Charcot-Marie-Tooth disease: CMT2, CMT4, and others
Sep. 10, 2023
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Traumatic cranial neuropathies are often seen with fractures involving the skull base. They most frequently involve the olfactory and optic nerves at the anterior skull base or the seventh and eighth cranial nerves in the temporal bone. In this update, the author updates the literature.
Samuel Thomas Soemmerring, a German medical student, established the present classification of cranial nerves in 12 numbered pairs two centuries ago. The basis of the classification was the different foramina in the base of the skull through which the nerves exit the cranial cavity (81). The first description of traumatic cranial nerve injury appears in the Edwin Smith Papyrus and describes a patient with a facial droop following head injury. The droop was on the same side as the fracture and was probably due to a contre-coup injury (24).
The 12 pairs of cranial nerves are injured in different orders of preference following trauma depending on the patient’s age and mechanism of injury. In general, the olfactory, facial, and vestibulocochlear nerves are damaged most frequently following blunt head trauma, with the lower cranial nerves (ninth, tenth, eleventh, and twelfth) being the least commonly injured (95; 35; 13). In severe head injury, the olfactory and ocular motor nerves are injured more often (78). In children younger than 10 years of age, abducens and facial nerves are commonly injured (58). Middle and lower cranial neuropathies are closely associated with basal skull fractures in blunt head trauma (92). In the case of firearm injuries and bullet trajectory, the injured cranial nerves are frequently different in suicide and homicide attempts: optic nerves are affected by the transverse trajectory of a suicide attempt, and the lower cranial nerves, the carotid, and the sympathetic trunk in the neck are affected by the anterior-posterior trajectory of a homicide attempt (19; 54). Hyperextension injuries of the cervical spine may interrupt the function of the sixth and twelfth nerves bilaterally. An injury to the pontomedullary junction, either directly or indirectly, through vertebral artery injury, would have a similar effect (54). With the exception of the oculomotor, abducens, and facial nerves, altered consciousness frequently obscures examination of cranial nerve injuries and diagnosis may be delayed significantly. Crush injuries often damage the middle cranial nerves as a result of skull fractures whereas stretching injuries damage nerves at points of fixed attachments.
Olfactory nerve. The olfactory nerve is not routinely assessed during the clinical examination. Injury is difficult to establish acutely after trauma, especially when nasal bleeding and swelling compound the examination. Olfactory nerve dysfunction parallels the severity of injury, particularly in occipital trauma (17), and has been estimated to occur in 25% of head injuries occurring after motor vehicle accidents (77). Summers and Wirtschafter suggest the overall incidence of olfactory nerve dysfunction to be 7%, increasing to 30% with severe head injuries or anterior cranial fossa fractures (92). In more than one third of cases, recovery follows within 3 months. Olfactory dysfunction may result from contusion of the sinonasal tract, olfactory nerve shearing and hemorrhage, and intracranial damage to the olfactory brain regions (77). Posttraumatic olfactory loss is thought to be the third most common etiology for olfactory disorders (17).
Optic nerve. The optic nerve is tethered in the optic canal and is subject to stretch injury during brain shifts. The bony architecture of the orbit directly transfers forces from the superolateral orbital rim to the optic nerve canal (34). A quarter of these injuries result from penetrating trauma, mostly gunshot wounds. Frontal blows to the orbit along the superior lateral rim also often damage the optic nerve, whereas occipital injuries rarely cause optic nerve injury (64). Most traumatic optic neuropathies are the result of severe head trauma, and altered consciousness in the patient delays the diagnosis. Traumatic optic neuropathy has been estimated to occur in 0.5% to 5% of patients with head trauma (103). Severe maxillofacial trauma may also result in traumatic optic neuropathy in the absence of brain injury up to 2.25% of the time (97). An afferent pupillary defect in an unconscious patient is useful in the detection of optic nerve injury, whereas testing for visual fields and acuity usually establishes the diagnosis in cooperative patients. About 10% of patients show signs of bilateral optic nerve injury or chiasmal damage, and the majority of chiasmal injuries are incomplete (87). Chiasmal injury is often associated with diabetes insipidus and decreased olfaction. The pupillary reflex is also useful in confirming nonorganic visual loss (69).
More recent research has identified evidence of indirect optic nerve injury even in mild traumatic brain injury. Chan and colleagues investigated military victims of both blunt trauma or concussive blasts and found abnormalities on optical coherence tomography compared to normal controls (15). Jashka and associates found a 20% rate of neuroophthalmologic injuries in a military cohort (49). These included optic nerve injuries in 44.3% and other cranial nerve injuries (eye movement palsies) in 38.2% with a 25.3% rate of diplopia. Final visual acuity was poorer than 20/200 in 34.3% of affected eyes.
Oculomotor nerve. Trauma is a common cause of oculomotor neuropathy, and associated facial and orbital injuries compound the diagnosis. Generalized orbital swelling with limitation of movements may be mistaken for oculomotor nerve injury. The third nerve may be injured at multiple sites, including the brainstem, exit zone from the brainstem, the entrance point to the dura, and in the orbital apex.
Trochlear nerve. The fourth nerve is often injured at its exit zone from the brainstem. Often a complaint of diplopia, following recovery from a severe head injury, leads to the diagnosis of trochlear nerve injury. Typically, the double vision is vertical, especially in downgaze (such as when the patient walks down a flight of stairs). Paralysis of the opposite trochlear nerve (bilateral trochlear nerve palsy) is suggested when there is alternating hypertropia with cross-cover testing and more than 10 degrees of excyclotorsion with double Maddox rod testing (56).
Abducens nerve. The sixth nerve may be injured because of its proximity to the petrous bone, which is prone to fracture. Injury may also occur in the brainstem or from stretch injury as the nerve passes through Dorello’s canal. Conjugate movements are controlled by the brainstem and sometimes make clinical examination and diagnosis of abducens nerve palsy difficult to establish in the presence of other brainstem injuries.
Trigeminal nerve. It is rare to have an injury to the trigeminal ganglion or main trunk within the cranial cavity, but the peripheral branches are often involved in facial lacerations and orbital fractures. Infraorbital nerve hypesthesia often accompanies orbital blowout fractures. Basal skull fractures may involve the trigeminal nerve and are usually combined with injury to the abducens nerve and facial nerve. Injury to the maxillary and ophthalmic divisions results in facial numbness, and involvement of the mandibular branch causes weakness of the muscles of mastication. The notion that pain is either verifiable or due to psychiatric disturbance has been questioned (26).
Facial nerve. Facial nerve palsy has been reported to occur in 5% of patients with head injury (73), with most being of the lower motor neuron type. The long, tortuous, intraosseous course of the facial nerve in the temporal bone makes this nerve highly susceptible to injury in temporal bone fractures. It is commonly injured in penetrating and blunt trauma to the head and face, as well as following parotid tumor surgery (38). Middle ear injury due to penetrating objects can easily damage the facial nerve. In about 50% of cases of transverse temporal bone fractures, the facial nerve within the internal auditory canal is damaged (12). With longitudinal fractures, the nerve is not directly involved, but a delayed paralysis may ensue secondary to edema.
Temporal bone fractures are classified as transverse, longitudinal, and mixed, based on the relation of the fracture line to the axis of the petrous pyramid (09; 51). Longitudinal fractures follow a blow to the temporoparietal bone and start from a weak point in the squama or the mastoid part of the temporal bone. They constitute about 80% of all temporal bone fractures and present with the classic Battle sign of bloody otorrhea and a bony step-off in the external auditory canal. The fracture line traverses anteromedially through the middle ear, disrupts the ossicular chain, and is deflected anteriorly by the hard otic capsule. It may terminate in the middle cranial fossa or extend medially to the sphenoid bone, crossing the midline in 30% of cases. Transverse fractures account for only 20% of temporal bone injuries. These require a force great enough to break the occipital bone at the foramen magnum, reaching the petrous pyramid. The intensity of this required force usually causes severe brain damage and disrupts the tough otic capsule, damaging both vestibular and cochlear components of the labyrinth. This fracture line generally spares the middle ear and has fewer external otological findings. Facial nerve injury occurs in about 50% of cases and the labyrinth is usually damaged by the fracture. Mixed fractures have a combination of these findings and are thought to be less common. However, 65% to 80% of fractures have been reported to be neither longitudinal nor transverse, with the fracture lines traversing the petrous pyramid in an oblique way (30). Severe head injury can also avulse the nerve root from the brainstem; the patient usually has features of brainstem dysfunction. Facial paralysis is usually striking in cases of infranuclear paralysis. Injury to the vertical mastoid segment produces loss of taste on the ipsilateral anterior two thirds of the tongue. A horizontal segment (middle ear portion of facial nerve) injury results in loss of the stapedius reflex (hypersensitivity to loud sound) and ipsilateral taste, although isolated loss of gustatory function after head trauma is rare (77). Additionally, a lesion of the labyrinthine segment results in impaired ipsilateral lacrimation. Trauma involving the internal auditory canal injures both facial and vestibulocochlear nerves, and facial nerve symptoms, loss of hearing, and vertigo are present (09).
Vestibulocochlear nerve. Longitudinal fractures traverse the anterior margin of the petrous pyramid, Meckel’s cave, and mastoid air cells, lacerating the tympanic membrane and producing a step-like deformity in the external auditory canal. This is often accompanied by sensorineural hearing loss and vertigo characteristic of inner ear concussion; however, the fracture typically does not involve the bony labyrinth (90). The less common transverse fractures damage both facial and vestibulocochlear nerves. These fractures involve the otic capsule, passing through the vestibule of the inner ear, tearing the membranous labyrinth, and lacerating both vestibular and cochlear nerves. The tympanic membrane is usually intact, and otoscopy may reveal blood in the middle ear cavity. Cerebrospinal fluid leakage is common, and meningitis may be a late complication. Sudden hearing loss following a blow to the head is often partially or completely reversible and is typically related to intense acoustic stimulation from pressure waves. Vertigo is the most common neurotologic sequel to head injury and is positional. This is seen in 47% of longitudinal temporal bone fractures and 21% of head injuries without skull fracture (08). This usually results from a dislodging of calcium carbonate crystals from the macula or the utricle that then become attached to the cupola of the posterior semicircular canal (89). Disruption of the labyrinth may cause a perilymphatic fistula, which is frequently associated with the sudden onset of an audible popping noise associated with hearing loss, tinnitus, and vertigo. Although they are more common after skull fractures, auditory and vestibular symptoms may still occur in the absence of a temporal bone fracture.
Vagus and glossopharyngeal nerves. Trauma to these cranial nerves is uncommon (02). Only a few cases of penetrating trauma and fracture of the occipital condyle have been reported (71; 96). Hyperextension neck injuries are also sometimes associated with injury to these nerves at the craniocervical junction (41). In severe head injuries, the incidence may be higher. Posttraumatic vagal dysfunction should be considered in patients with delayed gastric emptying and absent cardiac response to tracheal suction.
Spinal accessory nerve. Injury to the accessory nerve is uncommon. Avulsions may be associated with cervical spine trauma. A more common cause of injury is surgical trauma; iatrogenic injuries may follow surgery on deep cervical lymph nodes and the posterior cervical triangle (11). Hyperextension neck injuries, particularly in athletes (06), may occasionally involve the spinal accessory nerve and produce paralysis of the sternocleidomastoid and trapezius muscles, which may be delayed in presentation.
Hypoglossal nerve. The most frequent cause of hypoglossal nerve injury is iatrogenic, especially after carotid endarterectomy (85). Gunshot wounds and penetrating injuries to the nerve are common causes (04). Hyperextension neck injuries can produce blunt trauma to the nerve with or without fracture of the hypoglossal tubercle or occipital condyle (21). Tongue weakness ipsilateral to the nerve injury follows this type of injury; bullet wounds may produce bilateral tongue paralysis. Development of bilateral hypoglossal nerve injury has been reported following the use of a laryngeal mask airway (91).
Anosmia following trauma improves in one third of cases, usually during the first 3 months, though recovery may continue to occur for 5 years. It is only transient in 50% of cases and a sharp increase in recovery occurs about 10 weeks after injury (93). Delayed visual loss is potentially reversible. Direct injury results in immediate and often permanent visual loss. Gjerris reported that 40% to 50% of patients remained blind and nearly 75% of patients showed no improvement in their deficit (32). With a retained pupillary reflex, the prognosis is more favorable. Cook and colleagues discuss results of surgical and nonsurgical therapy for the optic nerve (18). A protocol of timely microsurgical decompression and high-dose corticosteroids resulted in significant visual improvement in 45% of patients and lesser recovery in 15%. Positive prognostic factors included surgery within 24 hours and hematoma of the nerve sheath (101). Recovery of the third nerve usually follows in 2 to 3 months, often with aberrant regeneration that manifests in the form of abnormal synkinesis of the extraocular and levator muscles, with ptosis often improving in downgaze. The slender trochlear nerve is often avulsed from the midbrain, and the prognosis is not favorable. Less severe injuries often recover within months after injury. Some recovery is expected in partial injuries to the trigeminal nerve. A positive percutaneous stimulation after 4 days of facial nerve trauma indicates excellent prognosis for recovery. In most series, facial palsy has a favorable rate of spontaneous recovery. Sensorineural hearing loss has a poor prognosis, and tinnitus is often disabling. Vestibular symptoms often take 6 to 12 weeks to subside, and an intractable postural vertigo with sensorineural deafness may be suggestive of perilymphatic fistula (68).
A 45-year-old man fell from a deer stand and immediately developed slurring of speech. Following arrival at the emergency ward, his consciousness rapidly deteriorated and he required intubation. He had lower motor neuron facial nerve paralysis on the left side with decreased gag reflex. This could not be adequately assessed due to the impaired sensorium. An urgent CT revealed a type III occipital condyle fracture and fractured foramen magnum with fractured and medially displaced petrous temporal bone.
The brainstem was compressed by the petrous and occipital bones and there was obstructive hydrocephalus. A diffuse subarachnoid bleed was noted, more prominent on the left side. An urgent ventriculostomy was performed for the obstructive hydrocephalus, and improvement in sensorium ensued. He was then referred to our center where he remained intubated and drowsy. On re-examination, he was noted to have bilateral abducens nerve paralysis with facial nerve palsy on the left side. Vestibulocochlear nerve function could not be examined because of the decreased sensorium. He had a decreased gag reflex and was tolerating the endotracheal tube, suggesting injury to the lower cranial nerves. He did not tolerate extubation and required reintubation. In view of the gross displacement of the middle and posterior cranial fossa base, an arteriogram was obtained to rule out injury to the carotid and vertebrobasilar system. An angiogram revealed a complete obstruction of the vertebral artery on the left side and partial obstruction of the carotid on the left side.
A ventriculoperitoneal shunt was placed for hydrocephalus, and a tracheostomy was performed for airway care, as he did not regain the cough reflex. Vascular insult to the brainstem was severe. The patient showed rapid neurologic deterioration and died.
This case illustrates both direct and indirect causes for cranial neuropathy. Though clinical examination was difficult due to an obtunded sensorium and the need for airway management, the likelihood of abducens, facial, and lower cranial nerve involvement was high. A fracture of the occipital condyle extending upwards to involve the petrous bone might have injured the lower cranial nerves and the seventh-eighth nerve complex. The bilateral abducens palsy (gaze paralysis might be easily overlooked in this clinical situation) may be due to raised intracranial pressure (from hydrocephalus, traumatic brain edema, or both) or due to ischemic injury of brainstem following vertebral artery involvement. Direct compression of the brainstem by the medially displaced fracture is also a possible cause.
The causes for cranial nerve palsy vary. Blunt head trauma in motor vehicle accidents and gunshot wounds are responsible for the majority of lesions. Other causes include penetrating stab wounds, hyperextension neck injuries, and operative trauma. Symmetric middle cranial neuropathies result from crushing injuries of the skull. Blunt head trauma is accompanied by basilar fractures and involves the cranial nerves near the fractures. Gunshot wounds take typical trajectories: horizontal in suicide attempts and anteroposterior in homicides. Avulsion and stretching of the nerve roots can occur with acceleration and deceleration trauma and in blunt injuries. These forces damage the nerves at their points of fixation or angulation (80; 14).
The common forms of direct injury to the cranial nerves are stretch injury, contusion, laceration, and disruption. Trauma to the vicinity of a cranial nerve leads to delayed injury, resulting in compression of the nerve or edema or hemorrhage of the nerve within its linings. Indirect injury can also be vascular with resultant ischemia that is either transient (transient visual loss for the optic nerve) or permanent with thrombosis or disruption of blood vessels. The optic nerve may be injured within or outside the cranial cavity; the most common location is the optic canal, with fractures traversing the ethmoids. The third nerve is damaged by a frontal blow to an accelerating head, resulting in stretching and contusion of the nerve; the usual point of injury is at its entry into the dura at the posterior end of the cavernous sinus (40). The fourth cranial nerve is usually injured by contusion or stretching at its entry zone to the dorsal midbrain near the anterior medullary velum. In an accelerating head, the dorsolateral midbrain is particularly vulnerable to injury, usually in a motor vehicle crash, with compression of the midbrain against the tentorial incisura and damage to one or both trochlear nerves (63). The abducens nerve is injured in crush injuries where anterior-posterior forces distort the skull, in fractures of the petrous bone (along with seventh and eighth nerves) that sever the nerve close to the petroclival ligament, or when vertical movements of the pons avulse the nerve at its exit zone. At the superior orbital fissure, fractures and stretch injuries may occur in combination with injuries to the third and fourth cranial nerves. Various ocular movement disorders including gaze palsy and internuclear ophthalmoplegia follow primary brain injury, especially with brainstem involvement. Trigeminal nerve injury follows fractures of the orbit and sometimes the petrous bone at Meckel’s cave. The facial nerve may be directly severed by fractures of the temporal bone or direct penetrating trauma whereas delayed paralysis is typically due to swelling of the nerve within its sheath, damaged vascular supply, or arterial spasm. Temporal bone fractures also involve the eighth nerve, and impaired hearing or vertigo have their origins in the cochlea and labyrinth (vide supra).
Fractures involving either the occipital condyles or the upper cervical spine can result in cranial neuropathies as well. Kaiser and Mehdian described permanent hypoglossal injury following occipital condyle and C1 posterior arch injury (52). Utheim and associates reported the presence of multiple cranial neuropathies with an occipital condyle fracture (98). In fact, appearance of a unilateral hypoglossal palsy after trauma should lead to the search for an unrecognized condylar fracture (82).
Necrosis of the nerve cells follows acute and irreversible injury. In the early stages, the neurons shrink and the Nissl substance stains are relatively normal. Many necrotic neurons are then removed by phagocytes to form microglial nodules. Cavitation follows as surrounding tissues become involved in the degeneration. Fragmentation and disappearance of the endoplasmic reticulum and disappearance of the Golgi apparatus follow dissolution of the Nissl bodies. These changes are accompanied by vascular congestion and perivascular inflammation. Phagocytes engulf these necrotic neurons and soon the nerve fibers show changes of degeneration, usually within 3 days of the death of their corresponding cell bodies.
Indirect or secondary damage to the cranial nerves can result as a consequence of traumatic vascular decompression. Carotid dissection can be associated with blindness and hemianopia from optic nerve involvement (07; 86). Lower cranial nerves can be vulnerable in extracranial carotid dissection, vertebrobasilar artery dissection, and even internal maxillary artery dissection (01; 72). Vascular dissection should be considered and ruled out in cases of traumatic cranial neuropathy.
The incidence of cranial nerve injury differs by age groups and demographics. Keane and Baloh provide a comparative tabulation of incidence (56). Anosmia is reported to occur in 7% of all head injuries (93). Injury to the optic chiasm is reported in only 0.7% of head injuries, and only 10% of head injuries have isolated bitemporal hemianopia (100). Ford and colleagues reported 26 cases in the pediatric population in the United Kingdom, with most injuries occurring in males. The incidence, natural history, and presentation were noted to be similar to those in the adult population (28). A 3% incidence of ocular palsy in closed head injury was noted by Russell (83). Hughes reported a 2.6% incidence of III nerve palsy, a 2.7% incidence of sixth nerve palsy, and 1.4% incidence of combined nerve injury in closed head trauma (46). Trochlear nerve trauma is least common, partly due to difficulty in diagnosis. Russell reported a 4.5% incidence of fifth nerve injury in his series of 1000 cases, whereas facial and eighth nerve injuries closely follow temporal bone fractures (83; 31). Some degree of hearing loss has been reported in nearly half of patients with serious head injury. The majority experience sensorineural hearing loss, whereas conductive loss is seen in only 3% (08). In pediatric head trauma, ocular motor nerves are most commonly affected, followed by the optic nerve, trigeminal, and facial nerve; vestibulocochlear and olfactory injuries are rare (48). Cranial nerve injury may occur with spinal traction. Background spinal traction is the application of a longitudinal force to the spinal column as a means of stabilizing a damaged or abnormal spine. Although not well documented in the ophthalmic literature, complications include cranial nerve palsies, with the sixth nerve being most commonly affected. Fourth nerve palsy may be underreported because of masking by another ocular motor palsy (75).
Traumatic cranial neuropathy can occur after either severe or minor closed head injury. Coello and colleagues reported on 49 individuals after minor closed head injury with cranial neuropathy (16). Single nerves were involved 78% of the time and multiple nerves in 22%. The olfactory nerve accounted for the greatest number of injuries, followed by the facial and oculomotor nerves. Skull base injuries were associated with the majority of the cases. Jin and colleagues reported an overall incidence of 9.1% for cranial nerve injuries in a population of single center head injuries (50). Optic nerve injuries were the most common, followed by olfactory, occulomotor, and facial nerve injuries. Overall mortality was significant when the lower cranial nerves were involved. Sixty-one percent of deficits improved without surgery, and 87% improved after surgical management.
Individuals suffering traumatic blast injuries may be particularly susceptible to vestibular nerve dysfunction. A review of 24 service members suffering blast-related traumatic brain injury showed abnormal nystagmus or oculomotor findings in half of symptomatic patients and rotational chair abnormalities characterized as peripheral in four and central in two patients compared to only one out of 10 asymptomatic patients (88).
A large database study looking for the incidence specifically of optic nerve injury found an overall rate of 2.2% of traumatic prechiasmal optic nerve injuries in a cohort of 84,627 overall patients (45). Risk factors included younger age, male gender, fractures, and pneumocephalus.
General guidelines for prevention of trauma apply to prevention of cranial nerve injury. In particular, prompt early diagnosis is emphasized to minimize neuronal damage. Iatrogenic injury may be avoided by following accepted surgical principles during operative procedures.
Traumatic cranial neuropathy is a difficult diagnosis and requires continuous and repeat examination in cases with high-risk factors, including basal skull fracture, bleeding from the nose or ear, and orbital injury. With the exception of some patients with oculomotor, abducens, and facial nerve injuries, it is often not easy to diagnose these lesions in comatose patients. The diagnosis requires a high degree of patient cooperation and perseverance on the part of the clinician. Nasal obstruction may be confused with olfactory nerve injury. Malingering may be difficult to exclude in optic nerve injury, where pupillary reaction should be assessed. Injury to the orbit along with swelling can mimic ocular motor nerve palsy. Evaluation of diplopia and examination of ductional eye movements is required, sometimes repeatedly. A blowout fracture of the orbit can give rise to facial numbness in the absence of trigeminal nerve trauma. Facial paralysis in coma may be coincidental and pre-existing. A stroke-like presentation of head injury with upper motor neuron facial paralysis needs to be considered in cases where correlation is difficult. Neuroelectrophysiological studies may be helpful in these instances. These diagnostic tests are also helpful for eighth nerve function. Hemotympanum injury results in conductive hearing loss and serious injury to the eighth nerve complex and suggests the need for audiometry. Central vestibular dysfunction similarly needs to be excluded in the absence of signs of significant brainstem injury, and electronystagmography may be helpful. Minor trauma may precipitate cranial nerve palsy in a nerve previously stretched by a compressive lesion such as an aneurysm or tumor (25).
It has been suggested that individuals with craniocervical instability with or without Ehlers Danlos syndrome may be particularly prone to cranial neuropathy after traumatic brain injury. Gami and Singman reported seven individuals with persistent neuro-ophthalmological symptoms after mild closed head injury and found clinical elements suggesting Ehlers Danlos in all seven (29). Henderson and colleagues have summarized neurologic manifestations of Ehlers-Danlos including lower cranial neuropathy from Chiari malformation and cranio-cervical instability (42). These associations remain controversial and often produce medicolegal dilemmas.
Clinical suspicion is important in trauma. High-risk factors include basal fractures, which are best demonstrated by high resolution CT scan with thin slices.
There is a good correlation between basal skull fractures and cranial neuropathies (09; 05; 14; 21; 10). MR imaging is utilized to visualize the cranial nerves. Yang and associates used diffusion tensor imaging to look for optic nerve fiber interruption, which was associated with lack of response to surgical decompression (104). Functional evaluation varies according to the nerve. Olfactory nerve injuries are evaluated electively by clinical examination. Optic nerve injury requires testing of visual acuity and fields as well as pupillary function. Evaluation of the anterior cranial fossa base using high resolution CT scan is suggested when injury to the optic nerve is suspected. Compression by ethmoid air cells in the optic canal and fractures across the optic canal were demonstrated in nearly half of the cases with optic nerve injury in one series (56). Ashar reported a high frequency of visual loss in midfacial fractures (05). Diplopia fields may be useful for diagnosis and follow-up of ocular motor function. High resolution CT and MRI are useful in differentiating the causes for delayed diplopia, including impending tentorial herniation, and midbrain contusions associated with trochlear nerve palsy (57; 54; 55). A good correlation exists between facial nerve injury or eighth nerve injury and the type of temporal bone fracture (12). For both of these nerves, electrophysiologic monitoring may be useful in the diagnosis and prognosis of injuries (56). For facial nerve injuries, demonstration of axonal degeneration on EMG or 90% to 95% loss of function on ENG may indicate need for decompression (106). Injury to the lower cranial nerves is uncommon, and clinical diagnosis helps in the case of accessory and hypoglossal nerves, sometimes combined with electromyography. Vagus and glossopharyngeal nerves are rarely injured and these lesions are often not easily diagnosed (36; 96).
Newer technologies may revolutionize the diagnostic workup of traumatic brain injury. Wright and colleagues showed that a virtual reality based analysis tool was more sensitive than conventional assessments in determining visual-vestibular dysfunction (102).
Olfactory nerve. Standardized tests are available to evaluate olfactory function. Two objective tests of olfaction are the olfactory respiratory reflex and olfactory electroencephalography. The former helps in excluding nonorganic loss, whereas EEG is a nonspecific alpha response to an odoriferous substance. A high-resolution CT of the ethmoids and frontal fossa with thin (1 to 2 mm) cuts is suggested in the evaluation of anosmia, especially with CSF rhinorrhea that frequently accompanies trauma to the anterior cranial fossa and nasal sinuses. MRI can demonstrate injury to the basofrontal lobes and the olfactory bulbs and tracts. In some cases of sinonasal damage, surgery can be helpful. However, there is no specific treatment for posttraumatic anosmia related to nerve damage other than counseling the patient and providing reassurance. Spontaneous recovery, thought to occur from recovery of damaged olfactory neurons, has been estimated at 30% within 6 to 12 months after injury. However, worsening of the deficit over time may also occur.
Optic nerve. Clinical evaluation and investigations such as electroretinography and visual evoked potentials may be helpful when evaluating severe head injuries. High resolution CT is typically better for evaluating bony injuries, and MRI is suggested for evaluating the optic nerve and its related soft tissue. Optic nerve injury is often categorized as direct (such as from penetrating injury) when there is direct nerve trauma or indirect (such as from blunt head injury) when the concussive force of trauma causes damage. Direct injury is often severe and generally associated with a grim prognosis. Visual loss is typically immediate from traumatic optic neuropathy; however, rarely there may be a delay in symptoms. Prompt treatment can be important in patients with a delayed onset of visual loss as it is potentially reversible; otherwise, no treatment provides definite benefit. High-dose corticosteroids (similar to dosage for acute spinal cord injury) have been used in the acute phase and have been studied largely in patients with indirect injury. Intravenous methylprednisolone 30 mg/kg as a loading dose followed by 5.5 mg/hour for 36 to 48 hours was used by the Graz group (65); Tandon and colleagues reported administering 80 mg/day of prednisolone for 3 weeks (94). Cook and colleagues describe different regimens used in various published series (18).
In patients with delayed onset loss of vision due to compression of the optic nerve and failed steroid treatment, operative approaches in transcranial, transethmoidal, transmaxillary, and transorbital routes have been described for optic nerve decompression. The details of outcomes in operative and nonoperative groups are provided by Cook and colleagues (18). In this meta-analysis, nontreatment was reported as effective as medical or surgical treatment of traumatic optic neuropathy. The presence of several confounding factors in the analysis of the outcomes led to the initiation of the International Optic Nerve Trauma Study, which was aborted, in part because of insufficient enrollment of patients (62). The International Optic Nerve Trauma Study, published in 1999, compared the visual outcomes of traumatic optic neuropathy treated with corticosteroids, optic canal decompression, or observation within 7 days of injury. It was found that, after adjustment for baseline visual acuity, there was no significant difference between any of the treatment options. The study concluded that there is no clear benefit of corticosteroids or optic canal decompression surgery in patients with traumatic optic neuropathy (62), and similar results have been reported in other studies (59). Thus, more emphasis has been placed on a combined approach, individualized to each patient, to manage this complex problem.
Rajinigandh and colleagues described a prospective, nonrandomized study to assess the visual outcome in 44 patients with traumatic optic neuropathy treated with a combined therapy protocol of methylprednisolone injections and endoscopic optic nerve decompression (76). The main outcome measure was visual acuity, which improved in 70% of patients when treatment was started within 7 days of the injury as compared to 24% improvement in patients whose treatment was delayed. In addition, CT evidence of canalicular and pericanalicular fractures was also found to be a significant prognostic factor.
Ropposch and associates reported a retrospective series of 42 patients with traumatic optic neuropathy. Steroids were used in half of the patients. Overall, 33% of patients improved with treatment. Surgery alone had a more beneficial effect than surgery plus steroids either in high or low doses (79).
Newer surgical approaches have sought to maintain excellent outcomes while lowering operative morbidity. Endoscopic decompression provides a less invasive alternative to open decompression. He and colleagues reported on 11 patients treated with 45% visual improvement at 3-month follow-up with little surgical morbidity (39). Yang and colleagues reported a large series of 96 patients treated with endoscopic optic nerve decompression and found improvement in 83% of patients with preoperative light perception and 26% of those with no preoperative light perception (105). Hemorrhage in the sinuses and a surgical delay of 3 days were also associated with poor outcomes. A systematic review of the literature found that endoscopic decompression was beneficial both in the early stages after injury and even when surgery was delayed greater than a week (23). Visual improvement was seen in 57% of patients with early surgery, 51% of patients with late surgery, and 41% of patients with no light perception preoperatively. Patients with some preserved optic nerve function fared better after surgery.
Part of the difficulty in the assessment of treatment outcomes is spontaneous improvement, which has been said to occur in 30% of patients (103). Furthermore, results from some studies (including the Corticosteroid Randomisation After Significant Head Injury study, which found a higher rate of death in the corticosteroid group when compared to placebo) suggest that the side effects of corticosteroids may outweigh their benefits. The management and recovery of traumatic optic neuropathy continues to remain difficult despite the ongoing research to manage this injury. Thus far, there have been no definite benefits of potentially neuroprotective agents, including glutamate inhibitors, nerve growth factors, crystallins, and inhibitors of nitric oxide (74).
A multicenter randomized trial was formulated to assess use of intravenous erythropoietin and steroid in indirect traumatic optic neuropathy (53). There was no significant difference between treatment with erythropoietin, steroid, or control. Color vision was improved with erythropoietin. Late treatment and initial finding of no light perception were adverse indicators of recovery.
Ocular motor nerves. The presence of third, fourth, or sixth nerve palsy after trauma is suggestive of more severe head trauma, including a lower Glasgow Coma Scale and more severe findings on CT, when compared to patients with closed head injury who do not have ocular motor nerve involvement (22). Pathologic evidence suggests that injury to these cranial nerves may occur in the brainstem, at the exit zone from the brainstem, and where the nerves enter the dura (22). Ocular motor palsy in the setting of minor trauma should be suggestive of other etiologies such as a tumor or aneurysm.
Alternate cover testing, CT, and MRI are helpful in establishing a diagnosis. However, there is no definitively effective treatment for injury to these nerves. Patching one eye eliminates diplopia whereas some level of recovery is expected to occur in 4 to 6 months. Extraocular muscle surgery, in cases of failed recovery, should generally be delayed for 6 to 12 months or until diplopia remains stable (84).
Botulinum toxin injected into the ipsilateral medial rectus muscle has also been proposed as a treatment option for traumatic sixth nerve palsy. A prospective multicenter trial, conducted by the North American Neuro-ophthalmology Society, however, revealed that patients with traumatic abducens palsy treated with either botulinum toxin or conservative measures had similarly high recovery rates (43).
Trigeminal nerve. Disabling neuralgia sometimes accompanies partial injuries to the sensory divisions. Carbamazepine, phenytoin, or gabapentin may help relieve the symptoms. In intractable cases, sectioning of the peripheral nerve may be required.
Facial nerve. High resolution CT is suggested in patients with facial nerve trauma. An excellent spontaneous recovery can be expected with delayed onset paralysis. With nonsurgical management, 90% of patients experience good recovery within 6 months (70). Absent facial nerve stimulation after 4 days may indicate the need for surgical exploration, especially with transverse fractures of the temporal bone and a discontinuous fallopian canal. Available electrodiagnostic tests that are used to evaluate facial nerve injury examine the nerve distal to the stylomastoid foramen. These tests cannot evaluate the nerve across the injury site because the nerve is mostly injured within the temporal bone. One group suggested transcranial magnetic stimulation of the nerve, proximal to the injury site. They found that such stimulation was able to assess the integrity of the nerve after trauma and could predict the potential for functional recovery. Issacson and colleagues retrospectively studied facial nerve outcomes in 229 patients undergoing resection of vestibular schwannomas with intraoperative facial nerve monitoring. They observed that proximal to distal amplitude ratio and proximal electric threshold were statistically significant predictors of the outcome for facial nerve functions (47). These methods, however, have not been in widespread clinical use (37). Mastoidectomy and decompression of the nerve under microsurgical techniques, with or without repair, using grafts may be beneficial (33; 61). Yetiser reported over 50% of patients with loss of facial nerve function and axonal degeneration on EMG had return of function to House Brackman grade 2 after facial nerve decompression by either a middle fossa or translabyrinthine approach (106). The early return of voluntary motor potentials on standard electromyography carries a favorable prognosis (27). Electrical stimulation and massage of denervated muscles may help avoid contractures. Iatrogenic facial nerve injury should be addressed as soon as can safely be performed after the injury occurs and may require nerve grafting (20).
More recent studies have attempted to elucidate the timing and efficacy of surgical repair of facial nerve injuries. Marzal and associates treated nine patients with complete facial nerve injuries, with five longitudinal and three transverse fractures (66). Surgical reconstruction was attempted in all cases but could not be completed in one. Of the eight surgical repairs, two yielded very good results and four had good results. Surgery in some cases was delayed due to medical issues and timing had no effect on recovery. Novel surgical techniques may ameliorate recovery. Alicandri-Ciufelli and colleagues reported endoscopic repair in six individuals with recovery to House-Brackmann grade 1 or 2 in five of six cases (03).
Adequate lubrication of the eye with artificial tears and ointment may be important, depending on the degree of involvement of the orbicularis oculi. More severe lagophthalmos may necessitate surgical procedures with gold weight implantation or tarsorrhaphy.
Vestibulocochlear nerve. Conductive hearing loss can be treated surgically by correcting the middle ear and has a good prognosis. There is no specific treatment for sensorineural hearing loss; however, some improvement often ensues with partial injuries. MRI can help delineate the central auditory pathways. Conversely, a perilymphatic fistula with loss of eighth nerve function suggests the need for surgical exploration. Repositioning maneuvers, which are designed to displace the calcium crystals from the semicircular canals, are often helpful for traumatic positional vertigo. In refractory posttraumatic vertigo, a labyrinthectomy or translabyrinthine eighth nerve section or selective vestibular nerve section (in cases with preserved hearing) may provide relief (44; 67).
Vestibular dysfunction is seen frequently following concussion. A review of vestibular rehabilitation shows both clinical and symptomatic improvement in both subjective and objective vestibular testing (99).
Lower cranial nerve palsy. Unilateral paralysis of the ninth through twelfth nerves, also known as Collet-Sicard syndrome, implies the need for investigation of an underlying cause, which most commonly occurs from penetrating neck wounds from bullets or stab wounds. In vascular injuries, such as vertebral artery fistulae, and in fractures of the occipital condyle, treatment may be directed to rectify them. Otherwise, the treatment for cranial nerve palsy is expectant and symptomatic (36; 96; 21). Lower cranial nerve damage may also occur through iatrogenic trauma (60), especially the hypoglossal and vagus nerves (85).
Medical and surgical treatment of cranial neuropathy should observe standard precautions for pregnancy and lactation.
Surgical treatment is required in selective situations and anesthesia follows routine trauma protocol, except for an allowance for intraoperative neuroelectrophysiological monitoring where short-acting muscle relaxants and inhalation anesthetics are favored.
All contributors' financial relationships have been reviewed and mitigated to ensure that this and every other article is free from commercial bias.
Richard S Polin MD
Dr. Polin of George Washington University has no relevant financial relationships to disclose.See Profile
Randolph W Evans MD
Dr. Evans of Baylor College of Medicine received honorariums from Abbvie, Amgen, Biohaven, Impel, Lilly, and Teva for speaking engagements.See Profile
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