AssessmentsMuscle weakness: Clinical practice
USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 30-year-old man comes to the physician complaining of numbness in the upper extremities that started several days ago. The patient has no significant past medical history, but he did have an episode of trigeminal neuralgia 4 months ago, which resolved spontaneously. He does not have any urinary incontinence, but he mentions that he took fiber supplements last year due to several weeks of constipation that “resolved on its own.” Temperature is 37.0°C (98.6°F), pulse is 82/min, respirations are 15/min, and blood pressure is 125/75 mmHg. Physical examination shows decreased vibratory and positional sensation in both upper extremities but no other sensory loss. An MRI is performed and shows the following:
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Which of the following medications is likely to prevent future exacerbations of this patient’s condition?
Content Reviewers:Rishi Desai, MD, MPH
Muscle weakness is a decrease in muscle power. It’s sometimes confused for fatigue which is the inability to sustain an activity, or with bradykinesia, which is a slowness of muscle movement.
So to measure weakness, there should be a decrease in power compared to what you would expect.
Muscle power can be graded on a scale from 0 to 5. 5 is normal power. 4 is a slight decrease in power. 3 is active movement against gravity, but not to resistance. 2 is active movement, but not against gravity or resistance. 1 is a small trace contraction, like a flicker of a finger. And 0 is complete paralysis.
Now, muscle weakness can generally arise from a problem in a few locations: the upper motor neuron; lower motor neuron; and the neuromuscular junction, or the NMJ, which includes the synapse and the muscle itself.
The next part of the exam is assessing muscle tone; which is the resistance to passive stretch.
In upper motor neuron lesions, the tone is increased and it’s called spastic paralysis.
In lower motor neuron and NMJ lesions, the tone is decreased and it’s called flaccid paralysis.
Next is muscle bulk, which is usually normal in upper motor neuron and NMJ lesions, whereas severe atrophy occurs in lower motor neuron lesions.
Next, reflexes are usually hyperactive in upper motor neuron lesions, preserved in NMJ lesions, and hypoactive or absent in lower motor neuron lesions.
Next are fasciculations, which are small muscle twitches under the skin that only occur in lower motor neuron lesions.
Finally, the Babinski reflex and pronator drift are both unique clinical features of upper motor neuron lesions.
A Babinski reflex is induced by lightly stroking the lateral aspect of the sole of the foot.
A Babinski reflex is normal in the first 18 months of life because the corticospinal tracts aren’t fully myelinated until that point, but after that it’s considered abnormal.
Okay, now to induce pronator drift, the individual is asked to outstretch their arms with the palms facing upwards, and close their eyes. In an upper motor neuron lesion, the forearm pronators are stronger than the supinators, causing the affected arm to subtly drift into pronation.
Also, multiple sclerosis can affect both the brain and the spinal cord.
A stroke usually causes sudden-onset unilateral hemiparesis of the contralateral arm, leg or face. It’s often associated hemisensory loss, as well as other signs like aphasia, neglect, agnosia or visual field deficits.
On the other hand, space-occupying lesions like a brain tumor or abscess usually causes gradual-onset weakness.
Spinal cord lesions can present with weakness in the extremities in the absence of cortical signs or facial weakness.
For example, penetrating trauma can cause hemisection of the spinal cord, also called Brown-Sequard syndrome, which presents with ipsilateral hemiparesis, ipsilateral loss of proprioception and vibration, and contralateral loss of pain and temperature, all below the level of the lesion.
At the spinal level of the lesion, there is ipsilateral loss of all sensation and flaccid paralysis of the muscles supplied by the affected spinal cord segment.
Herniated discs, spinal epidural abscess, hematoma and malignancies can also compress the cauda equina or the spinal cord, causing unilateral or bilateral weakness of the lower extremities along with bladder or bowel dysfunction.
Occlusion of the anterior spinal artery can cause a spinal cord infarction, and can occur as a complication of aortic surgery. This usually affects the anterior two-thirds of the spinal cord in the affected segment.
Individuals present with sudden-onset bilateral flaccid paralysis that evolves into spastic paralysis after a couple of days.
In addition to weakness, individuals have bilateral loss of pain and temperature below the level of the lesion.
Importantly, because the dorsal columns are not involved, proprioception and vibration are spared.
Next is syringomyelia which is a developmental disorder of the spinal cord in which a cavity or “syrinx” grows from the center of the spinal cord and gradually spreads outward, destroying gray and white matter in its way.
As the syrinx expands, the anterior horn cells are affected, causing bilateral flaccid paralysis.
Now multiple sclerosis, or MS is a chronic, inflammatory demyelinating disorder that affects the white matter of the brain and spinal cord, causing weakness based on upper motor neuron damage, as well as paresthesias or sensory loss.
Additionally, fundoscopic examination can show pallor and atrophy of the optic disk.
INO impairs adduction in the ipsilateral eye, nystagmus of the contralateral eye when it’s abducting, and normal convergence.
So if the right MLF was damaged, then when an individual look left, the right eye won’t adduct, and the abducting left eye will have nystagmus. And when you ask the individual to look “cross-eye”, they can do it.
Now, bilateral INO is highly suggestive of MS.
Bladder dysfunction is due to loss of the cortical signal that causes relaxation, so there’s an overactive detrusor muscle causing urge incontinence. Having said that, some individuals also develop urinary retention.
Two interesting features in MS that are difficult to pronounce are Lhermitte’s sign and Uhthoff's phenomenon.
Lhermitte’s sign is an electric shock-like sensation that radiates down the back with neck flexion. Now this can also be a feature of other cervical spinal cord diseases.
Uhthoff’s phenomenon is a transient worsening of MS related neurological symptoms when the body temperature increases, like during exercise or a hot shower.
The main feature of MS is that these neurological symptoms are “disseminated in time and space” meaning different locations in the brain and spinal cord are affected at different times.
MS can have different patterns, the most common of which is relapsing-remitting MS. This is characterized by clearly defined attacks followed by full or incomplete recovery, and in between the attacks, there is no progression of the symptoms.
Secondary-progressive MS starts off as relapsing-remitting MS, and over time the symptoms and functional decline worsen over time.
Primary-progressive MS starts off with progressive symptoms and functional decline.
MS is diagnosed with a brain and spinal cord MRI, that shows hyperintense white matter plaques that enhance with the contrast gadolinium. These plaques are often periventricular, that is around the ventricles, or juxtacortical, that is near the cerebral cortex.
Also, a relatively specific sign in MS are plaques around the corpus callosum that look like fingers on sagittal MRI, and are called Dawson’s fingers.
Another test is the evoked potential conduction velocity, which shows slowing of the electrical events generated in the brain and spinal cord when a sensory organ is stimulated.
Treatment of MS starts with disease-modifying medications which reduce the attack rates of MS, and they include interferon-beta, glatiramer acetate, natalizumab, dalfapramide, fingolimod, and mitoxantrone.
Next, symptomatic therapy helps with symptoms. For example, spasticity can be alleviated using muscle relaxants like baclofen, cyclobenzaprine, or tizanidine. Urge urinary incontinence can be treated with amitriptyline, while urinary retention is treated with bethanechol. Neuropathic pain can be treated with anticonvulsants like carbamazepine, gabapentin, or pregabalin, or antidepressants like desipramine and venlafaxine.
Okay, now a - relapse, attack, flare, exacerbation - whatever you want to call it, is an episode of focal neurological disturbance lasting longer than 24 hours with a preceding period of stability for at least 30 days, without an alternative explanation like Uhthoff's phenomenon or an infection that causes transient worsening of symptoms.
A workup includes a CBC, a urinalysis to look for a potential UTI, and in some cases an MRI is obtained.
If there’s a relapse, treatment depends on the severity of symptoms.
Mild symptoms may not require treatment, whereas paralysis or visual loss would require treatment with high-dose IV corticosteroids like methylprednisolone for 5 days, followed by a taper of oral corticosteroids over 2 weeks.
If there’s an inadequate response to corticosteroids, then plasma exchange is done.
Next up are problems with the anterior horn motor neuron, starting with amyotrophic lateral sclerosis, or ALS.
ALS affects the anterior horn and cortical motor neurons, so it can cause both upper motor neuron and lower motor neuron problems like spasticity in one limb and absent reflexes in another.
ALS is a purely motor disorder, so there are no sensory, autonomic, or cognitive symptoms. In fact, one of the greatest minds in human history, Stephen Hawking, had ALS.
ALS can cause focal limb weakness, diplopia, difficulty with speech or swallowing. Eventually, the disease progresses to involve the breathing muscles and can lead to respiratory failure.
Treatment of ALS is mainly supportive, like muscle relaxants for spasticity and a percutaneous gastrostomy tube for feeding.
Because individuals with ALS retain their cognitive abilities, they’re capable of making their own decisions regarding end-of-life care.
Next up are spinal muscular atrophy, or SMA, and poliomyelitis - both of which only affect lower motor neurons.
SMA is actually a group of hereditary disorders that cause degeneration of the anterior horn motor neurons.
SMA type 1, also called Werdnig-Hoffmann disease is the most common subtype, and causes hypotonia in neonates.
Poliomyelitis is an enterovirus infection that directly attacks the anterior horn motor neurons. Individuals usually develop a 1 to 2 week period of prodromal symptoms that include fever, headache, nausea and vomiting, followed by weakness. However, one feature that distinguishes poliomyelitis from SMA and ALS is that the weakness is usually asymmetric. Additionally, individuals may develop weakness years after the initial infection, which is called post-polio syndrome.
Another viral infection that can affect the lower motor neuron is West Nile fever, which is transmitted through mosquitoes. Individuals develop fever, rash, myalgias, and meningitis or encephalitis, along with an acute asymmetric flaccid muscle weakness.
Alright, now conditions that affect the peripheral nerves are called polyneuropathies if they affect multiple nerves, and mononeuropathies if they affect one nerve.
A helpful mnemonic for polyneuropathies is: “DANG, My THERAPIST”.
“D” is for diabetic neuropathy, the most common type.
“A” is for alcoholic neuropathy, the second most common type.
“N” is for nutritional, which includes vitamins B1, B6 and B12 deficiencies.
“G” is for Guillain-Barré syndrome.
“M” is for medications.
“T” is for toxins, like lead and arsenic poisoning.
“HE” is for hereditary causes, such as Charcot-Marie-Tooth syndrome.
“R” is for renal failure, also called uremic neuropathy.
“A” is for amyloidosis.
“P” is for porphyrias.
“S” is for other systemic disorders that don’t fit in our mnemonic, like hypothyroidism and amyloidosis.
“T” is for tumors, which includes plasma cell disorders like multiple myeloma.
Diabetic neuropathy can occur in both diabetes type 1 and type 2, and can affect the sensory, motor, or autonomic systems.
Sensory symptoms like loss of sensation or paresthesias usually appear first, affecting the distal extremities in a symmetric fashion, also called the “stocking and glove” neuropathy.
The diagnosis includes fasting blood glucose and hemoglobin A1C levels.
Now, the best way to prevent and treat diabetic neuropathy is optimizing glucose control, but some medications can help symptoms like antidepressants like venlafaxine or duloxetine, anticonvulsants like gabapentin or pregabalin, or capsaicin cream.
Okay, Guillain-Barré syndrome is an autoimmune demyelinating disorder of the peripheral nerves. It often occurs after an acute inflammatory diarrhea caused by Campylobacter jejuni, as well as many other infections.
The most common variant of Guillain-Barré syndrome is acute inflammatory demyelinating polyneuropathy, or AIDP.
In AIDP, about 2 to 4 weeks after an infection, individuals develop rapidly progressive, symmetric, ascending weakness in the distal lower extremities that evolves over hours to days. Symptoms are at their worst around 4 weeks later.