Differential Diagnosis for Sudden Onset Unilateral Diplopia
Single Most Likely Diagnosis
- Fourth Cranial Nerve (Trochlear Nerve) Palsy: This condition is a common cause of sudden onset unilateral diplopia, especially in older adults. The trochlear nerve controls the superior oblique muscle, which is responsible for rotating the eye inward and downward. A palsy of this nerve can lead to difficulty moving the eye in these directions, resulting in diplopia. The fact that the patient has normal visual acuity and intraocular pressure, and an otherwise normal eye examination, supports this diagnosis.
Other Likely Diagnoses
- Third Cranial Nerve (Oculomotor Nerve) Palsy: Although less common than fourth nerve palsy, third nerve palsy can also cause diplopia. This nerve controls several eye muscles, including those responsible for upward and downward gaze. However, third nerve palsy often presents with additional symptoms such as ptosis (drooping eyelid) and pupillary dilation, which are not mentioned in the scenario.
- Sixth Cranial Nerve (Abducens Nerve) Palsy: This condition affects the lateral rectus muscle, leading to difficulty abducting the eye (moving it outward). While it can cause diplopia, it is less common than fourth nerve palsy and often associated with other symptoms like increased intracranial pressure.
- Orbital Myasthenia Gravis: This is an autoimmune disorder that can cause fluctuating weakness of the eye muscles, leading to diplopia. However, it typically presents with other symptoms such as ptosis and variability of symptoms throughout the day.
Do Not Miss Diagnoses
- Giant Cell Arteritis: Although less likely, this condition is critical to diagnose due to its potential to cause blindness if not treated promptly. It can present with diplopia, but usually, there are other symptoms like headache, jaw claudication, and visual loss.
- Diabetic Third Nerve Palsy: This condition can cause sudden onset diplopia and is more common in diabetic patients. It is essential to consider this diagnosis, especially if the patient has a history of diabetes.
- Aneurysm or Stroke: These are life-threatening conditions that can cause sudden onset diplopia due to compression or damage to the cranial nerves. Although unlikely given the normal eye examination, they must be considered due to their severe consequences.
Rare Diagnoses
- Thyroid Eye Disease: This condition can cause diplopia due to inflammation and swelling of the tissues around the eye, including the muscles. However, it typically presents with other symptoms like exophthalmos (bulging eyes) and eyelid retraction.
- Orbital Tumors: Tumors in the orbit can compress or invade the cranial nerves, leading to diplopia. These are rare and usually present with other symptoms like proptosis (bulging eye) or visual loss.
- Wernicke's Encephalopathy: This is a rare condition caused by thiamine deficiency, often seen in alcoholics. It can present with ophthalmoplegia (weakness or paralysis of the eye muscles), leading to diplopia, but is usually accompanied by other neurological symptoms like ataxia and confusion.