Differential Diagnosis for Horizontal Diplopia with Intact Extraocular Motions

Given that the extraocular motions are intact, the differential diagnosis for horizontal diplopia can be organized into the following categories:

• Single Most Likely Diagnosis

  • Fourth Cranial Nerve (Trochlear Nerve) Palsy: Although this typically causes vertical diplopia, a mild palsy can sometimes present with horizontal diplopia if there's a compensatory head tilt. However, given the context, this might not be the most fitting single diagnosis, and other conditions should be considered based on the specifics of the presentation.
  • Convergence Insufficiency: This condition, where the eyes have difficulty converging or focusing on near objects, can cause horizontal diplopia, especially at near distances. It's a common issue and could be considered a likely cause given the intact extraocular motions.

• Other Likely Diagnoses

  • Sixth Cranial Nerve (Abducens Nerve) Palsy: This condition affects the lateral rectus muscle, leading to horizontal diplopia. However, it typically presents with limited abduction of the affected eye, which might not align perfectly with "intact extraocular motions" unless the palsy is very mild or the question implies that the diplopia is present despite full range of motion being preserved.
  • Esotropia or Exotropia: These are forms of strabismus that can cause horizontal diplopia. They might not necessarily result from extraocular muscle weakness but from misalignment of the eyes.
  • Decompensated Heterophoria: Normally, heterophorias (latent strabismus) are compensated by the brain's fusion mechanisms. However, under certain conditions like fatigue, they can decompensate, leading to diplopia.

• Do Not Miss Diagnoses

  • Myasthenia Gravis: An autoimmune disease that can cause fluctuating muscle weakness, including the extraocular muscles, leading to diplopia. It's crucial to consider this diagnosis due to its potential for serious systemic implications and the fact that muscle weakness can be intermittent.
  • Thyroid Ophthalmopathy: Although typically associated with vertical diplopia due to involvement of the inferior rectus muscle, it can also cause horizontal diplopia. Given its potential for significant morbidity, it should not be missed.
  • Stroke or Brainstem Lesion: Certain lesions in the brainstem can affect the cranial nerves controlling eye movements, leading to diplopia. These conditions are medical emergencies.

• Rare Diagnoses

  • Graves' Disease without Significant Proptosis or Vertical Misalignment: While more commonly associated with vertical diplopia, in rare cases, it can present with horizontal diplopia.
  • Ocular Myopathies: Conditions like chronic progressive external ophthalmoplegia can cause diplopia due to weakness of the extraocular muscles, though they typically present with bilateral involvement and progressive limitation of eye movements.
  • Skew Deviation: A vertical misalignment of the eyes due to a brainstem or cerebellar lesion, which can occasionally present with or be perceived as horizontal diplopia, especially if there's a torsional component.