Weakness has a broad differential diagnosis. A paradigm for organizing possibilities is to consider what part of the nervous system is involved, ranging from brain, spinal cord, nerve roots, and peripheral nerves to the neuromuscular junction. The clinician can consider internal versus external causes. Some neurologic conditions have subtle presentations yet carry a risk of short-term decompensation if not recognized. It is helpful to consider whether an emergency department presentation of weakness is a new disease process or represents an exacerbation of an established condition. Emergency presentations of weakness are challenging, and one must carefully consider potential serious causes.
Introduction, history, definitions, and background
Weakness can represent the initial stage of many emergency conditions. The differential diagnosis is broad and includes many etiologies that are decidedly non-neurologic. In this review, we discuss a general emergency medicine approach to neurologic causes of weakness. Our focus is on both common neurologic conditions and uncommon conditions with a high likelihood of morbidity or mortality if the diagnosis is delayed.
We consider a variety of external and internal causes of neurologic weakness. We define weakness as decreased or lost muscular strength resulting in an inability to act with normal or desired force. Neurologic weakness, specifically, is due to specific dysfunction in any of the following locations: the neuromuscular junction, peripheral nerves (including brachial plexus), spinal cord, or brain. We do not discuss generalized malaise, often referred to as “weakness” by patients, because this entity is most frequently attributable to or associated with a primary medical cause such as anemia or hypoglycemia. We define external causes to represent infectious diseases, trauma, and toxins (including drug effects). Internal causes broadly include autoimmune, vascular (including stroke), neoplastic, metabolic, and genetic. Idiopathic causes might represent as yet undefined internal or external causes. Localization is the process by which we use the signs and symptoms to determine what part or parts of the nervous system are likely to be the source of the new problem.
The now-known causes of weakness were, at some point in history, newly discovered or described. This continues today, because new diseases may present with weakness of unclear etiology. Acute flaccid myelitis, particularly in children, is an example. Evolving technologies, such as methods to isolate viruses and other pathogens or molecular techniques to detect pathologic antibodies that lead to autoimmunity, continue to uncover such previously unknown or unclear etiologies. This historical context importantly highlights that the inability to identify a cause in the emergency department does not mean there is no medical or organic etiology.
As with all neurologic conditions, a careful, focused history can assist in organizing areas to query further with physical examination and diagnostic testing. Ask the patient or accompanying history-provider about the location of the weakness, duration and pattern of symptoms, and association with other symptoms such as blurry vision or pain. Ask about particular tasks or movements that are most impaired or that highlight the weakness. For example, difficulty with standing from a seated position or ascending stairs points to an etiology resulting in a proximal muscle weakness. Inquire about any medications, such as diuretics, that may cause pertinent electrolyte disturbances, with particular emphasis on newly started, changed, or discontinued medications.
Where is the weakness (note unilateral vs bilateral involvement)?
When did this start?
Is the weakness persistent or intermittent or is there some consistent pattern?
What activities make the weakness more noticeable?
What other symptoms accompany the weakness?
The clinician’s first duty is to assess for emergent or urgent conditions necessitating timely action. For patients with a suspected neurologic cause, respiratory status is particularly at risk given the propensity for altered mental status or diaphragmatic or accessory respiratory muscle weakness. Note that tachypnea often presents sooner than, and may herald other signs of, impending respiratory failure.
Abnormal or poor mentation
Difficulty with speech or weak voice
Drooling or other indication of difficulty handling secretions
Inability to lift head off the bed
Weak, rapid, or shallow breaths or use of accessory muscles
If rapid sequence intubation is deemed necessary, consider avoiding depolarizing neuromuscular blocking agents such as succinylcholine. These agents carry the risk of potentially fatal hyperkalemia in those with neurologic disease by amplifying the succinylcholine-related cellular potassium release. , For most relevant conditions, this response occurs in those with symptoms or illness for 3 or more days. Succinylcholine is considered safe in myasthenia gravis, although it should be noted that the emergency physician may not always be certain of the etiology at the time rapid sequence intubation is performed. Nondepolarizing agents such as rocuronium and vecuronium may be preferable in such suspected or unknown cases. On a related note, there may also be some degree of autonomic dysfunction or instability secondary to the suspected or known neurologic process that requires some support or resuscitation until the underlying issue can be identified and managed.
In the absence of emergent or critical findings, a more thorough neurologic examination can be illuminating. This process includes an assessment of the cranial nerves, reflex testing, sensory and motor examinations, coordination, and gait. Any one or combination of these components may be key in narrowing the differential and guiding testing, as well as determining any necessary emergent interventions.
The localization of the culprit lesion as an upper versus lower motor neuron (UMN and LMN, respectively) can be made partly or even completely on the basis of the physical examination. Hyperreflexia, increased muscle tone (spasticity), and a positive or present Babinski sign (the extensor plantar response resulting in a fanning of the toes) indicate UMN involvement. Decreased or absent reflexes and/or muscle tone and an absent Babinski reflex indicate LMN involvement. Note that, in early UMN disease, there may initially be hyporeflexia and decreased muscle tone before the more characteristic spasticity and hyperreflexia develop. These distinctions, coupled with pertinent anatomy, can be helpful in the emergency department. For example, recall that in the adult the spinal cord terminates at L1. Below this point is the cauda equina, at which point an insult would result in an LMN-patterned examination. A patient presenting with bilateral leg weakness, hyperreflexia, and a positive Babinski sign (consistent with a UMN-level lesion) would thus be extremely unlikely to have a lesion at L1 or lower. Consequently, limiting this patient’s imaging to an MRI lumbar spine, presumably to identify cauda equina syndrome, would not reveal the etiology. We provide an overview of a localization paradigm as Fig. 1 .