A hyperosmolar ionic contrast medium, ioxithalamate (Telebrix®), was inadvertently injected intrathecally to a dog during myelography. The resultant severe adverse effects were myoclonus, uncontrollable seizures, and hyperthermia. These symptoms have been described by some authors as “ascending tonic–clonic seizure syndrome.” The dog completely recovered within 24 h. The literature on 47 humans receiving intrathecal ionic contrast medium after 1966, one dog and one horse was reviewed, including the drugs involved, the circumstances of their use, and the symptoms, treatment and outcome of patients who received the drugs intrathecally. Recommendations to prevent such a misuse are given. The present report and review are a reminder that ionic contrast media are absolutely contraindicated for myelography. Only nonionic contrast media can be used intrathecally. All of the hyperosmolar contrast media are ionic and therefore contraindicated for myelography.