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Ptosis
This long delayed quiz concerns Horner's syndrome again but this time, there is only one question. The answer can be easily found in standard texts but it must first be reasoned out. The question is one of those that challenges what we think we know as basic knowledge, at least it did when it struck me; The question is simply this.
Why is ptosis a component of Horner's syndrome? A hint, if the answer was simply paralysis of the levator palpabrae muscle, I would not be asking the question. To answer the question, you first need to ask yourself what is the underlying pathology behind Horner's syndrome. After this, it should be a fairly simple matter to find the answer.
Horner's syndrome is caused by sympathetic paralysis or paresis. The associated ptosis cannot be due to paralysis of the levator palpabrae muscle as this is innervated by the oculomotor nucleus not the sympathetic system. Consequently, another muscle must be involved. It must be a smallish muscle as generally, the ptosis is volitionally correctable by the lavator palpabrae. In either eyelid, there is a fibrocartilaginous plate called a tarsal. Attached to this plate is a small muscle called Muller's muscle. This muscle is innervated by the sympathetic system through the superior cervical ganglion. The effect of sympathetic stimulation and subsequent contraction of this muscle is to open the eyes wide. Therefore, when the system is paralysed or paretic, the superior eyelid droops. References:
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