Pupillary Reflex Pathologies

• 3rd nerve palsy, Adie's tonic pupil, and Horner's syndrome.

neurophysiology of pupillary constriction and relaxation.

Relevant Anatomy

Superficial structures of the eye:

• The palpebral (the eyelid)
• The iris
• The pupil
• The iris sphincter muscles are circumferentially-arranged to narrow pupil size.
  • They are parasympathetically-innervated muscles to constrict pupil size in bright light.
• The iris dilator muscles are radially-arranged to enlarge pupil size.
  • They are sympathetically-innervated muscles to widen pupil size in dim light.
• Sympathetic excitation causes upper eyelid elevation, which occurs via activation of the superior tarsal muscle. This establishes the size of the palpebral fissure, (which narrows in Horner's syndrome).

What happens when parasympathetic activation fails?

• The pupil dilates.
• The iris thins, and thus the pupil enlarges.

Pupillary constriction to light

Key non-ocular anatomy

• Midbrain
• Pretectal olivary nuclei
• CN3 Edinger-Westphal nuclei
• Ciliary ganglia

Circuitry

• Light strikes the retinae, which channel back via the optic nerves to the midbrain (we show only one optic tract lead to the midbrain, so we can see that each side produces a bilateral response.).
• The optic tract triggers the pretectal olivary nucleus.
• Each pretectal olivary nucleus triggers bilateral activation of CN3.
• Each CN 3 activates the ipsilateral ciliary ganglion.
• The ciliary ganglia send short ciliary nerves to the iris sphincter muscles to produce pupillary constriction.

Key Pathological causes of dilated pupil

• CN 3 injury
• Ciliary ganglionopathy or neuropathy (Adie's tonic pupil)
• Iris sphincter muscle failure

Adie's tonic pupil produces a supersensitivity to direct cholinergic agonists via denervation effects.

• So weak (dilute) pilocarpine (a direct cholinergic agonist) will causes pupillary constriction in an Adie's tonic pupil because it is super sensitive to even weak pilocarpine.
• In other conditions, there will be no response, meaning the pupil remains dilated.

3rd nerve palsy

• Strong (concentrated) pilocarpine will causes pupillary constriction in a 3rd nerve palsy, because it directly activates the iris sphincter muscle – it doesn't rely on CN 3.

Iris sphincter muscle failure

• If there is iris sphincter failure, such as from muscarinic antagonist (eg, atropine), the pupil will fail to constrict even with a direct cholinergic agonist.

We can remember atropine's effects with the story of atropa bella donna; this plant contains atropine, and women in ancient times used it to dilate their eyes, hence bella donna for "pretty woman".

What happens when sympathetic activation fails?

In brief:

• The pupil constricts
• The eyelid droops

The sympathetic pathology, called Horner's syndrome, manifests with, amongst other pathologic signs:

• Ptosis, which is eyelid droop; we can detect it if we compare the palpebral fissure size of the normal eye to the abnormal eye; here it will be more narrow because of the eyelid droop, from loss of superior tarsal muscle tone.
• Miosis, pupil constriction, from a loss of innervation to the dilator muscles.
• Anhidrosis, loss of sweating, from loss of sympathetic innervation to sweat glands to the face.

The acronym "PAM" is used to remember this triad.

Common causes of Horner's syndrome

Key Anatomy

• Sympathetic circuitry begins within the hypothalamus.
• Descends via the posterolateral hypothalamospinal tract.
• Innervates the ciliospinal center of Budge at C8 – T2.
• Innervates the superior cervical ganglion, which sends fibers that ascend the carotid artery to innervate the head and neck.

Key causes of Horner's syndrome are, from proximal to distal:

• Hypothalamic dysautonomia (called hypothalamic storm).
• Brainstem strokes (typically, medullary).
• Pancoast tumor (a form of apical lung tumor) and other paravertebral masses.
• Carotid dissection, which expands the vessel wall and injures the ascending sympathetic fibers.