The Approach to Neuromuscular Weakness

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Weakness is a common, though sometimes vague, presenting symptom in the ED. Neuromuscular (NM) weakness can have various causes, ranging from common and relatively easy to diagnose conditions (such as strokes and trauma) to less common and more difficult to diagnose ones (such as vascular pathologies, infections, autoimmune diseases, and neoplasms). This post focuses on how to localize weakness to differentiate types of NM weakness and presents classic cases of diffuse weakness caused by three diseases.

For each of the three disease processes, differentiation can be achieved through a thorough neurological exam, along with cardiac and respiratory assessments. As you assess your patient, it is crucial to correlate physical exam findings with their history. These diseases differ in terms of timing of presentation and pattern of distribution.

History and Physical Exam are Your Best Friends

As with any disease process, a thorough history and physical exam are key to diagnosing the patient’s condition accurately. Timing is especially important—symptom onset and speed of progression can rapidly narrow the differential diagnosis (Table 1).

A complete neurological exam is critical for diagnosing neuromuscular weakness. When conducting the exam, look for patterns of weakness and constellations of findings that fit a specific disease state. In addition to a motor strength exam, assess mental status, cranial nerves, sensation, reflexes, and coordination. Complete the physical exam with a thorough cardiac, circulatory, and respiratory assessment.

Recognizing Patterns of Focal Weakness

Identifying specific patterns of weakness helps refine your differential diagnosis. If you find that distal muscles are weaker than proximal ones, you may be observing a pyramidal pattern of weakness. In this pattern:

  • In the upper extremities, extensors are weaker than flexors.

  • In the lower extremities, flexors are weaker than extensors.

Patients with this pattern may also show signs of spasticity and hyperreflexia, indicating upper motor neuron pathology.

Other central nervous system weakness patterns include spinal, subcortical, and cortical:

  • Spinal weakness often presents with weakness below the level of the spinal injury.

  • Subcortical weakness involves the face, upper, and lower extremities, often affecting the internal capsule, a subcortical structure.

  • Cortical weakness typically presents as weakness involving the face and upper extremities, or the lower extremities without face and upper extremity involvement. Cortical patterns of weakness are often associated with middle cerebral artery (MCA) or anterior cerebral artery (ACA) pathology. Knowledge of intracranial vascular territories and neuroanatomical pathways can help localize lesions and understand the functional deficit.

For example, anterior circulation disruption can lead to ophthalmic and speech deficits.

Some spinal cord injury patterns, like central cord syndrome, present with bilateral upper and lower limb weakness, with upper limb weakness greater than lower limb weakness. Patients may also have bilateral loss of pain, temperature, and light touch sensation.

Rian Kabir, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

Patients Presenting with Diffuse Weakness

Some patients present with diffuse weakness. To simplify understanding how a disease is affecting the patient, consider how it impacts the motor unit pathway. Based on exam findings, the affected part of the pathway can often be predicted:

  • If the patient has diffuse weakness with pain and sensory findings, the issue is likely affecting the nerve or nerve root.

  • If the patient has diffuse weakness without pain or sensory loss, the disease is likely affecting the anterior horn cells, the neuromuscular junction (NMJ), or the muscles themselves.

By using these patterns along with exam data and patient history, you can refine your differential diagnosis and plan appropriate management and disposition.

Daniel Walsh and Alan Sved, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

Three Clinical Diseases That Cause Diffuse NM Weakness

Myasthenia Gravis

  • Caused by IgG antibodies against acetylcholine receptors in almost 90% within the neuromuscular junction

    • Motor weakness without sensory loss or pain

  • Diagnostic (ice pack reversing ptosis in most cases)

  • Treatment / management of crisis

    • Supportive; respiratory function should be monitored

    • For Myasthenia crisis do NOT use succinylcholine in the need for intubation may require more to paralyze

    • Timing is key; Rocuronium is non-depolarizing. Although it takes longer to paralyze, it does prolong paralysis

Botulism

  • Caused by toxin-mediated (Type A and B) blockade of neuromuscular transmission in cholinergic nerve fibers

    • Inhibits acetylcholine release at the presynaptic clefts of the myoneural junctions

  • Clinical diagnosis - Motor without sensory loss or pain

  • Treatment / management of crisis

    • Respiratory function should be monitored

    • If suspected (descending paralysis beginning with cranial nerves), immediately contact the local or state health department to investigate outbreak

    • Supportive care, intubation and mechanical ventilation when necessary

      • Only specific therapy for botulism is botulinum antitoxin (> 1 yr old)

      • < 1 yr old human based botulism immunoglobulin

    • Patients can survive, even without antitoxin

Guillain Barre

  • Caused by postinfectious, immune-mediated cross-reaction where antibodies form against proteins on host that are molecularly similar

    • Cross-react with ganglioside in myelin

  • Presentations vary

  • Diagnosis via lumbar puncture

  • Treatment / management of crisis

    • Respiratory failure can occur without symptoms of dyspnea and therefore respiratory function should be monitored in all patients

    • Intravenous immunoglobulin and/or plasma exchange

      • Both shown to be equally effective

    • Do NOT use succinylcholine

References

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  2. Neel, R. Neuro and Neuromuscular Emergencies Neel 010621. University of Cincinnati MediaSpacehttps://uc.mediaspace.kaltura.com/media/Neuro+and+Neuromuscular+Emergencies+Neel+010621/1_r7bxl47p (2021).

  3. Murphy, S. JX. & Werring, D. J. Stroke: causes and clinical features. Med. Abingdon Engl. UK Ed 48, 561–566 (2020).

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  6. Li, Y., Arora, Y. & Levin, K. Myasthenia gravis: Newer therapies offer sustained improvement. Cleve. Clin. J. Med. 80, 711–721 (2013).

  7. Bennett, W. Botulism Treatment & Management. (2024).

  8. Rao, A. K., Sobel, J., Chatham-Stephens, K. & Luquez, C. Clinical Guidelines for Diagnosis and Treatment of Botulism, 2021. MMWR Recomm. Rep. 70, 1–30 (2021).

  9. Habib, D. Annals of B Pod - Infantile Botulism. Taming the SRU https://www.tamingthesru.com/blog/annals-of-b-pod/b-pod-cases/infantile-botulism (2020).

  10. Andary, M. Guillain-Barre Syndrome. (2024).

  11. Leonhard, S. E. et al. Diagnosis and management of Guillain–Barré syndrome in ten steps. Nat. Rev. Neurol. 15, 671–683 (2019).

  12. Knudsen-Robbins, C. Diagnostics and Therapeutics: The Who, What, Where, When and Why of Lumbar Punctures. Taming the SRU https://www.tamingthesru.com/blog/diagnostics/lumbar-puncture (2024).

Authorship

Written by - Anthony Onuzuruike, MD PGY-2 University of Cincinnati Department of Emergency Medicine

Peer Review, Editing, Posting - Jeffery Hill, MD MEd, Associate Professor, University of Cincinnati Department of Emergency Medicine

Cite As - Onuzuruike, A., Hill, J. The Approach to Neuromuscular Weakness. TamingtheSRU. www.tamingthesru.com/blog/2024/9/11/the-approach-to-neuromuscular-weakness. 9/12/2024