Bilateral mydriasis in cats

©Barnett, KC & Crispin, SM Feline Ophthalmology (2002) Saunders

Bilateral mydriasis with intact vision has been associated with a variety of neurological conditions:

Dysautonomia

Although not as common in the UK now as when originally reported cases of feline dysautonomia still occur in Europe, and less commonly, the USA. The sympathetic and parasympathetic systems are both affected. Bilateral, dilated, unresponsive pupils, often accompanied by third eyelid protrusion, reduced tear production and photophobia, are common ocular signs. Other evidence of generalised dysautonomia is usually evident (e.g. depression, anorexia, weight loss, dry mucous membranes, intermittent regurgitation, megaoesophagus, ileus, constipation, urinary retention, bradycardia), but in some cases classical clinical signs may be absent or less prominent. Degeneration of autonomic nerves and ganglia is seen on post-mortem examination.

Application of one drop of 0.1% pilocarpine to the eye of an affected cat may produce miosis (constriction of pupil) and third eyelid retraction within minutes as a result of denervation hypersensitivity in established cases, whereas topical application of 0.25% physostigmine sulphate should have no effect.

The management of dysautonomia cases requires good nursing care, with parenteral feeding or feeding via a pharyngostomy tube during the acute phase and moist food thereafter, until the mucous membranes are no longer dry. Tear replacement therapy is also necessary until tear production has returned to normal. The prognosis is guarded in severely incapacitated cats and nursing care is required for months, even in successfully treated cases. Recovery may be partial in some cats and stress may trigger apparent relapses in others.

Hepatic encephalopathy

During hepatic encephalopathic episodes, some cats will exhibit bilateral mydriasis. This is invariably accompanied by other neurological signs such as ataxia, altered behaviour and hypersalivation

Feline spongiform encephalopathy

Bilateral mydriasis and blindness have been a feature of some cats with feline spongiform encephalopathy. The pupillary light reflex may be poor or absent. Other ocular manifestations are usually absent. A range of neurological signs such as persistent and progressive ataxia, altered behaviour (e.g. extreme apprehension) and ptyalism (drooling) are also present. The diagnosis is confirmed by brain histopathology.

Thiamine deficiency

Clinical signs of thiamine (Vit B1) deficiency in cats are characteristic and include anorexia, cerebellar-type ataxia, ventroflexion of the neck and bilateral mydriasis. Fundus examination may reveal peripapillary neovascularisation, but these are not specific for thiamine deficiency as inflammatory and neoplastic conditions may produce similar changes. Treatment includes systemic corticosteroids to reduce the oedema in addition to thiamine to correct the deficit; both may be given intravenously initially.

Acute brain disease

Mydriasis may accompany acute generalised diseases of the midbrain such as swelling or compression. If the mydriasis is bilateral it carries a grave prognosis. 

Cavernous sinus syndrome

The cavernous sinus is a venous sinus on either side of the pituitary fossa, but also contains fibres of all the cranial nerves, except the optic nerve, that innervate the globe. Many disease processes can affect the cavernous sinus; for example, neoplasia, inflammatory diseases and vascular disorders can affect this site and cause unilateral or bilateral fixed (slightly mydriatic) pupils and loss of ocular motility. There may also be ptosis and third eyelid prominence and, occasionally. strabismus will be present depending upon specific or multiple cranial nerve palsies. Cavernous sinus syndrome is the commonest reason for complete ophthalmoplegia (paralysis of extraocular and intraocular muscles).