Iatrogenic Hypoglossal Nerve Palsy

The hypoglossal nerve or twelfth cranial nerve (CNXII) innervates all of the intrinsic and extrinsic muscles of the ipsilateral tongue, except palatoglossus. Therefore, injuries to this nerve have the potential to affect a patient’s articulation and swallow substantially. The nerve can be transected during trauma or surgery but there are also several reports in the otolaryngology, anesthesia, and neurology literature concerning iatrogenic hypoglossal nerve palsy without direct nerve injury following tonsillectomy,^1,2 laryngoscopy,³ transoral intubation,⁴ use of the laryngeal mask airway (LMA)⁵ tooth extraction,⁶ or poor body position.⁷ A full listing of cases is listed in Table 1.

CASE REPORT

A 32-year-old man with a past medical history of diastolic congestive heart failure, morbid obesity (BMI 66), obesity hypoventilation syndrome and obstructive sleep apnea, hypertension, and diabetes mellitus type 2 presented with respiratory distress secondary to an acute CHF exacerbation. During his hospital stay, the patient was found to be unresponsive and a secure airway was placed emergently. Multiple intubation attempts failed, due to the patient’s body habitus and extremely short neck. Following intubation attempts, blood was found to be in the pharynx, likely secondary to trauma. Finally, an LMA #5 was secured with cuff inflated with 20mL of air, however position was noted to be poor, and the patient was subsequently taken to the OR for an emergent tracheotomy. Anesthesia premedicated the patient with Versed 5mg and Rocuronium 50mg x2. Anesthesia was maintained with oxygen and MAC (conscious sedation). A size 8 endotracheal tube was used to fashion an extra long tracheotomy tube, due to the large amount of subcutaneous fat on the patient’s neck. The duration of the procedure was 133 minutes. After the procedure, the patient was transferred to the ICU with a Shiley #8 tracheotomy tube.

The patient was maintained with the tracheotomy tube for the next 16 days. Prior to removal of the tracheotomy tube, he received a consultation from speech language pathology. The therapist noted that the patient was unable to protrude, elevate, or lateralize his tongue. The patient was able to trigger a swallow with a small piece of ice, however the swallow effort was noted to be weak. Speech was also noted to be dysarthric, due to inability to move his tongue. The patient tolerated a Passy-Muir valve and was able to produce a weak voice.

Neurology was consulted and determined that the patient had absent tongue movement; the tongue was flaccid. There was a question of mild atrophy; no fasciculations were observed. Sensation of the tongue to light pin and touch stimulus was intact. The remainder of the cranial nerves as well as general neurological examination was unremarkable. Needle EMG testing of the tongue revealed grade 3+ fibrillations with positive sharp waves of the right and left hyoglossus. There was no acute denervation seen at the cricothyroid and thyroarytenoid muscles. These findings confirmed a bilateral hypoglossal nerve palsy. Clinically, the patient fully recovered function within three months with no residual weakness and essentially normal speech and swallow function.

PATHOPHYSIOLOGY

The hypoglossal nerve originates from the hypoglossal nuclei in the medulla at the inferior aspect of the fourth ventricle.^8,9 It then exits the skull base from the hypoglossal canal in close association with CN IX, X, and XI and medial to the internal jugular vein and lateral to the internal carotid artery.¹⁰ It then descends vertically between the vessels and anterior to the vagus until the angle of the mandible. The nerve then crosses the lingual artery a little above the greater cornu of the hyoid. It then curves anteriorly and inclines upwards on hyoglossus, passing deep to the stylohyoid, tendon of the digastric, submandibular gland, lingual nerve, and the posterior border of the mylohyoid. It finally passes onto the lateral aspect of genioglossus where it continues as far as the tip of the tongue. A stylized illustration of this anatomy is presented in Figure 1 depicting where the tip of the laryngoscope blade can exert maximal force on the hypoglossal nerve as it enters the base of tongue laterally.

Hypoglossal nerve injuries occurring from intubation are often neuropraxic in nature, though they may also be due to axonotmesis. Neuropraxic injuries are mild and may result from ischemia or mechanical compression; these injuries should resolve within three months. With axonotmesis, the axon is irreversibly damaged as a result of crush injuries or nerve stretch injuries, necessitating a slower recovery due to distal axon sprouting.¹¹ Winfree and Kline wrote about peripheral nerve injury associated with intraoperative positioning.¹² They noted that severe stretching may tear the intraneural connective tissue, resulting in intraneural hemorrhage and possible necrosis. Severe compression may elevate the intraneural venous pressures and produce endoneurial edema, resulting in impaired axoplasmic flow resulting in nerve dysfunction for hours to weeks. Continued compression may lead to Schwann cell damage and demyelination or even axonal loss and Wallerian degeneration. Michel and Brusis published a cadaver study showing that the hypoglossal nerve was stretched by up to 1.3cm due to insertion of a laryngoscope on the lateral tongue base.² They additionally found more nerve distension with more extension of the neck, more lateral placement of the laryngoscope on the tongue, and more force placed on the tip of the blade.

Multiple different etiologies have been proposed for the development of hypoglossal nerve palsy associated with anesthesia. These etiologies include forceful laryngoscopy, hyperextension of the head, cricoid pressure, tight throat packs in the oropharynx, overinflation of the LMA, and the effect of nitrous oxide on the LMA cuff.^5,13,14 Lumb, et al. published a study that found a consistent, linear increase in LMA cuff pressure during anesthesia with nitrous oxide due to its ability to diffuse much more rapidly through the cuff walls.¹⁵ This pressure increase in some patients was as high as 50 percent more after a 30 minute procedure. No further difference in etiology has been postulated in those patients that have developed bilateral hypoglossal nerve palsy as opposed to unilateral.

In addition to hypoglossal injury, there are multiple reports of concomitant injuries to other nerves in the vicinity that can be affected by anesthesia or oropharyngeal manipulation. Tapia’s syndrome is a hypoglossal nerve injury with ipsilateral vagal nerve injury of the recurrent laryngeal nerve with associated vocal fold paralysis. This has been attributed to pressure neuropathy of both nerves due to inflation of the endotracheal tube cuff within the larynx.¹⁶ The lingual nerve, which supplies taste and sensation to the anterior two-thirds of the tongue, can also be damaged along with the hypoglossal nerve due to its similar course along the hyoglossus muscle and subsequent ability to be stretched in the same manner.¹³

DIAGNOSIS

The diagnosis of peripheral hypoglossal nerve injury can be made through a combination of history, physical, and ancillary tests. The key differentiation should be made between a supranuclear, nuclear, or peripheral lesion.¹⁷ A supranuclear lesion will most often be in the motor strip of the precentral gyrus but can also involve the internal capsule and pons. The lesion will typically be unilateral, and the tongue will have an initial deviation away from the cerebral lesion without evidence of atrophy or fasciculation but due to the crossed innervation from corticobulbar fibers the effects become minimal with time. Bilateral upper motor neuron involvement can occur, again without evidence of atrophy or fasciculations. Swallowing can be inhibited due to inability to manipulate food for mastication as well as due to trouble initiating the oral phase of swallowing. Electromyogram (EMG) will show a spastic paralysis of the tongue musculature that will distinguish it from lower lesions. Nuclear lesions will cause a paresis and atrophy of the tongue, ipsilateral to the site of the lesion. Because of the close proximity of hypoglossal nuclei bilateral lesions can occur. Patients will usually only have transient dysarthria as well as trouble initiating swallowing. Fasciculations can be seen later in the course. Nuclear lesions are best differentiated from peripheral lesions due to presence of other brain stem findings relating to involvement of motor and sensory long-tracts, resulting for example in contralateral hemiplegia and/or loss of vibration sense. Peripheral lesions will cause ipsilateral paresis, atrophy and a flaccid paralysis with deviation towards the side of the lesion. Because of proximity to other lower cranial nerve (IX, X, XI) in posterior cranial fossa one or more these nerves can be involved depending on the anatomical site of pathology. The lingual nerve (branch of V) can be co-involved with deep tongue lesions. Needle EMG can be performed of the tongue to ascertain presence of acute denervation (fibrillations and fasciculations), which typically have onset three weeks after axonal injury. Needle EMG can also be performed of vocal cord musculature to determine associated recurrent laryngeal nerve/vagal nerve involvement. Flexible endoscopic imaging by speech language pathology or otolaryngology will also be able to determine a vagal nerve palsy by the presence of vocal fold paralysis.

After the clinical exam and EMG are utilized, an MRI is probably the most useful imaging tool to help localize the lesion or rule out associated pathologies, due to its ability to follow the nerve in its entirety.¹⁸ Computed tomography (CT) is best at imaging the skull base foramina if there is suspicion of skull base pathology.

PREVENTION / TREATMENT

Many of the reported cases of hypoglossal nerve injury following intubation were due to improper cuff usage. Steps to reduce this risk include monitoring cuff pressure, inflating the cuff with the least amount of volume required to seal the airway, and deflating the cuff in all patients undergoing anesthesia with nitrous oxide for more than 30 minutes.^5,7,15 It has also been suggested that malpositioning of the patient’s body during surgery, particularly hyperextension of the neck, may result in nerve stretching and injury.¹⁹ Similarly, previous case reports have suggested using a shorter blade on the laryngoscope or intermittently releasing and repositioning the mouth gag or laryngoscope during longer cases.¹ In addition, two predisposing factors have been identified: calcification of the ligamentum stylohyoideum and deformities of the skull base.^4,16

From cases reported in the literature, complete recovery of hypoglossal nerve function is expected within the first six months.²⁰ Due to this progressive pattern of recovery, it has been postulated that the nerve experiences a neuropraxic injury and would therefore not benefit from intervention.²¹ Most case reports describe the use of steroids for several days in the immediate postoperative period following identification of hypoglossal nerve palsy, but there is no support within the literature. Patients will benefit from the inclusion of a speech language pathologist to help them with their loss of function, especially for swallow. Patients can be counseled to only chew and manipulate food on their non-involved side.

The treatment of bilateral hypoglossal nerve paralysis, on the other hand, is more involved. These patients will have total loss of tongue movement, making speech and the voluntary initiation of swallow extremely difficult.¹⁷ Therefore, alternative feeding methods should be utilized, such as a nasogastric tube or gastrostomy tube to maintain nutrition. Additionally, if patients are unable to initiate a swallow, they may develop difficulty handling their secretions. Tympanic neurectomy, botulinum toxin injection to the submandibular glands, and salivary diversion procedures can be utilized to decrease the flow of saliva depending on the clinical scenario. If the patient still has intractable aspiration, a tracheotomy can be placed along with closure of the larynx if no return of function is expected.

CONCLUSION

Hypoglossal nerve palsy represents a not uncommon finding in patients after surgery or anesthesia intervention even when no direct transection has occurred. Many proposed etiologies are preventable with careful attention to reducing compression of the lateral tongue base during prolonged surgery or intubation. However, the patients that have unilateral palsy have a good prognosis and most will recover full function in weeks to months.

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