Do any of you feel comfortable localizing Horner’s syndrome in a dog? If you do…skip the first section and read the data from a recent study about cervical myelopathy and Horner’s syndrome. If not, please carry on and join us for an interesting look at Horner’s syndrome with cervical myelopathies.
First, Anatomy
The sympathetic pathway to the eye is a 3-neuron system. Neuron 1 starts in the thalamus, travels through the brainstem and cervical spinal cord to T1-T3 thoracic spinal cord segments where it synapses. Neuron 2 starts here and travels cranially, through the ansa subclavia along the vagosympathetic trunk (right next to that jugular vein you’re about to do venipuncture on!) to the caudal aspect of the bulla. From there, the 3rd order neuron takes a path through the tympanic bulla, along the ventral aspect of the skull (in the cavernous sinus) and hops a ride with CN 5 (trigeminal) to make a beeline to the eye. This neuron innervates the muscles of the iris, eyelids and orbit. It is the most indirect path anyone could design but I might argue that you can break it down into several key parts when localizing Horner’s Syndrome.
Intracranial
Cervical
Brachial plexus
Jugular groove
Tympanic bulla
CN 5
Cervical Myelopathies and Horner’s Syndrome
After reviewing the anatomy, it might be easy to see how a cervical lesion may cause Horner’s syndrome, right? The 1st order neuro travels from the intracranial structures via the cervical spinal cord to the upper thoracic spinal cord segments. Interestingly, a recent study looked at Horner’s Syndrome and cervical myelopathies* and found an incidence of only about 10% of Horner’s syndrome with concurrent cervical myelopathy. Therefore, although the anatomy makes sense, it is a fairly protected neuronal pathway and therefore a cervical lesion rarely causes Horner’s signs.
What Causes Horner’s Syndrome and a Cervical Myelopathy?
I’m glad you asked! According to this study, more dogs with Horner’s syndrome had noncompressive lesions compared to the control group (dogs with cervical myelopathy without Horner’s syndrome). Noncompressive lesions are often caused by fibrocartilaginous embolism (FCE) but hydrated nucleus pulposus extrusion (HNPE) and intramedullary neoplasia, noncompressive spinal trauma, Syringohydromyelia and inflammatory myelitis were found to cause Horner’s syndrome. Also of note, Horner’s syndrome can be unilateral OR bilateral with cervical myelopathies.
Did having Horner’s Syndrome Affect Prognosis?
Nope. The underlying disease predicted prognosis and having Horner's Syndrome did not significantly negatively or positively influence outcome. However, seeing Horner’s syndrome on the neurologic examination would suggest that a surgical lesion is less likely to be identified.
Do you have a patient with Horner’s Syndrome? Do you suspect a cervical myelopathy? I’d love to help! Please reach out via email or schedule a consult online. Stay safe on these slippery roads and have a great week!
Reference from the TidBit Tuesday: https://onlinelibrary.wiley.com/doi/epdf/10.1111/jvim.16588