Seizures and blindness onset zone in 7 horses undergoing radiographic myelography in lateral recumbency under inhalation anesthesia.

Overview

• This study investigated the occurrence, classification, and brain regions involved in seizures and blindness in horses undergoing radiographic myelography performed under inhalation anesthesia while lying on their sides.
• The research identified specific seizure and blindness onset zones in the brain and described the clinical features, management, and recovery times for these complications.

Background

• Myelography is a diagnostic imaging technique that involves injecting contrast dye into the spinal canal to visualize the spinal cord and surrounding structures.
• While useful, myelography can lead to adverse neurological side effects such as seizures and blindness, although these are not well understood or documented in horses.
• This study aims to better characterize these complications, their brain onset zones, and clinical outcomes in horses undergoing this procedure.

Study Design and Subjects

• This was a retrospective study reviewing cases of horses that developed seizures or blindness linked to radiographic myelography.
• A total of seven young adult horses were included: four Warmbloods and three Thoroughbreds, aged 3 to 16 years with a median of 9 years.
• Among the horses, three presented with general proprioceptive ataxia (a neurological deficit affecting movement awareness), and four were neurologically normal before the procedure.
• All underwent myelography with the injection of 50 to 60 mL of iohexol (a non-ionic iodine-based contrast agent) intrathecally, dosed at approximately 0.09-0.1 mL/kg.
• Myelography was performed in lateral recumbency, meaning horses were lying on their sides under inhalation anesthesia.

Findings: Seizures

• Six of the seven horses experienced focal facial motor seizures on the side opposite (contralateral) to the side they were lying down on during the procedure.
• One horse progressed from these focal seizures to generalized seizures affecting the entire body.
• Electroencephalography (EEG) localized the seizure onset zone to the right parietal cortex — a region of the brain involved in processing sensory information and motor control.
• Seizures were treated with short-acting drugs such as diazepam (in 3 horses), midazolam (in 2 horses), or both (in 1 horse), with long-term antiseizure medication phenobarbital used in two cases.
• All seizures resolved fully within 2 to 8 hours after the myelography procedure.

Findings: Blindness

• Five horses showed an absent menace response (a blink reaction to an approaching object) in the left eye, but bilateral normal dazzle reflexes (a blink to bright light) and pupillary light responses were observed.
• This clinical pattern suggests cortical blindness — blindness due to brain cortex dysfunction rather than eye problems — localized to the right occipital cortex, the brain region responsible for vision.
• These horses’ blindness was transient, resolving within 8 to 48 hours post-myelography.

Interpretation of Results

• The seizures and blindness occurred contralateral to the side of recumbency (the side the horse was placed on during the procedure) but ipsilateral to brain cortex regions identified via EEG and clinical exam.
• The focal facial seizures were linked to activity starting in the parietal cortex, while cortical blindness was linked to dysfunction in the occipital cortex.
• The localized brain effects are likely related to the positioning during anesthesia and the intrathecal administration of contrast, possibly causing transient irritation or chemical effects on specific brain areas.
• The fact that all symptoms resolved fully within hours to two days indicates these were reversible, transient adverse effects rather than permanent damage.

Clinical Implications

• Veterinarians performing radiographic myelography in horses should be aware of potential transient neurological complications, specifically contralateral focal facial seizures and cortical blindness.
• Recognizing the typical lateralization and characteristics of these events can help timely diagnosis and management.
• Supportive and antiseizure treatments effectively controlled seizures, and blindness spontaneously resolved, allowing practitioners to reassure owners about the prognosis.
• Modifications in anesthetic management or contrast administration techniques may reduce these risks but require further study.