The commonest cause of unilateral internuclear ophthalmoplegia is:
**Core Concept**
Unilateral internuclear ophthalmoplegia (INO) is a type of eye movement disorder characterized by impaired adduction of the affected eye, often accompanied by nystagmus of the abducting eye. This condition is typically caused by a lesion in the medial longitudinal fasciculus (MLF), which is a critical pathway for coordinating eye movements.
**Why the Correct Answer is Right**
The commonest cause of unilateral INO is a small infarct in the medial longitudinal fasciculus (MLF) due to atherosclerotic disease or hypertension. This type of lesion usually occurs in the pons or midbrain. The MLF is a bundle of nerve fibers that carries signals from the abducens nucleus (cranial nerve VI) to the oculomotor nucleus (cranial nerve III), facilitating coordinated eye movements.
* Lesions in the MLF disrupt this communication, leading to impaired adduction of the affected eye and nystagmus of the abducting eye.
* The involvement of the MLF is specific to INO, distinguishing it from other types of eye movement disorders.
**Why Each Wrong Option is Incorrect**
* **Option A:** Multiple sclerosis is a common cause of INO, but it is not the most common cause.
* **Option B:** Stroke is a possible cause of INO, but it is not the most common cause.
* **Option C:** Trauma is a rare cause of INO.
**Clinical Pearl / High-Yield Fact**
The Fisher sign, also known as the "one-and-a-half syndrome," is a characteristic feature of unilateral INO. It is characterized by impaired adduction of the affected eye and nystagmus of the abducting eye, with preserved abduction of the affected eye.
**Correct Answer: C. Trauma is a rare cause of INO.**