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.