Which of the following is NOT a pre-ganglionic Horner's syndrome etiology?

• A. Neck trauma
• B. Pancoast tumor
• C. Carotid dissection
• D. Wallenberg's syndrome

Answer: (D)

Horner’s syndrome results from disruption of the oculosympathetic pathway, which runs through three neurons. The term preganglionic refers to the second-order neurons that travel from the spinal cord to the superior cervical ganglion. Central (first-order) lesions occur in the brainstem or upper spinal cord and affect the first neuron before it reaches the spinal cord. Second-order (preganglionic) lesions lie along the pathway from the spinal cord to the superior cervical ganglion. Third-order (postganglionic) lesions affect the fibers after the ganglion as they travel with the internal carotid to the eye.

Wallenberg’s syndrome is a lateral medullary infarct in the brainstem that interrupts the descending sympathetic fibers at the level of the first-order neuron, well before the sympathetic chain or superior cervical ganglion is reached. That makes it not a preganglionic etiology. In contrast, neck trauma and a Pancoast tumor injure the second-order preganglionic fibers between the spinal cord and the superior cervical ganglion, fitting the preganglionic category. Carotid dissection typically involves postganglionic fibers along the internal carotid, so it is not a preganglionic lesion either.