Horner's syndrome has many possible aetiologies that can involve a lesion anywhere in the course of the sympathetic tract from hypothalamus to brainstem and upper thoracic cord, sympathetic trunk, stellate ganglion, carotid artery and, finally, long ciliary nerve
to the eye.
A14.2.01.027 Long ciliary nerves
A14.2.01.028 Posterior ethmoidal nerve A14.2.01.029 Anterior meningeal branch A14.2.01.030 Anterior ethmoidal nerve A14.2.01.031 Internal nasal branches A14.2.01.032 Lateral nasal branches A14.2.01.033 Medial nasal branches A14.2.01.034 External nasal nerve A14.2.01.035 Infratrochlear nerve A14.2.01.036 Palpebral branches A14.2.01.025 N.
From here, they divide into two long ciliary nerves
to reach the iris dilator muscle.
Sympathetic fibres travel via the nasociliary nerve and the long ciliary nerves
until they reach the ciliary body and dilator pupillae muscle.
The sensory nerves reach the eye through the nasociliary branch of the ophthalmic nerve , which branches typically into two long ciliary nerves
, one nasal and the other temporal, which course directly to the posterior pole of the eye, and a communicating branch carrying sensory fibers to the ciliary ganglion .
The third-order or post-ganglionic neuron travels on the surface of the internal carotid artery into the intracranial space and cavernous sinus to arrive at the iris dilator through the long ciliary nerves
. Damage can happen anywhere along this pathway and a series of pharmacological tests can be performed to determine whether the lesion is pre- or post-ganglionic.