Horner’s syndrome

28 Horner’s syndrome


Horner’s syndrome describes the constellation of signs resulting from paralysis of the oculosympathetic pathway anywhere along its considerable length. It is a common cause of anisocoria in small animals.


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.

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Sep 3, 2016 | Posted by in SMALL ANIMAL | Comments Off on Horner’s syndrome

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