Approach to differentiating lesions (motor neuron): Clinical sciences

Motor neuron lesions are conditions associated with a loss of strength and movement due to upper or lower motor neuron injuries. Upper motor neurons originate in the cerebral cortex, with axons that descend through the subcortical white matter and synapse with lower motor neurons in the brainstem and anterior horn cells in the spinal cord. These lower motor neurons then send axons out of the central nervous system via cranial nerves or spinal nerve roots to reach muscle cells.

Now, some conditions affect only lower motor neurons, such as acute flaccid myelitis and adult-onset spinal muscular atrophy; while other conditions can affect both upper and lower motor neurons, such as amyotrophic lateral sclerosis.

Now, if your patient presents with a chief concern suggesting motor neuron disease, first perform an ABCDE assessment to determine if they are unstable or stable.

If unstable, stabilize the airway, breathing, and circulation. In some cases, you might even have to intubate the patient and start mechanical ventilation. Next, obtain IV access and consider starting IV fluids. Finally, put your patient on continuous vital sign monitoring, including heart rate, blood pressure, and pulse oximetry, as well as cardiac telemetry; and if needed, provide supplemental oxygen.

Now, let’s go back to the ABCDE assessment and focus on stable patients. In this case, obtain a focused history and physical examination. The patient will describe progressive bilateral weakness, which could affect muscles of the face, neck, arms, or legs. Some patients might describe trouble standing up and walking, while others might report trouble speaking, swallowing, or breathing.

On physical exam, you will notice weakness of the facial muscles, neck, trunk, or limbs, which may or may not be symmetric. Finally, the physical exam will reveal abnormal muscle tone and reflexes, and in some cases, cranial nerve dysfunction and an abnormal gait.

Next, assess for sensory involvement. If the patient reports numbness or tingling, and the exam reveals a loss of sensation, you should consider alternative diagnoses, because motor neuron conditions do not cause sensory deficits.

Instead, think about conditions that impair both motor strength and sensation, such as spinal cord lesions, or bilateral brain lesions from a stroke or demyelination. On the other hand, if the exam reveals pure weakness with normal sensation, consider motor neuron disease.

This can be isolated to either upper motor neurons, or lower motor neurons or affect both. In this case, your first step is to assess the pattern of motor neuron injury. If the physical exam reveals muscle atrophy; fasciculations, which are involuntary muscle fiber contractions; decreased tone; and hyporeflexia, it’s likely that your patient has a lower motor neuron lesion.

The next step is to assess the underlying cause. Now, if history reveals a child with weakness that progresses rapidly over hours to days; with a preceding fever and respiratory or gastrointestinal illness; consider acute flaccid myelitis.

Then, obtain a CSF analysis and an MRI of the spine. If CSF analysis reveals pleocytosis, usually less than 100 cells per microliter, with a lymphocytic predominance; and the spine MRI shows predominantly gray matter involvement;
diagnose acute flaccid myelitis in this case.

Now, here’s a clinical pearl to keep in mind! Acute flaccid myelitis typically occurs after a viral infection, especially enteroviruses D68 and 71, West Nile virus, and poliovirus.
Depending on the underlying infection, you can often identify the causative virus by testing serum, stool, nasopharyngeal, or CSF samples.

Infections from enteroviruses D68 and 71, and West Nile virus are more common in the summer and early fall, while the incidence of poliomyelitis has fallen significantly due to widespread polio vaccination.