Unequal & Abnormal Pupils Flashcards
How would you tell between normal anisocoria and pathological anisocoria?
Normal asymmetry between the pupils persists whatever the ambient illumination. Pathological anisocoria varies between dark and light conditions.
True or false: the normal pupil is located slightly nasal to the centre of the cornea.
True.
Pupil size is determined by a balance between which muscles, and what innervates them?
The sphincter pupillae and dilator pupillae muscles.
The parasympathetic nervous system, carried in the third cranial nerve, innervates the sphincter muscle. The transmitter is ACh.
The sympathetic system innervates the dilator muscle. The transmitter is noradrenaline.
How is cocaine 4% used as a test of pupil function?
Cocaine 4% prevents reuptake of noradrenaline at the NMJ into the sympathetic nerve ending.
As more noradrenaline will be available at the NMJ, this will dilate the normal pupil but not a pupil in which there is loss of sympathetic innervation.
What is denervation hypersensitivity, and which agents can be used to differentiate pupil dysfunction in postganglionic parasympathetic vs postganglionic sympathetic lesions?
When post-ganglionic fibres are damaged, there is a reduction of transmitter secretion and release. In response, the understimulated end-organ (in this case the iris muscle), develops an excess of receptor and becomes hypersensitive such that it will react to only a small quantity of transmitter = denervation hypersensitivity.
Postganglionic parasympathetic lesion (post-ciliary ganglion in the orbit): pupil will constrict to 0.1% pilocarpine (a cholinergic agonist) = ADIE’S PUPIL, with a normal pupil unaffected.
Postganglionic sympathetic lesion (post-superior cervical ganglion in the neck): pupil will dilate to 0.1% adrenaline = HORNER’S PUPIL, whereas normal pupil will not.
What are posterior synechiae and why might they cause unequal pupils?
Adhesions between the iris and the lens occur as a result of intraocular inflammation (iritis). The adhesions are only rarely complete, so the pupil margin becomes irregular in shape.
This is most obvious when the pupil has been dilated. Light reactions should be normal, but difficult to detect.
How would acute glaucoma cause unequal pupils?
During an attack of acute glaucoma, the IOP may become so high that ischaemic damage results, particularly at the 3 and 9 o’clock positions, causing the pupil to become vertically oval. It reacts poorly to light.
What type of pupil does a third nerve palsy cause?
Causes a dilated pupil that fails to react to light or accommodation.
The anisocoria is most obvious in bright light, when the normal pupil constricts but the abnormal pupil remains dilated.
Other features include a ptosis and divergent squint.
Acute rise in ICP, after head injury for example, may cause s rapidly evolving third nerve palsy, manifested as a sudden dilation of the pupil. This has given rise to the phrase “the pupils have blown”.
During recovery from a third nerve palsy, nerve fibre regeneration may be aberrant. Typically, there is what finding?
Typically, there is pupil constriction on adduction of the affected eye.
What is Adie’s pupil, and how may you confirm this with a pharmacological test of pupil function?
Adie’s pupil is a disorder of postganglionic parasympathetic fibres in the orbit. This causes a semi-dilated pupil that reacts poorly to direct light and to accommodation. Re-dilation is slow. There is denervation hypersensitivity.
*slow irregular contraction, constriction to 0.1% pilocarpine (ACh stimulant).
What is Horner’s syndrome, and how many you confirm this with a pharmacological test of pupil function?
Horner’s syndrome causes a miosis through damage to the sympathetic innervation, at any point along the pathway between the hypothalamus and the eye (eg. either pre- or post-ganglionic, incl. T1, superior cervical ganglion, etc). Requires Ix.
The anisocoria is most obvious in dim illumination, when the affected pupil fails to dilate naturally.
Cocaine 4% dilates the normal pupil, but not the Horner’s pupil.
Due to denervation hypersensitivity, 0.1% adrenaline will dilate only the Horner’s pupil (and not the normal pupil).
Why is there a ptosis in Horner’s syndrome?
Horner’s syndrome involves damage to the sympathetic innervation.
Because the Müller’s muscle component of the upper lid levator muscle is innervated by the sympathetic system, there will also be a mild ptosis (vs. third nerve palsy where there will be a significant ptosis).
Where might there be a lesion if anhydrosis is present in Horner’s syndrome?
If the sympathetic lesion occurs below the superior cervical ganglion, ipsilateral facial perspiration may be diminished and facial flushing will be present, although these features are usually transient.
The light and near reflexes are not affected.
When the sympathetic chain is damaged by a lung cancer (leading to Horner’s), what is the typical cancer which causes this?
Pancoast’s tumour (a tumour of the pulmonary apex).
What is light-near dissociation?
Light-near dissociation describes pupils that react poorly or not at all to light, but which constrict to a near stimulus (accommodate).
The causative lesion is in the midbrain, affecting the internuncial neurons which pass from the pre-tectal nucleus to the Edinger Westphal component of the third nerve nucleus.
