8.3 Neurologic Examination: Cranial Nerve and CNS Testing

8.3 Neurologic Examination: Cranial Nerve and CNS Testing

Neurologic examination questions on NBCE Part II test whether you can localize a lesion — peripheral nerve, nerve root, spinal cord, brainstem, or cerebral hemisphere — from a handful of objective findings. Cranial nerve testing and central nervous system screening maneuvers (Babinski, Hoffmann, clonus, pronator drift, Romberg) appear frequently because each maps to a specific anatomic pathway. The highest-yield skill is distinguishing upper motor neuron (UMN) from lower motor neuron (LMN) patterns and recognizing when a cranial nerve finding implies a central versus peripheral lesion. These distinctions drive referral decisions and are among the most heavily tested traps on Part II.

Cranial Nerve Overview for Part II

High-Yield Cranial Nerve Tests

Optic nerve (CN II)

Test visual acuity, confrontation visual fields, color vision (red desaturation), and the swinging flashlight test. An afferent pupillary defect (RAPD) — pupil dilates when light swings to the affected eye — indicates optic nerve or severe retinal disease. Papilledema on fundoscopy signals raised intracranial pressure and is a referral emergency.

Oculomotor, trochlear, abducens (CN III, IV, VI)

Have the patient follow your finger in an H pattern. CN III palsy: ptosis, eye down and out, possible dilated pupil if parasympathetic fibers are compressed (posterior communicating artery aneurysm until proven otherwise). CN IV palsy: vertical diplopia worse going downstairs (superior oblique). CN VI palsy: horizontal diplopia, cannot abduct eye (abducens nucleus/fascicle — false localizing sign in raised ICP).

Trigeminal (CN V)

Test light touch and pinprick in ophthalmic (V1), maxillary (V2), and mandibular (V3) distributions. Weakness of jaw clench or temporalis wasting suggests motor V involvement (motor nucleus in pons).

Facial (CN VII) — UMN vs LMN (critical Part II trap)

• UMN (central) facial palsy: Contralateral lower face weakness; forehead sparing because bilateral cortical input to the upper face remains intact. Suggests stroke or cortical lesion.
• LMN (peripheral) Bell palsy: Entire ipsilateral face weak, including forehead (cannot raise eyebrow, cannot close eye). Suggests facial nerve pathology at or distal to the stylomastoid foramen.

Vestibulocochlear (CN VIII)

Weber test: Tuning fork on forehead — lateralization. Rinne test: Compare air conduction (AC) to bone conduction (BC) at the mastoid. Conductive loss: BC > AC on affected side, Weber lateralizes to affected ear. Sensorineural loss: AC > BC but reduced, Weber lateralizes to unaffected ear.

Glossopharyngeal and vagus (CN IX, X)

"Say ah" — palate should elevate symmetrically. Uvula deviates away from the lesion (LMN weakness on the affected side). Hoarseness and dysphagia accompany vagal lesions.

Accessory (CN XI)

Test sternocleidomastoid (turn head against resistance) and trapezius (shoulder shrug). Weakness is ipsilateral to the lesion.

Hypoglossal (CN XII)

Tongue protrusion: deviates toward the lesion (LMN — weak genioglossus pushes tongue toward the weak side).

Meningeal Irritation Signs

• Nuchal rigidity: Passive neck flexion produces resistance or pain.
• Kernig sign: Supine, hip flexed 90 degrees; passive knee extension causes hamstring pain/resistance.
• Brudzinski sign: Passive neck flexion causes involuntary hip/knee flexion.

Positive meningeal signs suggest meningitis or subarachnoid hemorrhage — not musculoskeletal neck strain. Part II may embed these in a headache vignette.

Upper Motor Neuron vs Lower Motor Neuron Signs

Babinski sign

Stroke the lateral sole from heel to ball and across the metatarsals. Upgoing great toe with fanning of other toes is positive in adults and indicates corticospinal tract (UMN) dysfunction. Normal flexor response in infants under ~12–18 months.

Hoffmann sign

Flick the middle fingernail downward; reflex flexion of thumb and index finger suggests cervical myelopathy or UMN pathology in the upper extremity.

Clonus

Sudden dorsiflexion of the foot (or wrist) produces rhythmic oscillations. Sustained clonus (more than 3–4 beats) indicates UMN disease.

Pronator drift

Arms extended, eyes closed; the affected arm pronates and drifts downward — subtle UMN weakness of the supinator and shoulder girdle stabilizers.

Romberg Test and Ataxia

The patient stands with feet together, eyes open then closed. Positive Romberg (instability only when eyes close) indicates proprioceptive loss (dorsal column — B12 deficiency, tabes dorsalis, peripheral neuropathy). Cerebellar ataxia is present with eyes open and closed; the patient may not fall but shows wide-based gait and dysmetria on finger-to-nose testing.

Part II Traps

Do not call a peripheral VII palsy a stroke — forehead involvement localizes peripheral. Do not ignore a dilated pupil with CN III palsy — compressive aneurysm is the emergency until imaging excludes it. Confusing Romberg-positive sensory ataxia with cerebellar ataxia is a recurring error. Remember that Hoffmann and Babinski are UMN signs; their presence with neck pain suggests myelopathy, not simple radiculopathy. Finally, meningeal signs override musculoskeletal neck treatment — fever plus nuchal rigidity demands medical referral.