Neuromyelitis optica (NMO) is an autoimmune disease that affects the central nervous system. Patients have relapses (also known as attacks) which are often quite severe and leave them with significant disability. Without treatment, within 5 years 50% of NMO patients are blind in one or both eyes or require walking assistance (cane, walker or wheelchair).

NMO has only been relatively recently described and is fairly rare. Most NMO patients' immune systems produce abnormal antibodies against aquaporin-4 (AQP4), which is found in certain cells in the central nervous system. When these AQP4 antibodies bind to AQP4, they trigger a cascade of events involving the immune system which eventually leads to damage to the nervous system. This ultimately leads to disability, some of which is permanent.

Until now, treatments for NMO have been mostly focused on decreasing production of this AQP4 antibody. However, recent experiments in animal models of NMO have shown the importance of what happens inside the central nervous system after the antibody binds to the nervous system cell. Specifically, researchers have noted the importance of a specific cell type, eosinophils, in causing damage in NMO lesions. In a recent study, researchers showed they could prevent damage from NMO by blocking eosinophils using cetirizine, which is a popular over-the-counter allergy medicine.

Cetirizine is already known to be safe and well-tolerated in the general population. In this study, the researchers plan to add cetirizine on to patients' current NMO treatment. The researchers aim to show that it is safe, well-tolerated, and that with cetirizine, NMO patients have less relapses and therefore less disability over the course of the year following initiation of treatment. The researchers also plan to study how cetirizine changes the immunological profile in NMO patients by examining blood and cerebrospinal fluid.

The researchers hypothesize that cetirizine, an allergy medication that acts as an eosinophil-stabilizer, will decrease the relapse rate when added to current standard therapy in patients with neuromyelitis optica.

Medication compliance will be assessed by the research coordinator at each visit through discussion with the patient and pill counting.

Inclusion Criteria:

• 18 years to 85
• Meet criteria for the diagnosis of neuromyelitis optica as outlined by Wingerchuk et al in 2006. Alternatively patients may be included if they have had an episode of myelitis or optic neuritis in combination with a positive NMO IgG antibody, as positive antibody with a first episode is highly associated with future relapse.
• Disease duration of at least 6 months
• Stable, without any NMO relapses, for the 3 months prior to the baseline assessment visit
• Ability to understand the purpose and risks of the study and provide signed and dated informed consent and authorization to use protected health information (PHI) in accordance with national and local subject privacy regulations.

Exclusion Criteria:

• Current therapy with daily cetirizine or another daily antihistamine for any indication
• Known hypersensitivity to cetirizine, hydroxyzine, or any component of the formulation
• Change in NMO disease-modifying therapy in the 3 months prior to baseline assessment
• Pregnancy or planning pregnancy during the study period
• Severe renal or hepatic impairment
• Inability to complete the study protocol for any reason

Small sample size and lack of a control group.