, oculomotor and bulbar testing is critical. Assess extraocular movements, including the eyelids, masseter muscle strength, facial expression, palatal movement, sternocleidomastoid, and trapezius muscles.



When facial weakness is present, examine the symmetry of forehead skin wrinkles and the strength of forehead muscles to distinguish a Bell’s palsy (peripheral 7th cranial nerve dysfunction) from a central process. In Bell’s palsy, the forehead will be weak, while with a central process, the forehead will be spared, due to bilateral cortical innervation. Importantly, an isolated “central 7th” lesion, with no other weakness or cortical deficits, is extremely rare, and most often represents a subtle form of Bell’s palsy. Weakness of the 9th or 10th cranial nerve can be detected by noting asymmetric movement of the soft palate (uvula is deviated), while weakness of the 12th cranial nerve will lead to the tongue pointing to the weak side. Assessing the symmetry of shoulder shrug will identify unilateral 11th cranial nerve weakness.

Assessing Reflexes

Reflex findings also aid in lesion localization. (See Table 3.) Tendon jerks are conventionally graded on a scale from zero to 4, with zero representing absent jerks and 4 representing hyperactivity with clonus. (See Table 7.)



If a lesion involves a cerebral hemisphere, anticipate asymmetry between the sides, with hyperreflexia on the affected side. The presence of symmetrical hyperreflexia and extensor plantar responses indicates an interruption of the corticospinal tracts bilaterally. This usually means a spinal cord lesion, but it could be the result of bilateral hemisphere disease or a brainstem process. If nerve root compression is suspected from the history, unilateral lack of a given lower extremity reflex can help localize the lesion. For example, loss of one ankle jerk indicates a lesion of the S1 nerve root. Alternatively, symmetrical absence of ankle jerks and decreased knee jerks indicate a neuropathy.

The Babinski sign is a UMN lesion finding and exists when the great toe dorsiflexes and the other toes fan out when the sole of the foot is stroked.32 (A less sophisticated, but equally unambiguous, way to refer to the presence of the Babinski sign is the phrase “up-going toe.”) Describe the response of the great toe as either flexor or extensor (e.g., extensor plantar response). It is important to distinguish a Babinski sign from simple withdrawal of the great toe, which will be the response of many normal patients who are ticklish.

A few other maneuvers will help the emergency medicine practitioner localize the lesion (and impress the neurologist). A positive Hoffman sign consists of reflex flexion of the fingers, most easily seen in the thumb, and is a UMN sign. Test this reflex by holding the patient’s middle finger between your second and third fingers, and flick the patient’s distal phalanx downward with your thumb. Recognize that the presence of a Hoffman sign alone does not imply pathology. As with any individual feature of the neurologic examination, it must be interpreted in the context of other tendon jerks and signs of a UMN lesion. Clonus represents the repetitive contraction of a muscle or muscle group and is most easily elicited at the ankle. While clonus is another UMN sign, one or two beats of reflex clonus can be obtained at the ankle in many normal individuals. Test for clonus with the patient in the supine position and hold the patient’s leg flexed at the knee. While maintaining this position, rapidly dorsiflex the foot and hold it in the dorsiflexed position. In a patient with a severe myelopathy, sustained clonus can be maintained virtually indefinitely as long as upward pressure is maintained on the foot. Finally, absence of the so-called cutaneous reflexes (the cremasteric reflex and anal wink) can help localize lesions. The cremasteric reflex (T12-L1) is elicited in the male by stroking the proximal inner thigh with a pointed instrument (or tongue blade) and observing the scrotal sac. A normal reflex involves a retraction of the scrotum. (Testicular torsion will also cause loss of the cremasteric reflex, but rarely will such a patient present with acute weakness.) The anal wink (S2-S4) is elicited by stroking the skin around the anal sphincter and watching it contract. Absence of either reflex is abnormal. Depending on the other neurologic examination findings present, spinal shock or damage of the lumbosacral cord or roots will be the cause.

Fatigability

The phenomenon of fatigability with initially normal strength is a specific characteristic of disorders of the NMJ.33 In cases where the history identifies the bulbar or eye findings characteristic of an NMJ process, perform repetitive or sustained oculomotor muscle testing to provoke ptosis or diplopia.

Assessing Tone

Because muscle tone must be subjectively assessed with the patient relaxed and because non-neuromuscular processes, such as bone or joint abnormalities, may confound testing, assessing tone may be challenging for the emergency medicine practitioner. Evaluate the LMN sign of flaccidity by looking for a difference from one side to the other if unilateral weakness exists.1,34 For the upper extremity, shake the forearm and observe the floppiness of the movements of the hand at the wrist. Alternatively, with the arms raised overhead, compare the degree of flexion or limpness of the wrist on each side. To test for flaccidity in the lower extremities, rapidly flex the thigh after instructing the supine patient to let the leg flop or relax. In a patient with normal tone, the heel may transiently slightly come off the bed and then drag along the top of the bed as the thigh is flexed. The heel of the flaccid leg will drag across the bed from the very beginning. Alternatively, the spastic leg will jerk upward and the heel may never fall back to the bed. This maneuver can be particularly helpful in distinguishing psychogenic paraplegia from an acute spinal cord emergency. To assess hypertonic spasticity, which is a UMN sign, examine the biceps and adductors of the thigh. For example, with the patient lying down, hypertonicity in the adductors of the thigh can be felt by rapidly rotating the thigh back and forth. For the arm, rapidly flex and extend the elbow to feel a catch or interruption of extension at the elbow. Recognize, however, that rigidity (vs. spasticity) can also cause increased tone, but this will almost always indicate extrapyramidal dysfunction. Spasticity differs from rigidity in that it selectively increases the tone in the flexor muscles of the arm and the extensors of the leg, whereas rigidity affects flexors and extensors equally.1 In particular, the “cogwheel rigidity” seen in Parkinsonism can be confused with spasticity. This term refers to resistance that stops and starts in a quick repetitive sequence as the extremity is passively moved through a range of motion. The severe muscle rigidity seen with neuroleptic malignant syndrome (classically referred to as “lead pipe rigidity”) is an unmistakable finding.

 Fasciculations

Since “muscle twitching” may be encountered as an ED complaint, some points regarding fasciculations are worth noting. Fasciculations are an LMN sign caused by disorders of the anterior horn cell or spinal nerve root compression, although they typically develop later in the LMN disease process. They are commonly experienced as a benign phenomenon in the absence of any disorder of the LMN.35 Importantly, benign fasciculations can generally be differentiated from fasciculations due to LMN processes by several features.1 They have a predilection for males, increase with age, and have a propensity to involve certain muscle groups, particularly the calves and thighs. When they occur in the arm muscles, they tend to be seen as a repetitive twitch in the same muscle fascicle, and they do not display associated weakness or atrophy. Alternatively, anterior horn or nerve root disease causes random twitches in many parts of the muscle.

 Assessing Gait

Walking is one of the most important components of any neurologic examination, and in the case of weakness, gait testing will aid in distinguishing UMN from LMN lesions.1 Spasticity caused by a bilateral UMN lesion will result in stiff, jerky movements of the legs, or if the lesion is unilateral, the spastic leg will be circumducted (a revolving around movement). Alternatively, distal weakness from an LMN lesion will produce a floppy foot or feet that slap(s) on the floor and must be lifted high to prevent the toe from dragging and tripping the patient (“foot drop”).

 Assessing Sensation

While the presence, pattern, or absence of sensory symptoms and signs can be helpful in confirming the neuroanatomic level producing weakness, the subjective nature of sensory findings can make interpretation daunting. Classically, a cortical lesion causes relatively mild hemisensory loss, which affects touch and proprioception more than pain. The patient may simply describe this as the arm or leg “feeling funny.” More specifically, sensory impairment known to require cortical processing, such as stereognosis and graphesthesia, will localize a lesion to the contralateral sensory cortex. Stereognosis relies on light touch and position sense and is tested by placing an object in a patient’s hand and having him or her identify it with eyes closed. Graphesthesia is tested by tracing a number or letter on the surface of the skin, usually the palm, and asking the patient to name the number or letter.

A spinal cord lesion will often affect sensation on both sides of the body, with the upper level of the sensory loss defining the lesion level. In contrast, cervical central cord lesions most commonly produce a “cape” sensory loss over the shoulders that affects pain and temperature sensations, but spares vibration and proprioception (socalled “dissociated” sensory loss). Lesions of the conus medullaris or cauda equina produce loss of sensation in the perineum. To test for vibratory sensation, a beeper set on the vibrate mode can be used in place of a tuning fork, but don’t expect this substitution to detect subtle deficits. An absence of sensory symptoms in the setting of suspected neuromuscular disease, other than muscle aches, suggests that the weakness is due to a myopathy or NMJ process.

Sorting Out Psychogenic Causes Of Weakness

A common pattern of feigned weakness is a sudden collapse of the limb after an initial, often normal effort (“giveaway weakness”). Typically this results in a phasic, ratchet-like collapse as the muscle is tested, most commonly the biceps. Another contrived pattern may be exhibited when the shoulder is abducted and then pushed downward by the examiner. The malingering patient will bend his trunk to that side, giving the impression that his arm is being pushed downward, when in fact the relationship with the trunk remains the same. In unilateral lower-extremity weakness, testing for Hoover’s sign can be helpful. With the patient lying supine, judge the effort put forth in the lower extremities by placing one hand between the heel and the examining table while testing the ability of the patient to raise the other leg off the table. Normally, with full effort the heel on the table is forced downward to support the raising of the opposite leg. With feigned weakness in one leg, when full effort is used to raise the “good leg,” normal downward pressure will be felt from the feigned “weak” leg. Alternatively, when the feigned “weak” leg is supposed to be raised, very little if any pressure will be felt under the opposite leg with presumably normal strength.

When evaluating a weak upper extremity, pay close attention to the response when testing for “pronator drift.” A true weak arm will pronate (and sometimes flex) when drifting downward, since supination is most affected by weakness. An arm that drifts or drops downward with no pronation indicates malingering. To distinguish psychogenic paralysis of an arm, observe the movement of the arm when shaking the patient’s shoulders. A truly paralyzed arm that is held limply at the side will flail from side to side when the patient’s shoulders are shaken back and forth. If, instead, the arm is observed to remain tightly held against the body, it has normal strength and tone.

Hysterical paraplegia can be distinguished from a true spinal cord emergency in several ways. It may initially be suspected if the patient reacts remarkably calmly about their severe deficit (so-called “la belle indifference”). On observation, the patient may display ease in rolling over and moving in bed, with confirmation on examination by the presence of normal reflexes. If a patient feigning “weak legs” can be convinced to undergo gait testing, suspect nonorganic weakness if the patient displays a markedly exaggerated effort to take a step. The malingerer may display extraordinary strength and coordination in a contrived effort to create an impressively abnormal gait. Despite these tests and patterns that may indicate a psychogenic cause of the patient’s weakness, diagnosing a patient with “malingering” or “psychosomatic disorder” in the ED is fraught with pitfalls. Unless you are absolutely certain of this diagnosis, it is prudent not to label the patient with a psychogenic disorder. Instead, ensure follow-up to confirm the absence of serious pathology.

Autonomic Nervous System Dysfunction

Among the most obvious impairments of sympathetic innervation is Horner’s syndrome, which consists of ipsilateral ptosis (lid droop), miosis (small reactive pupil), and anhidrosis (lack of sweating) of the face. Interruption of descending preganglionic sympathetic fibers in the brainstem or spinal cord can lead to Horner’s syndrome. More peripherally, since postganglionic sympathetic fibers course along the carotid arteries, an internal carotid artery dissection can produce an ipsilateral partial Horner’s syndrome (without anhidrosis). The miosis and ptosis occur because of a compressive effect of the expanding intramural hematoma from the internal carotid artery dissection on the sympathetic fibers. Facial anhidrosis is not present because the facial sweat glands are innervated by the sympathetic plexus surrounding the external carotid artery.

Like sympathetic disruption, parasympathetic disruption can occur from one of several lesions. If a midbrain lesion impacts the third nerve nucleus or nerve, it can disturb the parasympathetic pupillomotor fibers, resulting in paralysis of the pupillary sphincter, producing pupillary dilatation. If an acute transverse spinal cord lesion exists, autonomic dysfunction, especially loss of sweating, can provide clues to the level of the lesion, since sweating will be lost below the lesion. To detect this, slightly rub the dorsal surface of the forefinger  long the skin, starting below the expected level of the lesion, and stroke upward.1 The finger will slide easily over the smooth, dry skin below the lesion, but stick momentarily as it meets the normal moist skin at the upper border of the lesion.