Vestibular Neuritis: This condition, also known as labyrinthitis, is an inner ear disorder that can cause nystagmus due to inflammation of the vestibular nerve. The presence of horizontal nystagmus, particularly with smooth pursuits to the right, suggests a peripheral vestibular disorder, with vestibular neuritis being a common cause.

Benign Paroxysmal Positional Vertigo (BPPV): Although BPPV typically presents with rotational vertigo and nystagmus, some cases can manifest with horizontal nystagmus, especially if the condition affects the lateral semicircular canal.
Labyrinthine Concussion: Trauma to the head can cause a labyrinthine concussion, leading to vestibular symptoms including nystagmus. The horizontal nature of the nystagmus could be consistent with this diagnosis, especially if there's a history of recent head injury.
Meniere's Disease: This inner ear disorder can cause episodes of vertigo, hearing loss, and tinnitus. Nystagmus can occur during acute attacks, and while it's more commonly rotational, horizontal nystagmus can be seen, particularly in the context of smooth pursuits.

Stroke or Cerebellar Infarct: Although less common, a stroke affecting the brainstem or cerebellum can cause nystagmus. It's crucial to rule out a central cause, as stroke requires immediate medical attention. The presence of other neurological deficits or risk factors for stroke would increase the suspicion for this diagnosis.
Multiple Sclerosis: This demyelinating disease can affect the brainstem and cause nystagmus. Given the potential for significant morbidity if untreated, multiple sclerosis should be considered, especially in younger patients with other neurological symptoms.

Congenital Nystagmus: While typically identified in childhood, some forms of congenital nystagmus may not be diagnosed until later in life. However, this would usually be associated with a lifelong history of nystagmus and other visual disturbances.
Medication-Induced Nystagmus: Certain medications, such as anticonvulsants, sedatives, and alcohol, can induce nystagmus. This diagnosis would be considered if there's a temporal relationship between medication use and the onset of nystagmus.
Brain Tumors: Rarely, brain tumors affecting the brainstem or cerebellum can cause nystagmus. This would typically be associated with other neurological symptoms and signs of increased intracranial pressure.