Differential Diagnosis for Gradual Onset Diplopia

Single Most Likely Diagnosis

• Chronic Progressive External Ophthalmoplegia (CPEO): This condition is characterized by a gradual decline in eye movement, often presenting with bilateral ptosis and limited eye movements. The absence of significant improvement in ptosis with an ice pack and the lack of diurnal variation or enhancement of ptosis with sustained upgaze are consistent with CPEO. The normal saccadic velocity and smooth pursuits also support this diagnosis.

Other Likely Diagnoses

• Myasthenia Gravis: Although the patient does not exhibit typical features like diurnal variation of symptoms or improvement with ice pack, myasthenia gravis can present atypically, especially in older adults. The presence of bilateral ptosis and limited eye movements could still suggest this diagnosis, warranting further investigation.
• Thyroid Eye Disease: This condition can cause restrictive myopathy leading to limited eye movements and diplopia. However, the absence of proptosis, disc edema, or pigmentary retinopathy makes this less likely, though it cannot be entirely ruled out without thyroid function tests.

Do Not Miss Diagnoses

• Brainstem or Cranial Nerve Lesions: Although the CT orbit and head are negative, it's crucial to consider lesions affecting the brainstem or cranial nerves (III, IV, VI) that could cause diplopia and limited eye movements. Further imaging, such as an MRI, might be necessary to rule out these conditions.
• Wernicke's Encephalopathy: This is a medical emergency that can present with ophthalmoplegia, among other symptoms. The absence of ataxia and confusion does not rule out this diagnosis entirely, especially if the patient has a history of alcohol abuse or malnutrition.

Rare Diagnoses

• Ocular Myopathy (e.g., Kearns-Sayre Syndrome): This is a mitochondrial myopathy that can present with chronic progressive external ophthalmoplegia, pigmentary retinopathy, and heart block. The absence of pigmentary retinopathy and heart block makes this less likely, but it remains a consideration in the differential diagnosis.
• Congenital Fibrosis of the Extraocular Muscles: This is a rare condition that could present with limited eye movements. However, it typically has an earlier onset and is often associated with other systemic features, making it less likely in this case.