Horner's syndrome occurs due to a disruption in the sympathetic pathway, which can be categorized into three orders:
First-order neuron (Central): Originates in the hypothalamus and descends to the spinal cord at the level of T1.
Second-order neuron (Preganglionic): Travels from the spinal cord, exits at T1, and ascends to the superior cervical ganglion.
Third-order neuron (Postganglionic): Runs from the superior cervical ganglion to the eye muscles and skin.
Neuron | Pathway |
First-order neuron (Central) | Hypothalamus → Spinal cord (T1) |
Second-order neuron (Preganglionic) | Spinal cord (T1) → Superior cervical ganglion |
Third-order neuron (Postganglionic) | Superior cervical ganglion → Eye (pupillary dilator muscle, Müller’s muscle) and face skin |
A Hydroxyamphetamine test helps localize the lesion by assessing the integrity of the postganglionic neuron (third-order neuron). Hydroxyamphetamine causes the release of norepinephrine from intact nerve terminals.
If the third-order neuron is intact, hydroxyamphetamine will cause dilation of the pupil.
If there is no dilation, it indicates that the lesion is in the third-order neuron.
Figure- Left-sided Horner's syndromeReference:
1. Comprehensive ophthalmology, A K Khurana, 7th edition, P. No. 329