Signs

Miosis

Anisocoria is most obvious in darkness or during excitement or fear when the failure to dilate fully is noted. It is less obvious when the animal is fatigued or drowsy as pupils naturally constrict as sleep approaches.

Ptosis

Denervation of the smooth muscle of the upper eyelid creates ptosis. The palpebral fissure may be narrowed further by denervation of the smooth muscle of the lower lid which causes it to be raised.

Enophthalmos

Loss of sympathetic tone to orbital smooth muscle and the resulting unopposed action of the retractor bulbi muscle causes the eye to sink further into the orbit.

Protrusion of third eyelid: Denervation of smooth muscle in the eyelid along with an enophthalmic position results in a prominent third eyelid.

Vasodilation of facial, aural, or conjunctival blood vessels may be noted. Ipsilateral coat colour change has been reported in Siamese cats secondary to the local temperature increase.

Figure 28.1 shows a left Horner’s syndrome.

Figure 28.1 Left Horner’s syndrome.

Pathway

The sympathetic innervation of the eye originates in the hypothalamus, descends through the brainstem and spinal cord to synapse within thoracic spinal cord segments T1–T3. This section of the pathway is referred to as the First Order. Axons exit via T1–3 nerve roots and pass through the thorax and several ganglia and travel rostrally up the neck in the vagosympathetic trunk to synapse in the cranial cervical ganglion. This anatomical division is referred to as the Second Order. Finally, the axons travel through the middle ear, along the floor of the skull and exit via the orbital fissure to innervate the smooth muscle of the eye, including the pupil dilators. The postganglionic pathway from the cranial cervical ganglion to the pupil dilators is referred to as the Third Order.

Lesions of the oculosympathetic pathway (Fig. 28.2) can be categorized as first, second, or third order according to their anatomical site which can be most reliably discovered by finding concomitant neurological deficits. Pharmacological testing may help localize the lesion. It makes use of the phenomenon of denervation hypersensitivity which develops 8–10 days after a postganglionic lesion develops. A denervated receptor responds to a dilute concentration of neurotransmitter whereas a normal receptor would not. This may be helpful in differentiating second from third order lesions. thoracic radiography is used to investigate second order lesions.

Figure 28.2 Oculosympathetic pathway.