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).


  1. 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.


  1. 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.


  1. 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.


  1. 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).


  1. 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.


  1. 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.


  1. 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.


  1. 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.


  1. 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.


  1. 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.


  1. 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).

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