Author(s): Blessen C. Eapen, MD, Michelle Q. Jin, MD
Originally published: September 20, 2014 Last updated: November 1, 2023
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Disease/Disorder

Definition

There are 12 pairs of cranial nerves emerging from the brain and radiating from its surface.1 The 12 pairs of cranial nerves provide motor and sensory innervation to the head, neck, glands, vasculature and viscera1,2  Cranial nerve (CN) injuries are a common complication after traumatic brain injury (TBI).3 Therefore,understanding the anatomy and function of each individual cranial nerve is essential for accurate diagnosis and effective treatment of these injuries.

Etiology

There are various causes of CN injury which may include:

  • Mechanical – e.g., Skull fracture, traumatic brain injury, intracranial mass lesion, brain stem herniation, compression from increased intracranial pressure, traction, or transection
  • Vascular – e.g., Ischemic event from an infarct, vascular occlusion, aneurysm, intracranial hemorrhage4
  • Motor neuron disorders – e.g., Amyotrophic Lateral Sclerosis, Myasthenia gravis
  • Demyelinating diseases – e.g., Multiple Sclerosis
  • Other – Acceleration-deceleration injury, shearing injury, or meningitis4–6
  • Rheumatological conditions
  • Metabolic (e.g., diabetes mellitus)
  • Infectious (e.g., syphilis, HIV, Lyme disease, possible SARS-CoV-2 infection involvement in olfactory nerve injury7
  • Pregnancy  – 3x risk compared to the general population 8 

Epidemiology including risk factors and primary prevention

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.4 CN injury can be a result of an acceleration-deceleration injury, shearing forces, skull fracture, intracranial hemorrhage, aneurysm, intracranial mass lesion, brain stem herniation, or meningitis.4–6  

While the exact incidence of cranial nerve injuries is unknown the following is the reported order of frequency after cranial nerve injury.5

  • Olfactory nerve (CN I)
  • Facial (CN VII)
  • Vestibulocochlear nerve (CN VIII)
  • Optic nerve (CN II) and oculomotor nerves (CN III)3,5
  • Trigeminal (CN V) and lower cranial nerves (CN IX-XII) are rarely injured5

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 cranium1

Patho-anatomy/physiology4,9

Cranial patho-anatomy and clinical features4,9

Cranial-Nerve-Visual-and-Hearing-Dysfunction-in-Disorders-of-the-CNS-Table-1Download

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.

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

Cranial nerve injuries may present immediately or in a delayed fashion, depending on the cause of injury. For example, complete laceration of a nerve via skull fracture may cause immediate paralysis; gradual compression may cause delayed paralysis.

In addition, patients may present with multiple cranial neuropathies, and there is a wide differential for pathologies that manifest as cranial nerve dysfunction.12The natural history of cranial nerve injury depends on the underlying cause, which may include infectious, inflammatory, neoplastic, vascular, toxic, and trauma. One such example is polycranial neuritis, an oculopharyngeal subtype of GBS, which presents as ocular and pharyngeal weakness without limb weakness or ataxia.13

Incomplete cranial nerve lesions often improve with time, while complete lesions are much less likely to recover. The olfactory nerve, in particular, has a capacity for regeneration and recovery, especially in with olfactory training with occupational therapy.14

Essentials of Assessment

Review of systems

  • Review 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.
  • Patient should be queried regarding history of trauma, recent craniofacial procedures, bleeding/drainage from the ears, fractures, and bruising patterns.15

Physical examination

The CN examination should be performed in conjunction with a thorough neurological examination to determine whether the deficit is isolated or diffuse.

Cranial-Nerve-Visual-and-Hearing-Dysfunction-in-Disorders-of-the-CNS-Table-2Download

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

Functional assessment

Cranial neuropathies can affect multiple domains of an individual’s life. The effect on vision, hearing, and balance should be evaluated by checking gait stability and ability to negotiate tight and crowded spaces. A driving assessment allows evaluation of the patients’ vision, reaction time and road safety. When a patient displays signs and symptoms of dysphagia a swallowing assessment not only serves diagnostic purposes but also helps with establishing a treatment program.  The swallowing assessment may include a clinical evaluation, video fluoroscopy and endoscopy. Finally, the impact of the neuropathy on psychological and social functioning and participation needs to be assessed.

Laboratory studies

The following may be considered depending on history and physical examination.

  • Infectious – CBC, cultures, ESR, serologies, CSF, or DNA PCR
  • Autoimmune – circulating antibodies in serum or CSF
  • Endocrine and electrolytic disorders

Imaging

  • 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.
  • Ethmoid tomography may be warranted when Battle’s Sign (mastoid ecchymosis), raccoon eyes, or other signs of basilar skull fracture are present.
  • MRI often provides visual confirmation of abnormalities in the olfactory bulbs and tracts, as well as in the inferior frontal lobes.

MRI is the imaging modality of choice for cranial nerve pathologies. It can diagnose mass-effect-lesions such as vascular lesions or tumors. MRI is also the best imaging modality to detect demyelinating diseases such as multiple sclerosis. Gadolinium administration allows the detection of cranial nerve enhancement which can be seen in malignancies, inflammation, demyelination, ischemia, radiation and axonal degeneration.

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.15,16
  • Electroencephalography should be employed if seizures are implicated.15
  • Electrodiagnostic testing with electromyography and nerve conduction studies.17
  • CSF analysis
  • Pain assessment (headaches, trigeminal neuralgia)

Early predictions of outcomes

A patient may expect moderate recovery initially for 4-6 weeks with resolution of edema and hematoma. 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.15

A study examining cranial nerve recovery in patients with traumatic superior orbital fissure syndrome demonstrated that of the most commonly affected CN’s in such injuries (CN III, IV, VI), CN VI demonstrated the greatest recovery in the first 3 months.18 However at 9 months following the injury, CN VI demonstrated the lowest level of recovery whereas CN IV demonstrated the highest recovery. Functional recovery for all cranial nerves reached plateau at 6 months after initial trauma.

Cranial-Nerve-Visual-and-Hearing-Dysfunction-in-Disorders-of-the-CNS-Table-3Download

Environmental

Environmental barriers to function can be vastly amplified by cranial nerve dysfunction.

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. Increased caregiver needs may place strain on the patient’s family.

Age

A study conducted among the short-living African turquoise killifish demonstrated that age is inversely correlated with the speed of optic nerve regeneration, likely due to the age-related decline in axonal regeneration.19 Similar animal/human studies are lacking for other cranial nerves, however could be extrapolated to possibly demonstrate similar association between age and regenerative potential.

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.

Rehabilitation Management and Treatments

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 essential4Avoiding 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.

Treatments based on deficits:

Cranial-Nerve-Visual-and-Hearing-Dysfunction-in-Disorders-of-the-CNS-Table-4Download

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 for select cranial nerve injuries

  • For olfactory (CN I) injury: Patients may be unable to detect smoke, gas, harmful chemicals or spoiled foods, which may cause harm. Use reminders and fixed schedules to 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.
  • For facial nerve (CN VII) injury: To protect the cornea, use lubricating eye drops regularly, as often as every 30-60 minutes. Additional options to reduce dryness are using an eye bubble or taping the eyelid.28
  • For glossopharyngeal (CN IX) or vagus (CN X) nerve injury: Explain the rationale for specific diets to families of patients with dysphagia, 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.25
  • For spinal accessory nerve (CN XI) injury: Physical therapy is important for reducing chronic pain from displacement of the humeral head, a consequence of loss of sternocleidomastoid and trapezius muscle function.28

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/ Emerging and Unique Concepts and Practice

In 2015, Morries, Cassano, and Henderson published Treatments for traumatic brain injury with emphasis on transcranial near-infrared laser phototherapy, which included a retrospective case series involving ten patients with chronic TBI.29This study used high-power near-infrared (NIR) light laser phototherapy, and demonstrated improved symptoms of headaches, sleep disturbance, cognition, mood dysregulation, and anxiety. The proposed mechanism underlying light therapy hypothesizes that NIR light reaches cytochrome c oxidase in mitochondria, which activates a signaling cascade that ultimately stimulates the production of inflammatory mediators and growth factors.29

In 2017, Kashkouli et al. published Traumatic optic neuropathy treatment trial (TONTT): open label, phase 3, multicenter, semi-experimental trial.30 This study included 100 patients with traumatic optic neuropathy (TON) and showed that best corrected visual acuity was significantly improved in patients that received either IV erythropoietin, IV steroid, or observation. No significant differences were found between the treatment and control groups. Given the lack of side effects and relative safety of EPO, the authors suggested EPO be recommended as an option in treatment of patients with TON, as well as further research on various EPO doses and dosing schedules.

In 2018, Do et al. published Comparison of hospital based and home based exercises on quality of life, and neck and shoulder function in patients with spinal accessory nerve injury after head and neck cancer surgery,31 This clinical trial included 40 patients, with half randomized to home exercise programs including range of motion, massage, stretching and strengthening, and half to hospital-based exercise groups for 40 minutes three times a week for 4 weeks. Results showed that the hospital based group had significantly increased range of motion compared to the home-based group. However, the quality of life was not significantly different between the two groups.

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.

References

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  28. Adunka OF, Buchman CA, eds. Otology, Neurotology, and Lateral Skull Base Surgery: An Illustrated Handbook. Georg Thieme Verlag; 2011:b-002-85469. doi:10.1055/b-002-85469
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  31. Do JH, Yoon IJ, Cho YK, Ahn JS, Kim JK, Jeon J. Comparison of hospital based and home based exercise on quality of life, and neck and shoulder function in patients with spinal accessary nerve injury after head and neck cancer surgery. Oral Oncol. 2018;86:100-104. doi:10.1016/j.oraloncology.2018.06.019

Original Version of the Topic

Blessen C. Eapen, MD, Thomas Shaw, Jeremy Gallant, MD, Carlos A. Jaramillo, MD, PHD. Cranial nerve, visual and hearing dysfunction in disorders of the CNS. 9/20/2014

Previous Revision(s) of the Topic

Jeannie Zhang, MD, Blessen C. Eapen, MD. Cranial nerve, visual and hearing dysfunction in disorders of the CNS. 5/2/2020

Author Disclosures

Blessen C. Eapen, MD
Nothing to Disclose

Michelle Q. Jin, MD
Nothing to Disclose