CN

Cranial Nerve I: Olfactory Nerve

• Function: Special sensory (smell)

• Innervation: Olfactory epithelium (nasal cavity)

• Clinical Assessment:

  • Ask the patient to identify familiar odors (e.g., coffee, peppermint) with each nostril separately.

• Potential Dysfunction:

  • Anosmia (loss of smell) – Can result from trauma, viral infections, neurodegenerative diseases (e.g., Parkinson’s, Alzheimer’s).

Cranial Nerve II: Optic Nerve

• Function: Special sensory (vision)

• Innervation: Retina of the eye

• Clinical Assessment:

  • Visual acuity test (Snellen chart)

  • Visual field testing (confrontation test)

  • Fundoscopic exam (examine retina and optic disc)

• Potential Dysfunction:

  • Blindness or visual field defects – Optic neuritis (multiple sclerosis), glaucoma, stroke, optic chiasm tumors (e.g., pituitary adenoma).

Cranial Nerve III: Oculomotor Nerve

• Function:

  • Somatic motor – Eye movement (superior, inferior, and medial rectus; inferior oblique; levator palpebrae superioris).

  • Autonomic (parasympathetic) motor – Pupil constriction (sphincter pupillae) and lens accommodation (ciliary muscle).

• Clinical Assessment:

  • Pupil reaction to light (direct and consensual response).

  • Accommodation test (near response).

  • Extraocular movements (test all directions except lateral and downward diagonal).

• Potential Dysfunction:

  • Ptosis (drooping eyelid), dilated pupil, impaired eye movements (down and out position) – Seen in oculomotor nerve palsy (e.g., diabetes, aneurysm, herniation).

Cranial Nerve IV: Trochlear Nerve

• Function: Somatic motor (superior oblique muscle – depresses and intorts the eye)

• Clinical Assessment:

  • Have the patient look down and in (toward the nose).

• Potential Dysfunction:

  • Vertical diplopia (double vision), head tilt to compensate – Seen in trochlear nerve palsy (e.g., trauma, congenital, stroke).

Cranial Nerve V: Trigeminal Nerve

• Function:

  • General sensory – Face, scalp, cornea, oral/nasal mucosa.

  • Somatic motor – Muscles of mastication (chewing).

• Clinical Assessment:

  • Facial sensation (test ophthalmic, maxillary, mandibular branches).

  • Corneal reflex (blink when touching cornea).

  • Jaw movement (clench teeth and move jaw against resistance).

• Potential Dysfunction:

  • Trigeminal neuralgia (severe facial pain), loss of facial sensation, weak jaw movement.

Cranial Nerve VI: Abducens Nerve

• Function: Somatic motor (lateral rectus muscle – abducts the eye)

• Clinical Assessment:

  • Test lateral eye movement.

• Potential Dysfunction:

  • Diplopia, inability to abduct eye (eye turns inward) – Common in increased intracranial pressure, diabetes, stroke.

Cranial Nerve VII: Facial Nerve

• Function:

  • Somatic motor – Facial expressions.

  • Autonomic (parasympathetic) motor – Lacrimal, submandibular, and sublingual glands.

  • Special sensory – Taste (anterior 2/3 of tongue).

• Clinical Assessment:

  • Ask patient to raise eyebrows, smile, frown, and puff cheeks.

  • Test taste on anterior tongue.

• Potential Dysfunction:

  • Bell’s palsy (facial weakness, loss of taste, dry eyes/mouth, inability to close eye) – Can be due to viral infection (e.g., herpes simplex).

Cranial Nerve VIII: Vestibulocochlear Nerve

• Function:

  • Special sensory (hearing) – Cochlear branch.

  • Special sensory (balance and equilibrium) – Vestibular branch.

• Clinical Assessment:

  • Hearing tests (Rinne and Weber tests).

  • Balance and coordination tests (Romberg test).

• Potential Dysfunction:

  • Hearing loss (sensorineural), vertigo, nystagmus, tinnitus – Common in Ménière’s disease, acoustic neuroma.

Cranial Nerve IX: Glossopharyngeal Nerve

• Function:

  • Special sensory – Taste (posterior 1/3 of tongue).

  • General sensory – Pharynx, tonsils, middle ear.

  • Autonomic motor – Parotid salivary gland.

• Clinical Assessment:

  • Test gag reflex (along with CN X).

• Potential Dysfunction:

  • Dysphagia (difficulty swallowing), loss of taste posterior tongue – Seen in stroke, brainstem lesions.

Cranial Nerve X: Vagus Nerve

• Function:

  • Somatic motor – Pharyngeal and laryngeal muscles (swallowing, voice).

  • Autonomic motor – Heart, lungs, digestive tract.

  • General sensory – External ear, larynx, pharynx.

• Clinical Assessment:

  • Say "ahh" and observe uvula movement (should rise symmetrically).

  • Test gag reflex.

• Potential Dysfunction:

  • Hoarseness, difficulty swallowing, asymmetric uvula deviation (away from lesion) – Seen in stroke, vagus nerve damage.

Cranial Nerve XI: Accessory Nerve

• Function: Somatic motor (sternocleidomastoid and trapezius muscles)

• Clinical Assessment:

  • Shrug shoulders against resistance.

  • Turn head against resistance.

• Potential Dysfunction:

  • Weak shoulder shrug, weak head turning – Seen in trauma, spinal cord injury.

Cranial Nerve XII: Hypoglossal Nerve

• Function: Somatic motor (tongue muscles)

• Clinical Assessment:

  • Stick out tongue and observe for deviation.

• Potential Dysfunction:

  • Tongue deviates toward side of lesion – Seen in stroke, hypoglossal nerve palsy.

Diagnosing Cranial Nerve Involvement from Symptoms

1. Case 1: Loss of Smell After Head Trauma

   • Symptoms: Inability to smell coffee or peppermint, no nasal congestion.

   • Likely Affected Nerve: Cranial Nerve I (Olfactory Nerve)

   • Possible Causes: Head trauma (shearing of olfactory nerve fibers), viral infection, neurodegenerative diseases (e.g., Parkinson’s, Alzheimer’s).

2. Case 2: Sudden Loss of Vision in One Eye

   • Symptoms: Blurry vision, no pupillary response in the affected eye, intact extraocular movements.

   • Likely Affected Nerve: Cranial Nerve II (Optic Nerve)

   • Possible Causes: Optic neuritis (multiple sclerosis), ischemic optic neuropathy, retinal detachment.

3. Case 3: Drooping Eyelid and "Down and Out" Eye

   • Symptoms: Ptosis (drooping eyelid), eye deviated downward and outward, pupil dilation, difficulty with accommodation.

   • Likely Affected Nerve: Cranial Nerve III (Oculomotor Nerve)

   • Possible Causes: Diabetes mellitus, aneurysm (posterior communicating artery), uncal herniation.

4. Case 4: Head Tilt and Vertical Diplopia (Double Vision)

   • Symptoms: Trouble looking down while reading, tilts head to opposite side to compensate.

   • Likely Affected Nerve: Cranial Nerve IV (Trochlear Nerve)

   • Possible Causes: Head trauma, congenital palsy, microvascular ischemia.

5. Case 5: Facial Numbness and Weak Jaw Movement

   • Symptoms: Loss of sensation on the forehead, cheek, and jaw; weak jaw movement; absent corneal reflex.

   • Likely Affected Nerve: Cranial Nerve V (Trigeminal Nerve)

   • Possible Causes: Trigeminal neuralgia, stroke, tumor (acoustic neuroma, meningioma).

6. Case 6: Eye Unable to Move Laterally

   • Symptoms: Diplopia when looking to the side, inability to abduct the affected eye.

   • Likely Affected Nerve: Cranial Nerve VI (Abducens Nerve)

   • Possible Causes: Increased intracranial pressure (hydrocephalus, brainstem tumor), stroke, diabetes.

7. Case 7: Facial Droop and Inability to Close Eye on One Side

   • Symptoms: Half of face paralyzed, inability to close eye, loss of taste on the front of the tongue, drooling.

   • Likely Affected Nerve: Cranial Nerve VII (Facial Nerve)

   • Possible Causes: Bell’s palsy (viral), stroke (if only lower face affected), parotid gland tumor.

8. Case 8: Hearing Loss and Ringing in One Ear

   • Symptoms: Progressive hearing loss in one ear, tinnitus, imbalance, dizziness.

   • Likely Affected Nerve: Cranial Nerve VIII (Vestibulocochlear Nerve)

   • Possible Causes: Acoustic neuroma (vestibular schwannoma), Ménière’s disease, ototoxic drugs.

9. Case 9: Difficulty Swallowing and Absent Gag Reflex

   • Symptoms: Trouble swallowing, choking on liquids, absent gag reflex, hoarse voice.

   • Likely Affected Nerve: Cranial Nerve IX (Glossopharyngeal Nerve) and Cranial Nerve X (Vagus Nerve)

   • Possible Causes: Stroke, brainstem lesion, Guillain-Barré syndrome.

10. Case 10: Hoarseness, Difficulty Swallowing, and Uvula Deviation

• Symptoms: Hoarse voice, difficulty swallowing, uvula deviates away from the affected side.

• Likely Affected Nerve: Cranial Nerve X (Vagus Nerve)

• Possible Causes: Medullary stroke, vagus nerve damage, surgery complications.

11. Case 11: Weak Shoulder Shrug and Difficulty Turning Head

• Symptoms: Weakness in shrugging the shoulder on one side, difficulty turning head to opposite side.

• Likely Affected Nerve: Cranial Nerve XI (Accessory Nerve)

• Possible Causes: Neck trauma, surgery (e.g., radical neck dissection), tumor.

12. Case 12: Tongue Deviation When Sticking It Out

• Symptoms: Tongue deviates to one side when protruded, difficulty with speech and swallowing.

• Likely Affected Nerve: Cranial Nerve XII (Hypoglossal Nerve)

• Possible Causes: Stroke, brainstem tumor, ALS (amyotrophic lateral sclerosis).