Middle Fossa Approach to Repair Cerebrospinal Fluid Leak

The middle fossa approach is indicated for procedures requiring access to the internal auditory canal, structures within the temporal bone, and adjacent structures. This is one of the three main approaches for the surgical repair of tegmental defects causing cerebrospinal fluid (CSF) leak. The middle fossa approach allows for an optimal view of the middle fossa floor for larger or multiple defects, ease of graft placement, and avoidance of the removal of ossicle to access the tegmen. Surgical intervention for CSF leak is indicated when conservative management fails or when spontaneous closure of a defect is unlikely. In this case, a middle fossa approach is used to surgically close a tegmen defect causing CSF otorrhea refractory to conservative management. This case highlights the step-by-step surgical techniques involved in this procedure including the surgical approach to expose the tegmen defect, repair of the tegmen defect using temporalis fascia and a bone graft, and craniotomy repair and closure.

Otolaryngology; surgery; cranial fossa, middle; cerebrospinal fluid otorrhea.

Cerebrospinal fluid (CSF) leaks occur from craniofacial trauma, iatrogenic causes, and other less common etiologies such as neoplasm, infection, and congenital defects.¹ The CSF plays a vital role in providing nutrients to the central nervous system and cushioning the brain and spinal cord. The tegmen is the osseous plate that forms the roof of the tympanic cavity and separates it from the middle cranial fossa. It also separates the subarachnoid space containing CSF from the air space of the middle ear. Therefore, defects in the tegmen can lead to abnormal communications between the subarachnoid space and air-containing spaces of the temporal bone causing CSF leakage.^6,8 On physical exam, patients with CSF leak commonly present with clear rhinorrhea, otorrhea, orthostatic headache, and additional symptoms based on etiology.⁵ For tegmental defect-based CSF leaks, patients may experience clear otorrhea, rhinorrhea, middle ear effusion, hearing loss, and headache.⁶ Serious complications of CSF leakage include meningitis and brain abscess if treated conservatively without surgical intervention.² Surgical treatment is first line for chronic CSF rhinorrhea, posttraumatic patients refractory to conservative or medical management, and patients with significant intracranial pathology.¹ The surgical approach to fixing a CSF leak is dependent on the location of the defect and etiology. For tegmental defects, the middle fossa approach is preferred due to its high success rate and long-term efficacy in control of CSF leak with several advantages over alternative approaches.³

This patient is a 57-year-old female who presented with a complaint of fullness and hearing loss in the left ear. She underwent a myringotomy with tube placement and subsequently had persistent drainage of clear watery fluid from the left ear canal. She was treated with multiple antibiotics and different topical preparations without improvement, and as a result of the persistence of her symptoms, she had a CT of the temporal bone performed, which revealed a defect in the tegmen overlying the mastoid of the left ear consistent with probable CSF leakage into the middle ear and mastoid.

On a physical exam, this patient had exam findings consistent with tegmen defect including clear otorrhea, middle ear effusion, and hearing loss from the left ear.

CT of the patient’s temporal bone showed a defect in the tegmen (5 mm in diameter) overlying the mastoid of the left ear.

The overall prognosis of CSF leaks is favorable especially in cases of craniofacial trauma where conservative management may be all that is required for resolution. However, most tegmental defects are spontaneous, which have a very low rate of spontaneous closure. This means that most of these patients require surgical closure. If left untreated, there is a risk of developing meningitis. Surgical repair of lateral spontaneous CSF leaks has a low average overall failure rate of 6.6%.⁷

There are several treatments for CSF leak depending on etiology including conservative and surgical management. Conservative management is often first line for CSF leak from craniofacial trauma, and these strategies include elevation of the head, antibiotics, and diuretics. Studies suggest that 68–80% of the posttraumatic CSF fistulas can be closed spontaneously within 48 hours and up to 85% closed within 7 days of initial injury.^1,12 Surgical management is considered as first-line treatment for patients with symptoms refractory to medical management for 3–7 days.¹ The location of the defect is the most critical factor in determining surgical approach. There are three main approaches to repairing tegmental defects: transmastoid approach, middle fossa approach, or a combination approach.³

The goal of treatment is to manage persistent symptoms of the CSF leak including CSF otorrhea as well as to prevent serious complications such as meningitis and meningoencephalocele.⁶ In this case, the patient underwent myringotomy and had persistent draining of water fluid from the left ear canal. Additional symptoms included feeling of fullness and hearing loss. She was treated with antibiotics and managed conservatively but had no improvement of symptoms. Imaging showed a defect in the tegmen, which has a low chance of resolving spontaneously. Therefore, surgery was indicated to close the defect in the tegmen overlying the mastoid air space to resolve symptoms and prevent additional complications.

For tegmental defects, the middle fossa approach is the most common and has a low failure rate. Its advantages include being able to see the entire skull base floor in the event of multiple defects or for the placement of large multilayer grafts, and avoidance of removal of ossicles for repair of tegmen tympani defects.^7,8 This approach has been shown to have low morbidity and high long-term efficacy.^9,10 However, this method is more invasive than other approaches as it involves a craniotomy and temporal lobe and dura retraction. Therefore, these patients have potentially longer recovery times and higher risk of epilepsy from the accompanying craniotomy.⁸

This middle fossa approach to the temporal bone was first described in 1891 by Frank Hartley as a route to the trigeminal ganglion. In 1958, this approach was adapted for neurosurgery procedures and was strictly used in that domain until it became more widespread as an approach for the temporal bone.¹¹ This has now become one of the three main approaches to repairing tegmental defects including: transmastoid, middle fossa craniotomy, or a combination approach.³ Its advantages include visualizing the entire skull base floor in the event of multiple defects, optimal placement of grafts, and avoidance of manipulation in the middle ear region such as removal of ossicles.^7,8 This approach has been shown to have low morbidity and high long-term efficacy.^9,10 Several institutional studies have found a high success rate in repair of CSF leak and prevention of recurrence with rates from 91–100% depending on usage of lumbar drains.^3,9 A systematic review of lateral skull base repairs with middle cranial fossa and transmastoid approaches reported a low overall failure rate of 6.6%.⁷

A general anesthesia with desflurane is required for brain relaxation. After prepping and draping the surgical field, an incision of the temporalis muscle in the pretragal area is required to access the temporal bone for craniotomy. A lazy S-type incision should be performed from the tragal area to the area above the ear in a vertical line tangential to the external auditory canal. The initial incision should be carried down through the subcutaneous layer to the level of the temporalis fascia. The superficial temporal vessels in the region anterior to the ear require careful manipulation and may end up being removed. The root of the zygoma is the inferior limit of the dissection. When the temporalis fascia is exposed, a graft can be harvested, cleaned, and set aside. Repairs often involve either autologous temporalis fascia, autologous bone grafts, or alloplastic grafts to repair tegmental defects. Among autologous grafts, temporalis fascia and muscle are easily available and have high efficacy making them popular choices for grafting. A Silastic block can be used to clean the graft and remove residual attached muscle fibers. This also allows the graft to dehydrate and form a flat surface for ease of manipulation. The temporalis muscle is divided up from the inferior end of the dissection at the root of the zygoma in a curvilinear fashion. A cuff of temporalis fascia on the edge of the muscle may be left to make reapproximation easier at the end of the procedure. The muscle can be elevated anteriorly and posteriorly to expose the temporal cortex for the craniotomy.

Next, the craniotomy is typically performed centered approximately 2/3 anterior and 1/3 posterior to the external auditory canal. A craniotomy bone flap measuring 4x5 cm to 4x5 cm on each side is made with a regular drill and cutting burr followed by a diamond burr when down to thinner eggshell consistency bone. Alternatively, a craniotome can be used to raise the bone flap. The bone should be taken down to the level of the dura with the diamond burr. It is critical to expose the dura at all edges to prevent tears when raising the bone flap. Dural bleeding can be managed with bipolar cautery; however, care should be taken to keep the bipolar parallel. When the elevation of the dura using a freer elevator begins, the floor of the middle fossa plate should come into view. A microscope can be utilized to see translucent areas with thin bone to improve exposure. The dissection should continue along the floor of the tegmen and dura from the petrous ridge, which is the extent of the dissection posteriorly. The dissection should continue anteriorly to the middle meningeal artery and foramen spinosum. Defects in the tegmen over the mastoid area can be visualized with this exposure.

Next, the defect should be measured to estimate the necessary size of the bone graft. A small graft is taken from the inner table of the bone flap. The inner cortex of the bone flap should be removed with just the outer cortex remaining. The graft should be shaped to be smooth on all the edges and contoured for the surface of the tegmen. Next, the temporal fascia graft should be taken and divided into two pieces. One piece should be placed over the defect first. The bone graft should be placed over the first layer of fascia. The second piece of fascia should be placed over the bone graft to secure and sandwich it in place. The upper fascia graft should be advanced to get good coverage of the bone graft as well as the tegmen medial to the bone graft. DuraGen can be utilized to hold down the fascia and support the repair. This will form a three-layer repair of fascia, bone, and fascia supported by DuraGen. Gelfoam can be placed to supplement the bigger gaps in the dura.

Specialized equipment includes:

• Drill system with cutting and diamond burrs.
• Craniotome, if desired.
• Stryker Plating System.
• DuraGen (collagen-based dural graft).
• Intraoperative facial nerve monitoring device (could be used to avoid iatrogenic traumatization of the facial nerve during the surgery).

Author C. Scott Brown also works as editor of the Otolaryngology section of the Journal of Medical Insight.

The patient referred to in this video article has given their informed consent to be filmed and is aware that information and images will be published online.