Differential Diagnosis for Sudden Onset Unilateral Diplopia
Single Most Likely Diagnosis
- Fourth Cranial Nerve (Trochlear Nerve) Palsy: This is the most likely diagnosis given the sudden onset of unilateral diplopia on right lateral gaze. The trochlear nerve controls the superior oblique muscle, which is responsible for rotating the eye downward and inward. A palsy of this nerve can cause diplopia, particularly when looking laterally. The condition can be congenital or acquired, with the latter often resulting from trauma, ischemia, or other compressive lesions. In an 80-year-old patient, an acquired cause, such as a microvascular ischemic event, is plausible.
Other Likely Diagnoses
- Sixth Cranial Nerve (Abducens Nerve) Palsy: This condition affects the lateral rectus muscle, leading to difficulty abducting the eye (moving it outward). While the primary symptom is esotropia (inward turning of the eye) with diplopia on lateral gaze, it's less specific to the direction of gaze mentioned but still a consideration.
- Third Cranial Nerve (Oculomotor Nerve) Palsy: This would typically present with ptosis (drooping eyelid), mydriasis (pupil dilation), and an inability to move the eye in most directions except laterally. However, a partial palsy could present with less severe symptoms, making it a consideration.
- Orbital Myositis or Other Inflammatory Conditions: Inflammation of the extraocular muscles can cause diplopia and pain, especially with eye movements. This could be a consideration, although the lack of pain or other inflammatory signs makes it less likely.
Do Not Miss Diagnoses
- Giant Cell Arteritis: Although less common, this condition can cause sudden onset of diplopia due to involvement of the cranial nerves. Given the patient's age, it's crucial to consider this diagnosis, as giant cell arteritis can lead to serious complications, including vision loss and stroke, if not promptly treated.
- Diabetic Cranial Neuropathy: Diabetes can cause neuropathies affecting the cranial nerves, including those controlling eye movements. This condition could present with sudden diplopia and is important to consider, especially if the patient has a history of diabetes.
- Cavernous Sinus or Orbital Apex Syndrome: These are rare but serious conditions that can cause diplopia due to involvement of multiple cranial nerves. They are often associated with other symptoms such as ptosis, facial numbness, or decreased visual acuity.
Rare Diagnoses
- Myasthenia Gravis: An autoimmune disease that can cause fluctuating muscle weakness, including the muscles controlling eye movements. While it can present with diplopia, it's less likely given the sudden onset and unilateral nature of the symptoms.
- Thyroid Eye Disease: Can cause restrictive strabismus leading to diplopia, but typically presents with other signs such as exophthalmos, eyelid retraction, and conjunctival injection.
- Skew Deviation: A vertical misalignment of the eyes due to a lesion in the brainstem or cerebellum, which could present with diplopia. However, this would typically be associated with other neurological signs.