If the smaller pupil dilates in response to instillation of apraclonidine eye drops, this suggests Horner's syndrome is present. In Horner's syndrome sympathetic nerve fibers have a defect, therefore the pupil of the involved eye will not dilate in darkness. Anisocoria which is worsened (greater asymmetry between the pupils) in the dark suggests the small pupil (which should dilate in dark conditions) is the abnormal pupil and suggests Horner's syndrome or mechanical anisocoria.Ĭauses of anisocoria range from benign (normal) to life-threatening conditions.Ĭlinically, it is important to establish whether anisocoria is more apparent in dim or bright light to clarify if the larger pupil or smaller pupil is the abnormal one. Alkaloids present in plants of the genera Brugmansia and Datura, such as scopolamine, may also induce anisocoria. Some examples of pharmacological agents which may affect the pupils include pilocarpine, cocaine, tropicamide, MDMA, dextromethorphan, and ergolines. Pharmacological agents with anticholinergic or sympathomimetic properties will cause anisocoria, particularly if instilled in one eye.In ischemic lesions of the oculomotor nerve, pupillary function is usually spared whereas in compressive lesions the pupil is involved. Oculomotor nerve palsy: Ischemia, intracranial aneurysm, demyelinating diseases (e.g., multiple sclerosis), head trauma, and brain tumors are the most common causes of oculomotor nerve palsy in adults.Tonic pupil is characterized by delayed dilation of iris especially after near stimulus, segmental iris constriction, and sensitivity of pupil to a weak solution of pilocarpine. It may be associated with loss of deep tendon reflex (Adie's syndrome). Adie tonic pupil: Tonic pupil is usually an isolated benign entity, presenting in young women.Mechanical anisocoria: Occasionally previous trauma, eye surgery, or inflammation ( uveitis, angle closure glaucoma) can lead to adhesions between the iris and the lens.In this condition, the difference between pupils is usually less than 1 mm. Physiological anisocoria: About 20% of population has a slight difference in pupil size which is known as physiological anisocoria.Preventionĭue to the nature of the condition, there is no known cure or a way of preventing the disorder.Anisocoria is a common condition, defined by a difference of 0.4 mm or more between the sizes of the pupils of the eyes. If medication is prescribed, the pet owner will need to ensure that all of the medication is given fully and as directed. Treatment will be fully dependent upon the underlying cause of the issue. Ultrasound can be used to detect lesions in the eyes, while computed tomography (CT) and magnetic resonance imaging (MRI) can be used to identify any lesions in the brain that may be causing the condition. When veterinarians are evaluating the dog's pupils, the primary goal is to distinguish between neurological and eye-related causes. There are several potential causes of an altered pupil size in dogs, including inflammation in the frontal region of the eye, increased pressure in the eye, diseases that are focused in the iris tissue itself, a poorly developed iris, scar tissue build up in the eye, medications, and cancer. The most noticeable symptom is when your dog has one pupil that is visibly smaller than the other. If you would like to learn more about how this disease affects cats, please visit this page in the PetMD health library. The condition or disease described in this medical article can affect both dogs and cats. With the proper detection of the disease's underlying cause, treatment plans are available that should resolve the issue. This condition causes one of the dog's pupils to be smaller than the other. Anisocoria refers to an unequal pupil size. The pupil expands when there is little light present, and contracts when there is a greater amount of light present. The pupil is the circular opening in the center of the eye that allows light to pass through.
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