WEB Vetnews Feb 2016

Feb 2016

In this part we look at the pupillary sympathetic efferent pathway, anisocoria and provide advice on how

to examine an eye to localise pupillary abnormalities.

Regulars

Eye column

Pupillary sympathetic

efferent pathway

he sympathetic

innervation of the iris

consists of a three-

neuron pathway. The

sympathetic nervous

system begins in the hypothalamus.

Upper motor neuron fibres (first

order) descend through the lateral

tecto-tegmento-spinal tract to the

first three thoracic spinal cord

segments, where they synapse on

the cell bodies of the lower motor

neurons in the intermediate grey

matter.

Preganglionic axons from these

cell bodies travel in the vago-

sympathetic trunk through the

thoracic cavity to the cranial cervical

ganglion, where they synapse on

the postganglionic cell bodies.

The postganglionic fibres continue

through the middle ear and reach

the eye in association with the

ophthalmic branch of the trigeminal

nerve, where they innervate the

dilatory muscles of the iris, as well

as smooth muscle in the peri-orbita

and Muller’s muscle.

Localising pupillary

abnormalities

Pupil symmetry and size should be

evaluated in both bright and dim

environments. Examination in bright

light when pupils should be miotic

is used to accentuate mydriasis

caused by parasympathetic lesions.

Examination in dim light when pupils

should be more mydriatic is used

to accentuate miosis caused by

sympathetic lesions.

For PLR’s done in dim light, the

swinging flashlight test is done by

moving a flashlight from the tested

eye to the other eye and observing

the direct and consensual response.

In a normal animal the pupil that is

not directly stimulated will constrict

and then constrict further when

directly lit with light.

A positive swinging flashlight test

is obtained when the pupil dilates

during direct stimulation instead of

constricting further as expected.

This is called the Marcus-Gunn sign.

A positive swinging flashlight test is

pathognomonic for unilateral retinal

disease or unilateral prechiasmal

optic nerve disease.

COLUMN

Neuro-ophthalmology

simplified

PART 3

Dr Antony Goodhead, Dr Izak Venter & Dr Lo-An Odayar

Specialist Veterinary Ophthalmologists, Johannesburg and Cape Animal Eye Hospitals

(www.animaleyehospital.co.za

)