Cranial Nerve Function, Testing and Disease Symptoms

The Cranial Nerves

The brain is the central processing point for all activity in the body. It is able to monitor and respond to changes in the body and control every organ either directly or indirectly. Nerves are like the electrical wiring that carry signals to and from the brain. Most of these nerves emerge pass through the spinal cord and are known as the spinal nerves. However, there are 12 pairs of nerves that emerge directly from the brain and are therefore known as cranial nerves. Some of these nerves are sensory or motor only while others are mixed containing a combination of sensory and motor fibers.

Names of the Cranial Nerves

Each pair of cranial nerves is numbered from one to twelve Roman numerals) and designated as CN.

• CN I – Olfactory nerve
• CN II – Optic nerve
• CN III – Oculomotor nerve
• CN IV – Trochlear nerve
• CN V – Trigeminal nerve
• CN VI – Abducens nerve
• CN VII – Facial nerve
• CN VIII – Vestibulocohclear nerve
• CN IX – Glossopharyngeal nerve
• CN X – Vagus nerve
• CN XI – Accessory nerve
• CN XII – Hypogloassal nerve

Functions of Cranial Nerves

In order to understand that nature of the symptoms in cranial nerve damage or disease, it is important to first know its functions.

• Olfactory nerve
  – sense of smell (olfaction)

• Optic nerve
  – sense of sight (vision)

• Oculomotor nerve
  – eyelid movements, most eyeball movements, constricts pupils and changes the shape of lens (accommodation for visual acuity).

• Trochlear nerve
  – outer and downward (inferolateral) eyeball movement.

• Trigeminal nerve has three branches :
  Opthalmic branch
  – sensation on the skin of the scalp, forehead, eyelids, cornea, nose, nasal cavity and paranasal sinuses.
  Maxillary branch
  – sensation on the skin of the cheeks, maxillary sinus, upper lip, maxillary teeth (upper jaw) and palate (roof of the mouth).
  Mandibular branch
  – sensation on the skin of the lower jaw, lower lip, mandibular teeth (lower jaw), temporomandibular joint (TMJ) and anterior two-thirds of the tongue.
  – muscles of mastication (chewing).

• Abducens nerve
  – moves eyeball to the outer side (laterally).

• Facial nerve
  – muscles that control facial expressions, scalp and stapedius muscle of middle ear.
  – taste from anterior two-thirds of the tongue and palate.
  – secretion of tears and saliva (except from parotid gland).

• Vestibulocochlear nerve
  – sensation of hearing.
  – balance and equilibrium.

• Glossopharyngeal nerve
  – muscles that assist with swallowing.
  – secretion of saliva from parotid gland.
  – taste sensation from the posterior one-third of the tongue.
  – sensation from external ear.
  – feedback on carotid blood pressure and blood gas concentration.

• Vagus nerve
  – feedback on aortic blood pressure.
  – muscles of the palate, throat, voice box and two-thirds of the esophagus.
  – muscles (involuntary) of the heart, trachea and bronchi, esophagus, stomach and intestines.
  – sense of taste from the palate and epiglottis.

• Accessory nerve
  – muscles for head and shoulder movements.

• Hypoglossal nerve
  – muscles of the tongue – swallowing and articulation (speech).

Video on Cranial Nerve Testing

Symptoms of Cranial Nerve Problems

Olfactory nerve

Problems with sense of smell :

• Anosmia – complete loss of sense of smell.
• Hyposmia – partial loss of smell.
• Parosmia – unusual odors.
• Cacosmia – foul odors.

Optic nerve

Problems with vision :

• Diplopia (monocular) – double vision with one eye.
• Blurred vision
• Bitemporal hemianopia – loss of of the outer left and right visual field (side or peripheral vision).
• Scotomata – disturbances in one part of the field of vision.

Oculomotor Nerve

Problems with vision, eye and eyelid movement :

• Ptosis – drooping eyelid.
• Nystagmus – repetitive oscillating (to-and-fro) eye movements.
• Pupillary light reflex abnormalities with size and symmetry.
• Eyeball deviation downward and to the outer side.

Trochlear Nerve

Problems with eyeball movement :

• Nystagmus
• Diplopia (vertical) – double vision with one image on top of another particularly when looking downward.

Trigeminal Nerve

Problems with skin sensation on the face, eye, lips, palate and jaw movements :

• Numbness
• Jaw movements abnormal with jaw deviating to one side on opening the mouth.
• Corneal reflex abnormality – blinking when the cornea is stimulated/irritated.

Abducens Nerve

Problems with eyeball movement :

• Nystagmus
• Eyeball deviation to the inside or cannot turn to the outside.

Facial Nerve

Problems with facial expression, loss of taste, salivation and tearing :

• Facial palsy – drooping or sagging of one side of the face, difficulty smiling, keeping mouth closed or closing eyelid.
• Ageusia – loss of taste.

Vestibulocochlear Nerve

Problems with hearing, balance and equilibrium :

• Deafness – partial or complete loss of hearing.
• Tinnitus – ringing in the ears.
• Nystagmus
• Vertigo – sensation of spinning when stationary.
• Poor balance – may have a history of falls.

Glossopharyngeal Nerve

• Ageusia – loss of taste at back of tongue.

Following symptoms overlap with the vagus nerve :

• Gag reflex impaired.
• Dysphagia – difficulty swallowing.
• Dysarthria – slurred speech.

Vagus Nerve

• Dysphonia – hoarse or whispering voice.
• Palatal elevation – uvula deviates to one side.

Following symptoms overlap with the glossopharyngeal nerve :

• Gag reflex impaired.
• Dysphagia – difficulty swallowing.
• Dysarthria – slurred speech.

Accessory Nerve

Problems with head and shoulder movements.

• Difficulty turning head to the side against force.
• Difficulty raising shoulders against downward force.
• Muscle wasting of sternocleidomastoid muscles and trapezius muscles.

Hypoglossal Nerve

Problems with tongue movement and slightly with articulation :

• Tongue deviation to one side when protruded.
• Tongue twitching when protruded.
• Dysarthria – difficulty saying “la” or “ta” rather than slurred speech as is seen with glossopharyngeal and vagus nerve problems.