Approach to weakness (focal and generalized): Clinical sciences

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Introduction0:00–0:38

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.

Unstable0:38–1:15

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.

Unilateral Weakness1:15–1:55

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.

Brain Lesion1:55–3:34

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.

Upper motor neuron lesion3:34–4:08

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.

Lower motor neuron lesion4:08–5:28

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.

Bilateral weakness5:28–6:03

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.
Once you identify bilateral weakness, assess for sensory involvement. If your patient reports symptoms like numbness and tingling and the exam reveals sensory loss, you should assess for cranial nerve involvement.

Brainstem lesion, acute polyneuropathy6:03–6:47

When there is cranial nerve involvement, be sure to check the patient's deep tendon reflexes. If they have hyperreflexia, think towards a brainstem lesion.
Keep in mind that lesions in different parts of the brainstem will present uniquely. However, if the patient has decreased or absent reflexes, you should think of acute polyneuropathy, such as Guillain-Barré syndrome.
On the other hand, if there’s no cranial nerve involvement, again, assess the reflexes and also for the presence of a sensory level, meaning a spinal dermatomal level below which there is loss of sensation.

Spinal cord lesion, chronic polyneuropathy6:47–8:17

Remember that the chest is at the level of the T4 sensory dermatome, and the umbilicus is at T10. If there is hyperreflexia or a sensory level is present, diagnose a spinal cord lesion.
Here is a clinical pearl to keep in mind! If the lesion is in the cervical cord, you will find weakness in the arms and the legs; but if the lesion is in the thoracic cord or below, only the legs will be weak.
A patient with a high cervical cord lesion might report difficulty breathing. Remember, “C3, C4, and C5 keep the diaphragm alive”!
This is because the phrenic nerve arises from these spinal nerve roots. Now, decreased or absent reflexes with no sensory level suggest chronic polyneuropathy, which predominantly affects the lower extremities.
Chronic polyneuropathies are seen in conditions like diabetes, chronic alcohol use, vitamin B12 deficiency, and hypothyroidism.
There are also hereditary causes, such as Charcot-Marie-Tooth disease. Okay, let’s switch gears and discuss patients with bilateral weakness but no sensory involvement.
The next step here is to assess for fluctuations of symptoms. If the weakness fluctuates, think of neuromuscular junction disorders.

Neuromuscular junction disorder8:17–9:20

Examples include myasthenia gravis or Lambert-Eaton syndrome. Keep in mind that patients with toxin-related neuromuscular junction disorders like botulism or organophosphate toxicity won’t have a fluctuating weakness.
Instead, they will present with rapidly progressive weakness after exposure. Time for a clinical pearl!
Channelopathies, such as hyperkalemic and hypokalemic periodic paralyses, can present with intermittent episodes of weakness in the setting of metabolic disturbances.
Most of these cases are hereditary with an autosomal dominant pattern of inheritance. Finally, if your patient reports no fluctuating weakness, assess the muscle tone and reflexes.
If the muscle tone and reflexes are normal, you should think of myopathies. On the flip side, abnormal muscle tone and reflexes suggest motor neuron disease.

Myopathy, motor neuron disease9:20–10:06

This could be a pure anterior horn cell disorder, such as poliomyelitis, or a mixed upper and lower motor neuron condition, such as amyotrophic lateral sclerosis.
Depending on the motor neuron disorder, some patients will also have weakness in the facial, laryngeal, and respiratory muscles, as well as the neck and trunk.
Alright, as a quick recap… If your patient presents with chief concerns suggestive of muscle weakness, first, determine if the weakness is unilateral or bilateral.

Review10:06–10:57

If unilateral, then you should consider brain lesions and conditions like radiculopathy, plexopathy, and mononeuropathy.
On the flip side, if the weakness is bilateral, then assess for sensory involvement. Sensory involvement and cranial nerve dysfunction suggest a brainstem lesion or acute polyneuropathy, while sensory involvement with no cranial nerve dysfunction suggests spinal cord lesions and chronic polyneuropathy.
Finally, if there is bilateral muscle weakness with no sensory involvement, think of neuromuscular junction disorder, myopathy, and motor neuron disease.
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