Physicians increasingly depend on laboratory tests and imaging, sometimes at the expense of a careful history and physical, to rule in and rule out one diagnosis or another.¹ Perhaps no presentation in medicine depends more on careful history taking and physical examination than acute motor weakness. Here, I’ll outline a practical and efficient six-step approach to help narrow your differential diagnosis for the patient who presents to the emergency department with acute motor weakness. It is my hope that by using this approach, you will arrive at a diagnosis well before any tests are considered.

Step 1: Determine if Weakness Represents a True Loss of Motor Power

The chief complaint of “weakness” is often vague and can mean malaise or fatigue or anhedonia to some people and true loss of motor power to others.² It is incumbent upon the emergency physician to elucidate whether a complaint of weakness represents a true loss of motor power. Some history-taking tips include asking about what activity of daily living or specific function has been lost or made more difficult. For example, patients may complain of being unable to pick up and control a coffee cup, or they may say they had difficulty standing up from a seated position. The word “power” is preferred when communicating with patients and other physicians, as it is more specific to neuromuscular strength than the word “weakness.”

Step 2: Find Pattern of Motor Power Loss

Outside of diffuse weakness, there are five patterns of loss of motor power that each correlate with a specific neuroanatomical source. Diffuse loss of motor power may be caused by systemic problems such as hypo- or hyperkalemia or thyrotoxicosis as well as polymyositis and dermatomyositis.² Paraplegia suggests a lesion in the thoracic or lumbar spinal cord or peripheral nerves, quadriplegia suggests a lesion in the cervical spine, and hemiplegia suggests a lesion in the contralateral cerebral cortex. A bilateral ascending pattern that starts at the feet and progresses toward the head suggests a peripheral polyneuropathy such as Guillain-Barré syndrome or a spinal cord lesion such as transverse myelitis, while a bilateral descending pattern suggests neuromuscular junction disease such as myasthenia gravis or a presynaptic disease such as Lambert-Eaton syndrome.³

Step 3: Investigate Timing, Course, and Fatigability

The onset of loss of power may help narrow the differential diagnosis. An abrupt onset should be considered an acute vascular event such as an ischemic stroke until proven otherwise. That said, small vessel lacunar infarcts may present in a gradual stuttering fashion over hours or even days; thus, a gradual onset should not rule out the possibility of a stroke.⁴ An onset over minutes to hours could also be caused by metabolic abnormalities or toxic exposures. Peripheral neuropathies, neuromuscular junction disease, and myopathies tend to develop over many hours to days. A fluctuating or relapsing course of illness suggests myasthenia gravis, multiple sclerosis, or periodic paralysis, while transient motor loss may be caused by peripheral nerve entrapment or a complex hemiplegic migraine. Finally, fatiguability or worsening motor weakness with repeated muscle contraction, such as chewing or talking, that seems to be more difficult through the day suggests a neuromuscular junction disease such as myasthenia gravis.⁵

Figure 1: In the Mingazzini sign (top), a standing or seated patient raises their arms with fingers spread and holds the position for 30 seconds. The affected arm will lower. In the Mingazzini maneuver (bottom), a supine patient flexes the hips so their thighs are at right angles to the trunk and the lower legs are horizontal and holds the position for 30 seconds. The affected thigh will lower.
ellepigrafica – stock.adobe.com

Step 4: Explore Associated Findings

There are five key associated findings that may accompany motor power loss that can help localize the lesion and narrow the differential diagnosis. Absence of reflexes suggests a peripheral neuropathy such as Guillain-Barré syndrome. Bladder dysfunction associated with loss of motor power is invariably caused by a spinal cord lesion. Bulbar symptoms such as diplopia, dysarthria, and dysphagia point to a brain stem lesion. To distinguish a left versus right cerebral cortex lesion, dysphasia suggests the former, while hemispatial and/or sensory neglect suggest the latter. Hemispatial neglect is sometimes obvious based on observation alone, but visual field testing confirms the finding.⁶ The “double extinction test” assesses for the presence of hemisensory neglect. This test is performed by tapping on the patient’s forearm on one side while asking which side they are able to feel the tap, right or left.⁷ The test is then repeated on the other side. Finally, both forearms are tapped simultaneously. The patient with hemisensory neglect will report sensation only on the non-neglected side.

Step 5: Distinguish Upper Versus Lower Motor Neuron Weakness

Clinical examination can distinguish between upper and lower motor neuron pathology. A Babinski reflex and increases in reflexes are seen with upper motor neuron lesions, while decreased or absent reflexes are associated with lower motor neuron lesions. An assessment of the degree and speed of movement may help distinguish upper from lower motor neuron disease.^8,9 Lower motor neuron disease weakness is generally more pronounced than upper motor neuron disease weakness. For example, the loss of power one gets as a result of a peripheral neuropathy, such as a foot drop, is generally more severe than the loss of power one gets from a central nervous system stroke. Another distinguishing feature is the speed of movement: While lower motor neuron lesions do not affect the speed of motor movement, upper motor neuron lesions usually result in slow movements, sometimes referred to as corticospinal tract slowness. To test motor speed, ask the patient to tap their foot or roll their forearms around one another at an increasing tempo.

Step 6: Differentiate the Types of Lower Motor Neuron Lesions

Peripheral neuropathies tend to present with sensory deficits before motor ones, with the longest nerves typically affected first, as is common in patients with Guillain-Barré syndrome.¹⁰ In contrast, neuromuscular junction diseases such as myasthenia gravis as well as myopathies usually present with pure motor deficits. Myopathies are painful, with proximal muscles being affected most (difficulty getting out of a chair, climbing stairs, or brushing one’s hair) as opposed to neuromuscular junction diseases, which tend to be painless, and peripheral neuropathies, which tend to attack the distal nerves first.³

Quick Screen Motor Exam

A detailed motor exam is often not practical in the emergency department given time constraints. However, there are six quick screening physical exam maneuvers that, if normal, can influence the need for a detailed segmental motor exam in all except patients in whom you suspect a spinal cord lesion. Two of them are familiar to emergency physicians: facial symmetry/power testing and arm pronator drift. The other four, although less familiar, are well-validated and can be time-saving.

The forearm roll test: Ask the patient to make fists with both hands and roll their forearms around each other quickly for 10 seconds in both directions.11,12 With an upper motor neuron lesion, the affected arm will be noticeably slower than the unaffected arm.

If the forearm roll test is normal (equal speed between the arms), ask the patient to roll their index fingers around one another. Again, the affected side will be slower than the unaffected side.

Many emergency physicians are unaware of the lower extremity drift test (the “Mingazzini maneuver,” see Figure 1), where the supine patient flexes the hips so their thighs are at right angles to the trunk and the lower legs are flexed so they are horizontal to the stretcher.^12,13 The patient is asked to hold this position for 30 seconds. The affected thigh will lower toward the stretcher, and the leg will lower in the patient with an upper motor neuron lesion.

Foot tapping test: The seated patient is asked to repeatedly tap their foot at a quick, steady tempo, and the clinician observes progressive slowing of the tempo. This suggests an upper motor neuron lesion.

Next time you evaluate a patient in the emergency department with the chief complaint of limb weakness, this six-step approach employing a careful history and this four-item quick screening motor exam will help you home in on the diagnosis before any tests are considered. When it comes to acute motor weakness, the history and physical are a not-so-secret weapon. 

References

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13. da Mota Gomes M. Jean-Alexandre Barré (1880–1967): his detection sign of subtle paresis due to pyramidal deficit (1919) and his work in line with that of Giovanni Mingazzini (1859–1929). Neurol Sci. 2019;40(12):2665-2669.