The most commonly involved cranial nerve in diabetes is?

PGIMS Rohtak NO - 2017

Medical Surgical Nursing

Easy

Appeared in: PGIMS Rohtak NO - 2017

Explanation

The oculomotor nerve (CN III) is the most frequently affected cranial nerve in diabetes mellitus.
This occurs due to ischemic mononeuropathy, where high blood sugar damages the small blood vessels (vasa nervorum) supplying the nerve, leading to a lack of oxygen.
Classic signs include sudden onset of diplopia (double vision), ptosis (drooping eyelid), and an eye that is deviated 'down and out'.
A key diagnostic feature is 'pupil sparing,' where the pupil remains reactive to light. This is because the parasympathetic fibers controlling the pupil are on the nerve's periphery and are often spared from the central ischemic damage.

Option B: The optic nerve (CN II) is affected by diabetic retinopathy, which is a chronic microvascular disease of the retina, not an acute ischemic mononeuropathy that typically presents as a palsy.
Option C: The trigeminal nerve (CN V) is much less commonly affected by diabetic mononeuropathy compared to the nerves that control eye movements (CN III, VI, and IV).
Option D: While the abducens nerve (CN VI) is commonly affected in diabetes, it is the second most common, not the most common. The oculomotor nerve (CN III) is affected more frequently.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Diabetic Cranial Mononeuropathy as background academic context rather than a clinical decision trigger.
A nurse assessing a patient with a sudden onset of ptosis and diplopia must immediately check the pupillary light reflex. This is a critical step in triage.
A normal, reactive pupil in this context points towards an ischemic cause like diabetes, which is less urgent. Management focuses on glycemic control and observation.
A dilated, fixed (non-reactive) pupil is a neurologic emergency. It suggests compression of the nerve, most dangerously by a posterior communicating artery aneurysm, and requires immediate escalation for neuroimaging and neurosurgical consultation.

How to Approach the Question

First, identify the key elements of the question: the disease ('diabetes') and the specific complication being asked about ('most commonly involved cranial nerve').
This is a factual recall question that tests knowledge of common neurological complications of diabetes.
Recall that diabetes causes microvascular disease, which can lead to ischemic injury (lack of blood flow) to individual nerves (mononeuropathy).
Remember that the cranial nerves controlling eye movements (oculomotor nerves) are particularly vulnerable.
Access your knowledge of the relative frequency of these palsies. The order of frequency is CN III (most common), followed by CN VI, and then CN IV.
Based on this, select the option corresponding to the oculomotor nerve (CN III).

Concept Tested & Keywords

Qdy9Bl1TBCYIsWOSBhkl74

E6 Medicine Harrison 22e Part 2 p. 1462-1464