What is a hypoglossal facial nerve anastomosis?
A hypoglossal facial nerve anastomosis is a surgical procedure to restore facial muscle function in patients with facial nerve paralysis. It describes the technique where a surgical connection (anastomosis) is performed between the hypoglossal nerve (Cranial nerve XII or CNXII) and the remaining facial nerve (Cranial nerve VII or CNVII) with its connection to the facial muscles. The hypoglossal nerve is responsible for movement of the tongue on one side. There is a hypoglosal and facial nerve on both sides of the body.
Why is a hypoglossal facial nerve anastomosis an important procedure?
Facial nerve paralysis is a devastating nerve condition and leads to many functional, psychological, and even socio-economic problems. Restoration of facial nerve function is therefore an important option that should be considered as early as possible after injury to obtain the best possible outcome.
Causes of facial nerve injury in which hypoglossal facial nerve anastomosis could be considered.
Hypoglossal facial nerve anastomosis should be considered in permanent facial nerve damage. Vestibular schwannoma, paraganglioma, skull base tumours, trauma, infections, surgical damage, and primary facial nerve tumours can cause permanent facial nerve damage. In cases where spontaneous improvement is possible the procedure should be postponed. Permanent damage can be confirmed with an electromyogram (EMG) of the facial muscles.
Options for facial nerve repair.
If the defect is small the nerve ends can be primarily connected. In larger defects another nerve, usually a less important sensory nerve such as the greater auricular or sural nerve can be used as a cable graft between the cut ends of the facial nerve. With defects where the end of the nerve closer to the brain (proximal part) cannot be accessed due to destruction or location, a connection can be established between the distal part of the facial nerve where it connects with the facial muscles and another functional motor nerve. The hypoglossal facial nerve anastomosis is such an example. The anastomosis can be performed in an end to end fashion where both nerves are cut and joined primarily or with a cable graft. Alternatively, the cut facial nerve can be anastomosed to the side if the hypoglossal nerve in such a way that half of the hypoglossal nerve is spared. We prefer the latter technique.
How does it work?
The success of hypoglossal facial nerve anastomosis relies on an important functional ability of the brain, namely neural plasticity. There is obviously a very detailed and complicated scientific explanation of the process that takes place in the brain for this procedure to work. It can be simplified as follows. There is a specific area in the brain, the command centre, of the facial nerve. It gives the instructions to the facial nerve to move the facial muscles. Since the facial nerve connection with the brain centre is disrupted this procedure utilizes another brain centre, that of the tongue, to become the new command centre for the facial nerve.
How successful is it?
To improve the quality of life of a patient the aim of a hypoglossal facial nerve anastomosis is to improve the tone, symmetry, and movement of the paralyzed face. The result depends on the timing of the procedure with those performed earlier performing better than those performed later. Hypoglossal facial nerve anastomosis can even be performed after 2 years of the time of injury but in general we prefer to perform it within 18 months.
In order to describe the results, the House-Brackman (HB) facial nerve classification system is used (see explanation of the House Brackman classification system for facial nerve function). The House Brackman facial nerve classification system describes 6 grades.
Normal facial nerve function is classified as a HB grade I and a complete paralysis as a HB grade VI. Overall, the suspected level of function obtained with this procedure is a HB grade III. This can be obtained in 75% of patients. In 5% of cases a HB grade II can be obtained which is really an excellent result. Any residual asymmetry can be improved with plastic surgery, botulinum toxin and mime therapy.
It is very important to realise that movement of the face does not occur immediately after surgery. Optimal results are typically reached more than 12 months after the procedure.
What is the risk to the tongue with the hypoglossal facial nerve anastomosis?
Since the hypoglosal nerve is violated it is expected that the one side of the tongue movement will be affected. As mentioned with an end-to-end anastomosis where the hypoglossal nerve is cut and sutured to the facial nerve, tongue weakness on the one side is expected to be 100%. With the end-to-side technique it is remarkable to see that the tongue is affected in less than 10% of cases. Even if this would occur it is not a serious complication and can be adequately managed with therapy. Swallowing and speech are usually not affected.