Cranial nerve, visual and hearing dysfunction in disorders of the CNS

Author(s): Blessen C. Eapen, MD, Thomas Shaw, Jeremy Gallant, MD, Carlos A. Jaramillo, MD, PHD

Originally published:09/20/2014

Last updated:09/20/2014



There are 12 pairs of cranial nerves emerging from the brain and radiating from its surface.1These cranial nerves have one or more types of functional components, which provide motor and sensory innervation to head, neck, glands, vasculature and viscera.1,2Cranial nerve (CN) injuries are a relatively common complication of traumatic brain injury (TBI).3Therefore, knowing the anatomy and function of each individual cranial nerve is essential for accurate diagnosis and effective treatment of injuries.


Cranial nerves are injured before, during or after their passage through the skull as a result of compression from increased intracranial pressure, traction or transection, ischemic event from an infarct, or vascular occlusion.4CN injury can be a result of an acceleration-deceleration injury, shearing injury, skull fracture, intracranial hemorrhage, aneurysm, intracranial mass lesion, brain stem herniation, or meningitis.4,5,6CN injury can also be a result of motor neuron disorders such as Amyotrophic Lateral Sclerosis or demyelinating diseases such as Multiple Sclerosis.

Epidemiology including risk factors and primary prevention

The exact incidence of cranial nerve injuries is unknown. The following is the reported order of frequency after cranial nerve injury: Olfactory nerve (CN I), facial (CN VII), vestibulocochlear nerve (CN VIII), optic nerve (CN II) and oculomotor nerves (CN III).3,5The trigeminal (CN V) and lower cranial nerves (CN IX-XII) are rarely injured. CN I, CN II, CN III, trochlear nerve (CN IV), CN V (first two branches), CN VII and CN VIII are at increased risk for injury from skull fracture secondary to their position next the cranium.1


Cranial patho-antomy and clinical features:4,7

  1. CN I: Shearing of olfactory nerve filaments, fracture of the cribiform plate or a frontal lobe lesion causing compression of the olfactory bulb/tract can result in anosmia or altered sense of smell and taste.
  2. CN II: Fracture of the sphenoid bone or compression of the optic nerve can result in unilateral blindness. Optic chiasm insults can lead to bitemporal hemianopsia.
  3. CN III: Basilar skull fracture, uncal herniation or compression can result in ptosis, inferolateral displacement of the ipsilateral eye and mydriasis.
  4. CN IV: Fracture of the sphenoid wing, can result in injury which presents as extorsion of the ipsilateral eye with diplopia with attempted downward gaze, but improved diplopia with head tilted to contralateral side.
  5. CN V: Lesion in cavernous sinus can lead to asymmetry of jaw on opening or weakness with mastication.
  6. CN VI: Injury caused to the cavernous sinus or fractures of the skull base can result in an extraocular palsy resulting by medial deviation of ipsilateral eye and diplopia that improves when the contralateral eye is abducted.
  7. CN VII: Temporal bone fractures can cause immediate facial nerve injury resulting in paralysis of facial expression. A peripheral lesion involves muscles of both the upper and lower face and can involve loss of taste from the anterior 2/3 of the tongue. A central lesion results in sparing of upper and frontal orbicularis occuli due to crossed innervation. Spared ability to raise eyebrows, wrinkle forehead helps differentiate a peripheral palsy from a central process.
  8. CN VIII: Temporal bone fractures can result in hearing loss, vertigo and nystagmus immediately after the injury.
  9. CN IX: Injury to the posterior fossa or jugular foramen can cause numbness of ipsilateral pharynx and dysphagia with absent gag reflex.
  10. CN X: Injury can lead to ipsilateral drooping of soft-palate with uvular deviation towards the affected side and may present with dysphagia, dysphonia and decreased gag reflex.
  11. CN XI: Damage can result in ipsilateral paralysis of sternocleidomastoid and trapezius.
  12. CN XII: Injury can lead to tongue protrusion away from side of lesion.

Disease progression including natural history, disease phases or stages, disease trajectory (clinical features and presentation over time)

Most cranial nerve lesions are present at initial presentation in TBI. However, peripheral CN VII lesions may present as immediate paralysis (usually due to a complete laceration of the nerve via skull fracture), or delayed, where a bone fragment compresses the nerve. Incomplete cranial nerve lesions often improve with time, while complete lesions are much less likely to recover.

Specific secondary or associated conditions and complications

Cranial nerve III, IV and VI lesions may result in secondary vision difficulties. CN VII lesions may result in mobility impairments. A cognitively impaired patient may not describe loss of the ability to smell in a CN I lesion, but may develop anorexia.



Pertinent historical information should be obtained by asking questions regarding sense of smell, visual acuity, visual fields, eyelid opening/closing, double vision, dry/painful eyes, facial sensation, chewing/mastication, taste, hearing, vertigo/balance, swallowing, salivation, gag reflex, speaking/vocal quality, and neck/shoulder muscle function. Inquiring about appetite and weight changes may raise suspicion for cranial nerve lesions that a cognitively impaired patient may otherwise have difficulty recognizing. Additionally, the patient should be queried regarding history of trauma, bleeding/drainage from the ears, fractures, and bruising patterns.8

Physical examination

  1. CN I: Familiar, non-noxious smells
  2. CN II: Visual fields/acuity, pupillary reactivity, opthalmoscopic examination
  3. CN III: Extraocular muscle function, convergence, saccades/smooth pursuit, pupillary reactivity, eyelid elevation
  4. CN IV/CN VI: Extraocular muscle function, convergence, saccades/smooth pursuit
  5. CN V: Facial sensation, mastication, corneal reflex
  6. CN VII: Taste, facial expression, external ear sensation, lacrimation/salivation, stapedius
  7. CN VIII: Tympanic membrane, hearing, postural responses, nystagmus
  8. CN X: Gag reflex, palate elevation
  9. CN XI: Ipsilateral shoulder shrug, resisted head rotation
  10. CN XII: Tongue deviation with protrusion, fasiculations, atrophy

See Pathophysiology section for additional information regarding clinical presentation of cranial nerve dysfunction.

Functional assessment

Functional assessment must be comprised of the patient’s visual and auditory awareness of environment as it pertains to mobility, transfers, activities of daily living, and safety. Balance testing must also be emphasized. Additionally, one should examine swallowing function, in terms of safety and efficacy for nutritional purposes. Functional communication is important to consider, as well.

Laboratory studies

A suspected infectious etiology may warrant investigation with a complete blood count, cultures, erythrocyte sedimentation rate, serologies, cerebrospinal fluid (CSF) evaluation, or polymerase chain reaction evaluation of DNA. Analysis for autoimmune disease may be indicated, including detection of circulating antibodies in serum or CSF. Additionally, one may employ laboratory investigation for endocrine and electrolytic disorders.


Utilization of imaging studies should be employed as directed by the history and physical examination. Computed Tomography (CT) has great utility in the acute setting after significant trauma, and can assist in assessment for temporal fractures, which are associated with Facial and Vestibulocochlear nerve injuries. Furthermore, ethmoid tomographymay be warranted when Battle’s Sign (mastoid ecchymosis), raccoon eyes, or other signs of basilar skull fracture are present.If suspicion is raised for olfactory dysfunction, magnetic resonance imaging (MRI) often provides visual confirmation of abnormalities in the olfactory bulbs and tracts, as well as in the inferior frontal lobes.9MRI is also warranted in the setting of suspected mass-effect-induced cranial nerve dysfunction, such as in malignancy, and in discerning multiple sclerosis.

Supplemental assessment tools

Evoked potentials, such as olfactory event-related potentials, visual evoked responses, and brainstem auditory evoked potentials, provide further diagnostic information in the setting of cranial nerve associated dysfunction in smell, vision, and hearing, as well is in lower neurologically functioning individuals.8,10

Electroencephalograophy should be employed if seizures are implicated.8Focal cranial nerve neuropathies are quite amenable to electrodiagnostic testing with electromyography and nerve conduction studies.11

Early predictions of outcomes

A patient may expect moderate recovery initially for 4-6 weeks with edema and hematoma resolution. Olfactory system dysfunction of central etiology is resistant to treatment, and sense of smell is regained by approximately 1/3 of patients within two years. Significant impending neurological compromise may be heralded acutely by pupillary fixation, and axonal injury to the optic nerve has a poor prognosis for recovery, given inability of those axons to regenerate. Vision following oculomotor nerve injury may begin to improve within two months of injury, and abnormal vision following isolated abducens palsy may resolve spontaneously. With regard to vertigo induced by CN VIII dysfunction, one can expect improvement in symptoms with time.8


Environmental barriers to function can be vastly amplified by cranial nerve dysfunction. Visual, vestibular, or hearing dysfunction necessitates investigation into the structural environment of the home and workplace, with regard to safety and function. Furthermore, reduced upper extremity muscle strength may impair performance in the workplace or function in activities of daily living.

Social role and social support system

Community integration and employment may be jeopardized by abnormal hearing, vision or communication, and should raise concern regarding the patient’s goals and demands for livelihood and civic involvement. Strain may intensify on the patient’s family with regard to transportation, communication, nutrition if swallowing dysfunction exists, and with basic activities of daily living.

Professional Issues

Particular care and sensitivity must be undertaken to maximize quality of life following cranial nerve dysfunction, given propensity for significant functional impairment and newfound disability resulting from visual, auditory, vestibular, alimentary, or communicative deficits. Concomitantly, safety must be prioritized concerning impacted functional mobility, self-care, and nutrition, with paramount attention to prevention of further disability.


Available or current treatment guidelines

There are few evidence-based guidelines regarding treatment of cranial nerve injuries in TBI. However, thorough examination of cranial nerves is essential and re-examination as the patient recovers may reveal previously undiagnosed deficits. Proper swallowing evaluation prior to allowing oral intake is essential.4Avoiding neurotoxic substances, maintaining normal electrolyte and blood glucose levels is encouraged to promote neural recovery. In cases of a delayed CN VII palsy, treatment with high dose steroids with a slow taper minimizes the risk of a rebound paralysis.

At different disease stages

There are no established effective treatments for traumatic olfactory dysfunction, but anosmic patients showed slight recovery of olfaction with intranasal steroids.12When olfaction or gustation are affected, one should minimize the use of medications that cause mucosal drying or xerostomia.4Anosmia has been associated with depression and reduced quality of life.12,13,14Education about the impairment should be considered (see below).

Visual deficits may result from optic nerve or extraocular muscle dysfunction. Depending on etiology, treatment may include patching, prisms, or ultimately neurolysis or surgical intervention. Lubrication and patching can protect the cornea when sensation and eye closure is impaired. Tarsorrhaphy may be performed if conservative measures are ineffective. Visual training may improve visual spatial disorders, balance, dizziness and posture.15Exercises for diplopia have demonstrated marginal results, and surgery is rarely indicated.16

Treatment for vestibular function involves medicine or therapies. Medicines should be used judiciously, for a short period and on an “as needed” basis, if at all.17Vestibular therapies aim to overstimulate the vestibular response, which may cause nausea, and may progress as patient cooperation improves.4Rehabilitation reduces vestibular symptoms and improves function up to 85% of the time.18Amplification devices and visual cues can compensate for hearing loss.4Various modalities have been used to address tinnitus with less than consistent results (see below).

Continued assessment of swallowing can guide dysphagia treatment, including exercises, appropriate diet (texture/viscosity), positioning, assistive maneuvers and supervision to reduce risk of aspiration.19

Neuropathic pain may be treated with medications used for peripheral neuropathic pain, including anti-convulsants and tricyclic antidepressants.20

Coordination of care

The interdisciplinary approach to treat TBI is well established. Specifically for cranial nerve lesions, vision specialists, audiologists and speech therapists can assist in evaluation and treatment. Occasional surgical referral may be indicated.

Patient & family education

Patients and families should be advised about safety considerations when patients have olfactory impairment. Inability to detect smoke, gas, harmful chemicals or spoiled foods may cause harm. Reminders and fixed schedules can improve hygiene, child or pet care when olfaction is impaired. Dietary changes may be required to optimize nutrition in patients with dysgeusia, including use of spices and textures to improve the appeal of foods.

Explaining the rationale for specific diets to families of patients with dysphagia is important, as this is often a source of confusion. Families may feel certain restrictions are preventing patient progress, and families often provide or prepare patient’s food.19

Emerging/unique Interventions

Treatment outcomes of cranial nerve injuries are most commonly assessed on a functional basis. Most cranial nerve deficits have been found to improve over the course of a few months to a year. Persistent or unexpected deficits may be evaluated with electrodiagnostic, vestibular and neuro-ophthalmologic studies described above.

Translation into practice: practice “pearls”/performance improvement in practice (PIPs)/changes in clinical practice behaviors and skills

Thorough examination of cranial nerves is essential on initial evaluation and throughout the course of TBI rehabilitation. Visual, hearing, vestibular and swallowing difficulties may be related to cortical or cranial nerve deficits and impede progress in the rehabilitation setting. Recognizing and addressing these deficits may expedite functional gains in a broad range of modalities. Providing reassurance and compensatory strategies may reduce the psychological effects of the deficits.


Cutting edge concepts and practice

Recent studies using intravenous erythropoietin for the treatment of traumatic optic neuropathy shows benefit.21 Currently in Phase 2 clinic trial is the Study of Visual Recovery After Erythropoietin (EPO) Injection, Steroid Injection or Observation in Patients With Traumatic Optic Neuropathy (TON).

Also, in Phase 2 clinical trial is the use of transcranial alternating current stimulation to improve visual field size in patients with optic nerve damage.


Gaps in the evidence-based knowledge

The exact incidence of cranial nerve injuries is unknown. On physical exam of patients with brain injury, patients with altered cognition or consciousness it is difficult to elicit a full and accurate CN exam. Currently there are numerous treatment options for CN injures but no standardized treatment guidelines exist.


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Author Disclosures


Author indicated he/she has no relevant financial relationships to disclose.


Author indicated he/she has no relevant financial relationships to disclose.


Author indicated he/she has no relevant financial relationships to disclose.


Author indicated he/she has no relevant financial relationships to disclose.

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