19.1 Introduction

Dermoid and epidermoid tumors are rare congenital non-neoplastic lesions. They are comprised of a capsule made of epidermal and dermal elements with an internal cystic component. The cyst slowly enlarges over time as the desquamated cellular debris from the capsular cells accumulates internally within the cyst. Dermoid and epidermoid cysts are believed to originate from totipotent ectodermal cells remaining in the developing neural tube. They are believed to be due to a defective closure of the neural tube between the third and fifth weeks of gestation.

Epidermoid cysts originate from primitive ectodermal elements. They have a smooth capsule composed of stratified squamous epithelium surrounded by an outer layer of collagenous tissue. ¹ The cysts are filled with soft, waxy, yellow material that is generated by the epithelial lining of the cyst. They are rare and make up 1% of all intracranial tumors and 7% to 9% of cerebellopontine angle (CPA) tumors. ²

Dermoid cysts originate from ectodermal and mesenchymal elements. They have a thick capsule that can present with many different dermal derivatives. Hair follicles, squamous debris, sebaceous, and sweat glands can all be found within a dermoid cyst. Dermoid cysts are most commonly located along the midline of the neuraxis, while epidermoid cysts are more commonly found laterally within the CPA and parasellar cisterns. However, both cysts can occur anywhere along the craniospinal axis.

Dermoid cysts are frequently midline and often present with an associated dermal sinus tract. This is most commonly found at the glabella or rhinion of the nasal dorsum. However, the sinus tract can also be located at the occipital or spinal level. The cutaneous opening is often present with a tuft of hair or a history of drainage. Yellow, clear, or purulent fluid is often expressed from the cutaneous opening, and patients may have a history of infection. Some patients have a cutaneous angioma beneath the skin marking the location of the sinus tract.

Epidermoid cysts are typically located laterally. Their most common location is the CPA and the parasellar cisterns. ^{1 .}As the otic and optic vesicles develop later in early gestation, the ectodermal cells of the future epidermoid cyst are pushed laterally, which may explain their typical lateral location.

Dermoid and epidermoid cysts are characterized by their slow growth rate as the desquamated debris gradually accumulates. Malignant degeneration is extremely rare. ³ Due to their location, they are often associated with compression of the surrounding neural and vascular structures. They can lead to hydrocephalus, infection, aseptic meningitis, and intracranial hypertension. They most frequently present in the third decade of life as their size slowly grows from birth until detection. Epidermoids tend to grow along the cisterns of the CPA and can cause a chronic inflammatory response. This chronic inflammatory reaction may explain epidermoids’ dense adherence to surrounding vessels and nerves found during surgical resection. Dermoid cysts are reported more frequently in childhood. Identification of the associated dermal sinus tract may explain why these lesions are recognized earlier in life than intracranial epidermoid cysts.

Due to the slow growth of the cysts, presenting symptoms can be vague and diverse. Headaches are the most common presenting symptom. Seizures, unilateral hearing loss, cranial nerve deficits, vision changes/loss, and behavioral changes have all been reported. Posterior fossa tumors present with cranial nerve involvement and cerebellar dysfunction. Hydrocephalus and meningitis are also presenting signs in some patients once the tumor has reached a size large enough to cause compression and obstruction of the cisterns.

Epidermoid tumors of the CPA or internal auditory canal typically present with unilateral hearing loss and headaches. In a recent pediatric series of CPA tumors, only 5% were epidermoid lesions and presented with a mean age of 12.8 years. ⁴ CPA tumors can also present with imbalance, vertigo, tinnitus, facial nerve paresis, otalgia, facial pain, or hypoglossal paresis.

On CT imaging, epidermoids and dermoids typically appear as a round/lobulated mass with a density similar to cerebrospinal fluid (CSF; ▶ Fig. 19.1). They are low-density masses that have no enhancement with contrast due to their low vascularity. Ten percent of intracranial epidermoids have calcifications.

On MRI, epidermoids will typically be hypointense on T1 and hyperintense on T2, with restricted diffusion (▶ Fig. 19.2). Internal heterogeneities seen on proton density and fluid-attenuated inversion recovery (FLAIR) images can help distinguish these cysts from arachnoid cysts. ⁵ They can be differentiated from arachnoid cysts on diffusion-weighted images because the epidermoid will appear bright (restricted diffusion) on these sequences (▶ Fig. 19.3). They do not unusually enhance with contrast imaging.

19.2 Management Options: Medical, Surgical, and Adjuvant

Surgical excision has historically been the mainstay of treatment for dermoid and epidermoid cysts. These cysts are benign intracranial lesions that lack radio sensitivity, and therefore surgical resection is the only viable treatment option. Complete resection of the cyst and cyst wall with no damage to vital neurovascular structures is the goal of surgery to avoid local regrowth of the lesion. However, radical resection is not always possible if the cystic capsule is adherent to surrounding nerves, vessels, or brain parenchyma and cannot safely be removed without extensive collateral damage. If complete removal of the capsule wall is not possible due to adherence, then opening of the cyst wall with the removal of its contents must be approached with caution. Surgeons must be attentive to prevent spillage of the cystic material into the subarachnoid spaces. This can lead to aseptic meningitis in the postoperative period. Subtotal resection must be followed with scans for regrowth of the lesion.

The surgical technique for intracranial epidermoid tumors starts with internal debulking (two-handed suction dissection technique) and decompression of the lesion. This provides improved access to the bulk of the tumor with collapse of the tumor capsule. The margin can then be dissected circumferentially from the surrounding structures. Epidermoid tumors tend to expand into the subarachnoid spaces and therefore create a surgical channel that can allow surgical access from one intracranial compartment to another.

Anterior cranial fossa dermoid lesions with intradural extension have traditionally been approached by a frontolateral/unilateral or a subfrontal approach utilizing a bicoronal incision. Transfacial approaches have also been used in the past to resect the cutaneous tract and the intracranial lesion as one specimen.

A retrosigmoid/lateral suboccipital approach has long been used to provide visualization and resection of epidermoid tumors of the CPA. It allows good access in most cases without excessive cerebellar retraction. A translabyrinthine approach can also be used if the patient has severe, unilateral, irreversible sensorineural hearing loss. This approach allows access to the ventral aspect of the posterior fossa, where epidermoid lesions tend to occur. The subtemporal approach has also been described for tumors that extend supratentorially into the middle cranial fossa, but there is an increased risk of temporal lobe damage with this approach. ⁶ With the advent of the endoscope, endoscopic-assisted microsurgery was introduced in the late 1990s to provide improved visualization and resection of these lesions. The endoscope can provide angled views of the tumor and help ensure complete resection of the lesion without the collateral damage caused by excessive neurovascular retraction and manipulation.

Tumor recurrence typically happens over a prolonged period. Since the cysts are made of epithelial cells, they grow at a steady rate of one generation per month. Therefore, symptom recurrence can take 6 to 9 months to occur and is frequently longer than that. Close neuroradiology imaging at 3- to 6-month intervals for the first 2 years is recommended. If there is no substantial growth within that window, imaging can progress to every 12- to 24-month examinations.

Complications related to surgical resection can range from mild to severe. Injury to surrounding cranial nerves, vascular injury (venous or arterial), aseptic meningitis, and CSF leaks can have devastating consequences to the patient. Hypopituitarism, vision loss, and ataxia can also result from radical resection, depending upon the size and local of the tumor. Communicating hydrocephalus can develop postoperatively. Sinus thrombus requiring anticoagulation therapy has also been reported. Minimizing these complications and providing a safe and effective means of resection is the goal of endoscopic endonasal surgery.

19.3 Role of Endoscopic Endonasal Surgery

Endoscopic endonasal surgery allows excellent visualization of epidermoid and dermoid tumors without any retraction of neurovascular or intracranial structures. It provides superior visualization for some tumors and may decrease risk of morbidity. There are no external incisions, and it allows for a faster recovery time. ⁷

The endoscopic endonasal approach provides direct access to dermoid tumors that are located midline in the anterior skull base. The endoscope allows excellent visualization of these midline tumors, and the lesion can be removed piecemeal without spillage into the subarachnoid space, thus preventing postoperative chemical meningitis.

The endoscopic endonasal technique can be used to work along multiple planes. In the sagittal plane, transfrontal, transcribriform, transplanum, transsellar, and transclival approaches can be used to access and resect midline tumors with some lateral extension. For laterally-based lesions, the transpterygoid, transmaxillary, and transpetrous approaches can help resect tumors in the coronal plane. ⁸

The endonasal transclival approach allows the surgeon to approach and resect epidermoid tumors ventral to the brainstem. The surgical corridor consists of opening the sphenoid sinus, drilling the sella and the clivus, and providing direct visualization of the clival dura. Opening of the clival dura and the arachnoid membranes provides direct access to the tumor.

Reconstruction of the skull base is a key part of the endoscopic endonasal approach. The reconstructive options have continued to evolve over the past 15 years. Inlay and onlay grafts as well as vascularized tissue flaps have decreased the rate of postoperative CSF leaks. The use of collagen matrix to close the dural defect space and then placement of an onlay graft followed by vascularized nasal flaps with an outer layer of sealant are the mainstays of skull base reconstruction. Free middle turbinate mucosa grafts can be used as onlay grafts for small defects. Larger defects require a multilayer repair with vascularized tissue. Fascia lata is an excellent choice for either an inlay or an onlay graft to repair a large dural defect. The workhorse of vascularized reconstruction is the nasoseptal flap. This can be rotated to provide coverage from the anterior cranial fossa to the clivus. The use of the nasoseptal flap has been well documented in the pediatric population as a reliable source of vascularized tissue for reconstruction. ^{9 ,​ 10 ,​ 11} A vascularized pericranial flap or lateral nasal wall flap can also be used if there is no tissue available for a nasoseptal flap due to tumor involvement or previous surgery. The placement of an abdominal fat graft between the fascia lata onlay graft and vascularized flap to fill the dead space created by a transclival approach has also been described to decrease the rate of postoperative CSF leaks and minimize the risk of pontine herniation. ^{12 ,​ 13}