Pathophysiology

In 1910, Grunwald described a theory for ND formation. Embryogenesis of the midface occurs between 4 and 8 weeks of gestation. At the end of that process, a protrusion of dura mater extends from the anterior cranial fossa through the foramen cecum to the space between the nasal bone and the deep layer of cartilaginous capsule (prenasal space). The dural diverticulum reaches and contacts the skin of the nasal tip. Subsequently, the fusion of the frontal and nasal bones occurs and the dural diverticulum involutes.

Failure during the regression of the dural diverticulum from the skin to the cranium results in a dermoid cyst or sinus. The contact of dura with the skin during embryogenesis may track skin elements from the nasal tip to the anterior cranial base. Proliferation of the trapped elements produces the typical lesion containing glands and hair. As the diverticulum courses from the cranial base to the tip of the nose, the ND can be extranasal, intranasal, intracranial, or a combination. Most commonly, NDs are restricted to the superficial nasal area. In case of intracranial extension, most often there is a communication through the foramen cecum to the anterior cranial base with extradural adherence to the falx.

Clinical

NDs usually present at birth or in early childhood with a cosmetic deformity of the midface. Adult cases have also been reported. Typically it is a midline mass and may be situated from the collumella to the glabella. Sixty percent are located on the lower nasal dorsum; 30% are intranasal, and 10% are combined.

NDs are typically a noncompressible mass and often present with nasal swelling or inflammation associated with a midline pit. Commonly, they end in a single subcutaneous tract with hair at the skin aperture. The presence of hair arising from the skin punctum is pathognomonic of ND. Rarely, multiple ostia are present, and midline cysts may exist at more than 1 level. Intermittent discharge of sebaceous material or pus when infected may be noticed.

Infection complications may occur as recurrent septum abscess and/or osteomyelitis. The tract communicating with the anterior cranial base creates a potential risk for infection to spread intracranially. The patient may present with meningitis or brain abscess.

Differential diagnosis for midline masses includes meningoencephaloceles and gliomas. Meningoencephaloceles consist of herniation of dura mater, cerebrospinal fluid (CSF), and brain matter through a bony defect. Gliomas are characterized by the presence of brain matter extracranially without dural connection. Because of those differences, encephaloceles typically enlarge with increases in intracranial pressure (Furstenburg sign), while gliomas and NDs do not change. Meningoencephaloceles are usually softer and may transilluminate. Unlike NDs, gliomas do not contain skin elements (sebaceous gland secretions and hair). Besides those clinical differences, some lesions may only be differentiated with an adequate investigation.

Clinical

NDs usually present at birth or in early childhood with a cosmetic deformity of the midface. Adult cases have also been reported. Typically it is a midline mass and may be situated from the collumella to the glabella. Sixty percent are located on the lower nasal dorsum; 30% are intranasal, and 10% are combined.

NDs are typically a noncompressible mass and often present with nasal swelling or inflammation associated with a midline pit. Commonly, they end in a single subcutaneous tract with hair at the skin aperture. The presence of hair arising from the skin punctum is pathognomonic of ND. Rarely, multiple ostia are present, and midline cysts may exist at more than 1 level. Intermittent discharge of sebaceous material or pus when infected may be noticed.

Infection complications may occur as recurrent septum abscess and/or osteomyelitis. The tract communicating with the anterior cranial base creates a potential risk for infection to spread intracranially. The patient may present with meningitis or brain abscess.

Differential diagnosis for midline masses includes meningoencephaloceles and gliomas. Meningoencephaloceles consist of herniation of dura mater, cerebrospinal fluid (CSF), and brain matter through a bony defect. Gliomas are characterized by the presence of brain matter extracranially without dural connection. Because of those differences, encephaloceles typically enlarge with increases in intracranial pressure (Furstenburg sign), while gliomas and NDs do not change. Meningoencephaloceles are usually softer and may transilluminate. Unlike NDs, gliomas do not contain skin elements (sebaceous gland secretions and hair). Besides those clinical differences, some lesions may only be differentiated with an adequate investigation.

Investigation

In case of clinical suspicious of ND, radiological imaging is essential to evaluate possible intracranial extension. Magnetic resonance imaging (MRI) seems to be superior compared with computed tomography (CT) scan for evaluation of intracranial extension. However, the combination of both examinations is more precise for a good operative planning. The cuts should be planned to evaluate the entire frontoglabellar region. The use of contrast is valuable to differentiate enhancing nasal mucosa or lesions (hemangiomas/teratomas) from nonenhancing dermoid cysts.

Typically, CT scan shows an enlarged foramen cecum and a bifid crista galli, which are suggestive of intracranial involvement. However, some authors have published that the presence of those findings are not pathognomonic of intracranial extension. The presence of fibrous tissue connecting the dermoid cyst to the dura may provoke an enlarged foramen cecum and bifid crista galli without true intracranial extension of the cyst. MRI is better to analyze soft tissue and consequent presence of intracranial involvement. It is important to note that the crista galli in infants is not ossified, and a high-intensity signal on T1-weighted images in that region should suggest intracranial dermoid.

Nasal endoscopy should be part of the investigation for ND. Nasal endoscopy is usually normal in ND with an extranasal mass. Intranasal lesions may present as a septal submucosal mass protruding from both sides of the septum. The nasal endoscopy is also helpful for the assessment of the degree of nasal obstruction caused by the mass and impact on sinus drainage.

Management of NDs

The treatment of ND is surgical resection. Radiation therapy and chemotherapy are not recommended in the management of these lesions. The goal of surgery is complete resection of the cyst and its tract. Failure to achieve a complete resection of the tract may result in recurrence. Reports in literature vary from 50% to 100% recurrence rate when the cyst/tract elements are not completely resected.

Several external approaches have been described for the treatment of ND (bicoronal or transfacial approaches). As an advantage, these approaches are sufficient to guarantee an adequate resection of extranasal, intranasal, and intracranial lesions. However, they have significant disadvantages: skin scar, alopecia (bicoronal), and brain retraction.

Recently, endoscopic approaches to treat ND have been published. The main advantages are good visualization, better cosmetic results, and avoidance of brain retraction for intracranial lesions. The endoscopic approach to ND can use the nasal cavity as a corridor (endoscopic endonasal) or subcutaneous tunnels through small incisions in the scalp, eyelid, or columella (endoscopic-assisted approaches).