Approach to weakness (focal and generalized): Clinical sciences

Weakness, also known as impaired motor strength, can be subdivided into unilateral and bilateral weakness. Unilateral weakness can be seen in conditions like brain lesions and peripheral nervous system lesions, while bilateral weakness can be associated with neurologic conditions like acute and chronic polyneuropathy, but also brainstem and spinal cord lesions. Other important causes of bilateral weakness include neuromuscular junction disorder, motor neuron disease, and various myopathies.

Now, if a patient presents with chief concerns suggestive of weakness, perform an ABCDE assessment to determine if they are unstable or stable. If unstable, stabilize their airway, breathing, and circulation. In some cases, you might need to intubate the patient and provide mechanical ventilation. Next, obtain IV access and consider IV fluids. Finally, put your patient on continuous vital sign monitoring, including blood pressure, heart rate, and pulse oximetry, as well as cardiac telemetry.

Okay, let’s go back to the ABCDE assessment and focus on stable patients. In this case, your next step is to obtain a focused history and physical exam. Let’s say your patient reports limb weakness on one side only, involving the arm, leg, or both, and the physical exam reveals unilateral weakness of the upper, or lower, or both extremities. These findings are suggestive of unilateral weakness, so be sure to assess for monoparesis. In other words, assess whether the weakness affects multiple body parts or one limb only.

If the weakness involves more than one limb or if the weakness involves a limb and the face, you should think of a brain lesion contralateral to the side of the symptoms. Keep in mind that with a brain lesion, you might also notice a sensory loss ipsilateral to the side of weakness.

Now, here's a clinical pearl! Weakness on one side of the body involving the upper and lower extremities is known as hemiparesis. Less commonly, ipsilateral hemiparesis sparing the face might be due to the damage to the corticospinal tract from a cervical hemi-cord lesion.

In this situation, spinal tracts on one side of the spine are interrupted. So, next to the corticospinal tract, the dorsal column is also damaged, leading to a loss of vibratory sensation and proprioception ipsilateral to the side of weakness.

Additionally, as the spinothalamic tract is damaged, there will be loss of pain and temperature sensation contralateral to the weakness. This happens because the spinothalamic tract crosses over to the opposite side soon after it enters the spinal cord, unlike the corticospinal tract and the dorsal column which decussate at the level of the medulla.

This combination of ipsilateral weakness, and loss of vibratory and proprioceptive sensation with contralateral loss of pain and temperature sensation is known as Brown-Séquard syndrome. Okay, let’s go back to our assessment. If the weakness is isolated to one limb only, you are dealing with monoparesis.

Your next step is to assess the pattern of weakness. If you notice spasticity, which is increased tone that increases with velocity of muscle movement, also known as velocity-dependent tone; and hyperreflexia; diagnose an upper motor neuron lesion, more specifically a brain or spinal cord lesion.

On the flip side, if your patient presents with decreased tone, hyporeflexia, muscle atrophy, and fasciculations, which are involuntary muscle fiber contractions; think of a lower motor neuron lesion. Next, assess localization with a detailed physical exam.

If the physical exam demonstrates weakness with or without sensory loss in the distribution of a spinal nerve root, diagnose a radiculopathy. If there’s a weakness with or without sensory loss in multiple contiguous nerve roots throughout a limb, which suggests a plexus distribution, you should think of plexopathy.

Remember that the brachial plexus consists of contributions from nerve roots C5, C6, C7, C8, and T1; while the lumbosacral plexus has contributions from L1 to L5, and S1 to S4 nerve roots. Lastly, if the exam demonstrates weakness with or without sensory loss in the distribution of one peripheral nerve, such as the median- or the sciatic nerve, diagnose mononeuropathy.

Alright, let’s go back to the focused history and physical and discuss different findings. History might reveal bilateral weakness of the arms, legs, or both. Similarly, physical exam will show bilateral weakness of the upper or lower extremities or both. With these findings, diagnose bilateral weakness. Remember that paraparesis refers to weakness of the bilateral lower extremities, while quadriparesis refers to weakness of all four extremities.