Vertebral malalignment & Subluxation atlanto-axial joint - USMLE Step 1, UWorld Questions, Neurology

Paralysis comes in two main categories. upper motor neuron lesions, and lower motor neuron lesions.

An upper motor neurone lesion will be in the central nervous system (brain and spinal cord). A lower motor neurone lesion affects anywhere from the anterior horn cell to the muscle.

Upper motor neuron lesions present with hypertonia and spastic paralysis. whereas, lower motor neuron lesions are usually associated with hypotonia and flaccid paralysis. This is because of the impaired ability of motor neurones to regulate descending signals, giving rise to disordered spinal reflexes. The central nervous system is involved in suppressing pathway activity. That is, the cortico-spinal tract helps in the conscious inhibition of muscle.

If upper motor neurons are damaged, there is a loss of inhibitory tone of muscles leading to constant contraction of muscles. This leads to the typical hypertonia, spastic paralysis and hyper-reflexia seen, when examining patients with upper motor neuron lesions. In contrast, if lower motor neuron lesions are damaged or lost, there is nothing to tell the muscles to contract, resulting in hypotonia and flaccid paralysis.

Long-standing rheumatoid arthritis can affect the cervical spine, causing vertebral malalignment (subluxation) that can affect the atlanto-axial joint. Extension of the neck during endotracheal intubation can worsen the subluxation, leading to acute compression of the spinal cord and/or vertebral arteries.

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