Topical Dexamethasone Counters Intravitreal Ivermectin-Induced Ocular Toxicity in a Rabbit Model

PURPOSE

Systemic use of Ivermectin has been reported to incite blindness in humans and veterinary patients. This study was designed to investigate the systemic and intravitreal effect of Ivermectin on ocular and retinal health and its attenuation with topical Dexamethasone.

METHODS

Systemic injection of Ivermectin@ 1.6 mg/kg S/C was administered, thrice a week for three weeks to New Zealand White rabbits (N = 4) with and without topical drops of Verapamil (N = 4). Pre and post-treatment ocular examination was conducted. At the end of three weeks the eyes were collected for histopathology.0.2 ml of Ivermectin solution (1.6 mg/ml) was injected intravitreally in one eye of the rabbit (N = 8), Half the rabbits received 0.1% dexamethasone drops thrice daily for 7 days, while the controls received PBS. Pre and post-treatment, detailed examination was conducted, which included the Schirmer Tear test, Fluorescein staining, Intraocular pressure, slit lamp biomicroscopy and fundus photography. The retina was harvested for histopathological and tunnel assay.

RESULTS

Systemic therapy with Ivermectin, with and without Verapamil did not incite any adverse response in the eye. Intravitreal Ivermectin evoked severe uveitis 4/4, cataract 3/4, corneal erosion 3/4 eyes and severe inflammatory response. Eyes that received dexamethasone were rescued from the adverse changes as demonstrated clinically, by histopathology and prevention of apoptosis.

CONCLUSIONS

Intravitreal Ivermectin incites severe inflammatory response. Topical dexamethasone counters the ocular toxicity incited by Ivermectin.

Manifestations of systemic disease in the retina and fundus of cats and dogs

The fundus is unique in that it is the only part of the body that allows for a noninvasive and uninterrupted view of vasculature and nervous tissue. Utilization of this can be a powerful tool in uncovering salient incidental findings which point to underlying systemic diseases, and for monitoring response to therapy. Retinal venules and arterioles allow the clinician to assess changes in vascular color, diameter, outline, and tortuosity. The retina and optic nerve may exhibit changes associated with increased or decreased thickness, inflammatory infiltrates, hemorrhages, and detachments. While some retinal manifestations of systemic disease may be nonspecific, others are pathognomonic, and may be the presenting sign for a systemic illness. The examination of the fundus is an essential part of the comprehensive physical examination. Systemic diseases which may present with retinal abnormalities include a variety of disease classifications, as represented by the DAMNIT-V acronym, for Degenerative/Developmental, Anomalous, Metabolic, Neoplastic, Nutritional, Inflammatory (Infectious/Immune-mediated/ischemic), Toxic, Traumatic and Vascular. This review details systemic illnesses or syndromes that have been reported to manifest in the fundus of companion animals and discusses key aspects in differentiating their underlying cause. Normal variations in retinal anatomy and morphology are also considered.

Ophthalmic effects of dexmedetomidine, methadone and dexmedetomidine-methadone in healthy cats and their reversal with atipamezole

OBJECTIVES

The aims of this study were to evaluate and compare the effects that dexmedetomidine and methadone, either alone or in combination, have on the ocular variables of healthy adult cats when administered intramuscularly, as well as their reversal with atipamezole.

METHODS

A randomized crossover blinded study of 10 healthy cats was used to assess the effect of 0.2 mg/kg methadone (MET), 7.5 μg/kg dexmedetomidine (D7), 10 μg/kg dexmedetomidine (D10), 7.5 μg/kg dexmedetomidine and 0.2 mg/kg methadone (DM7) and 10 μg/kg dexmedetomidine and 0.2 mg/kg methadone (DM10) on intraocular pressure (IOP), tear production and pupil diameter (PD). The animals were evaluated for 30 mins. Afterwards, atipamezole was administered and ocular variables were evaluated for 30 mins.

RESULTS

D10, DM7 and DM10 significantly decreased mean IOP but MET or D7 did not. Tear production decreased significantly in all treatments, corresponding to 18%, 59%, 63%, 86% and 98% in MET, D7, D10, DM7 and DM10, respectively. PD increased in all treatments, but MET showed the highest PD. Thirty minutes after atipamezole (RT30), IOP returned to baseline with no difference between groups, and there was a significant increase in tear production, but the means were still different from baseline.

CONCLUSIONS AND RELEVANCE

Dexmedetomidine decreases IOP and tear production but increases PD in healthy cats. Atipamezole can partly reverse those alterations. Low-dose dexmedetomidine (7.5 µg/kg) promotes sedation without changing the IOP. All protocols significantly decrease tear production, and Schirmer tear test after sedation is not representative of non-sedated values. Methadone induces quick onset mydriasis without changing the IOP.

Toxicology of Avermectins and Milbemycins (Macrocyclic Lactones) and the Role of P-Glycoprotein in Dogs and Cats

Overdoses of macrocyclic lactones in dogs and cats can result in such signs as tremors, ataxia, seizures, coma, and blindness. Dogs with the ABCB1-1Δ gene defect are predisposed to macrocyclic lactone toxicosis at lower dosages than dogs without the defect. Intravenous lipid emulsion therapy has been suggested for treatment of macrocyclic lactone toxicosis but evidence of efficacy is limited. Initial decontamination and supportive care remain the mainstays of therapy for macrocyclic lactone toxicosis.

Electroretinographic changes after intravenous lipid emulsion therapy in a dog and a foal with ivermectin toxicosis

This case report describes ivermectin-induced blindness in a dog and a foal with normal ophthalmic fundic examinations and attenuated electroretinography (ERG). Subsequent recovery in ERG was noted following intravenous lipid emulsion (ILE) therapy. A dog and a foal were evaluated for ivermectin-induced blindness. Clinical signs included dull mentation, absent pupillary light reflexes (PLRs), and absent menace on presentation. The animals had normal fundoscopic examinations; however, in both cases ERG was consistent with neurosensory retinal dysfunction. Following ILE therapy for ivermectin toxicosis, return of menace, PLRs, and normal mentation were noted, as was improvement in ERG and serum ivermectin levels. These are the first documented cases of ivermectin-induced blindness in a dog and a foal with normal fundic examinations and attenuated ERG. ERG improved in both animals after ILE therapy. ERG may assist in the diagnosis of ivermectin toxicosis in dogs and horses. ILE therapy may hasten recovery in treatment of ivermectin-induced blindness.