Reconstruction of Frontoethmoidal Encephalocele Defects

… nasofrontal lesions are uncommon, frequently diagnosed incorrectly, and treated in a haphazard fashion. —(Davis and Alexander, 1959)

Encephaloceles, historically termed cranium bifidum, were first documented in the medical literature in the sixteenth century. They are a result of a congenital defect in the skull base that allows the brain to herniate through the cranial floor. Encephaloceles are most commonly located in the posterior skull base (75%); however, anterior (sincipital) encephaloceles are considerably more deforming. The herniated cerebral contents may contain the meninges alone (meningocele), meninges and brain (meningoencephalocele), or in severe cases meninges, brain, and part of the ventricular system (hydroencephalomeningocele or meningoencephalocystocele).

There are many types of encephaloceles.

  • Classification of encephaloceles

  • Frontoethmoid encephaloceles

    • Nasofontal

    • Nasoethmoidal

    • Nasoorbital

  • Cranial vault encephaloceles

    • Interfrontal

    • Anterior fontanel

    • Interparietal

    • Posterior fontanel

    • Temporal

  • Skull base encephaloceles

    • Transethmoidal

    • Sphenoethmoidal

    • Transsphenoidal

    • Frontosphenoidal

  • Occipital encephaloceles

  • Cranioschisis

    • Those associated with cranial/upper facial clefts

    • Those associated with basal/lower facial cleft

    • Occipitocervical clefts

    • Acrania and/or anencephaly.

  • Suwanwela classification of anterior encephaloceles

  • Frontoethmoidal

    • Nasofontal

    • Nasoethmoidal

    • Nasoorbital

  • Interfrontal

  • Encephaloceles associated with craniofacial clefts.

  • Suwanwela classification of anterior encephaloceles

  • Frontoethmoidal

    • Nasofontal

    • Nasoethmoidal

    • Nasoorbital

  • Interfrontal

  • Encephaloceles associated with craniofacial clefts.

Presentation

Frontoethmoidal encephalocele have several hallmark findings on physical examination. A nasal mass located in the midline is the most common finding. It is often bluish in appearance, soft, compressible and often pulsatile as a result of its intracranial connection. They transilluminate and increase in size with valsalva, crying, or with compression of the internal jugular veins (Furstenberg test). Ortiz-Monasterio and Jackson independently popularized the terms long nose hypertelorism and long nose deformity to describe the nasal anatomy associated with frontoethmoidal encephaloceles. Nearly all patients present with a long, flat, wide nose that is more pronounced after the encephalocele is excised. In addition, there is a very specific orbital presentation in these patients. Telecanthus and interorbital hypertelorism are universal phenotypical findings associated with nasal encephaloceles but true orbital hypertelorism is rare. Some degree of deformational trigonocephaly is usually present. Other physical findings include medial eyebrow elevation, lacrimal obstruction, microencephaly, hydrocephalus, and anosmia.

Pathogenesis

To date, there is no known genetic mutation associated with the inheritance of the encephalocele trait. However, the incidence of frontoethmoidal encephaloceles is significantly higher in southeast Asia (1 in 6000 live births) compared with its incidence in Europe and North America (1 in 35,000 live births). One postulated theory for encephalocele formation is the primary failure of bone formation at the skull base allowing the brain to herniate. Alternatively, encephaloceles may result from adhesions between the brain, dura, and skin, which arrest bony development at the skull base. The most recent explanation for encephalocele development is increased intracranial pressure pushing the brain through the skull base and arresting bony development.

Embryologically, the facial primordial develops during the first 12 weeks of gestation. Between the third and the fourth week of fetal development, neural tube closure begins midembryo and propagates both cranially and caudally. As the neural tube closes cranially, neural crest cells migrate laterally and anteriorly into the frontonasal and maxillary processes. These cells are responsible for the mesenchymal structures such as the bones, cartilage, and muscles of the face. Several centers of mesenchymal structures fuse and ossify to form the skull base as well as the nasal and frontal bones. Potential spaces exist between these centers before their fusion and these spaces are important in the development of congenital midline nasal masses. These spaces consist of the fonticulus frontalis (space between the nasal and frontal bones), the prenasal space (between the nasal bones and the nasal capsule and cartilages/septum), and the foramen cecum (anterior neuropore at the anterior cranial vault floor). Abnormal development of these structures is believed to be responsible for the development of different midline nasal masses ( Fig. 1 ).

Fig. 1
Midline nasal mass pathophysiology. ( A ) Encephalocele: neural contents herniate through the foramen cecum. ( B ) Glioma: trapped neural contents with an intact skull base. ( C ) Dermoid: invagination of skin elements toward the skull base.

Differential diagnosis

Knowledge of embryology and craniofacial surgery is necessary to develop a comprehensive differential diagnosis and treat a child with a midline nasal mass. An understanding of how midline entities present is important in differentiating other midline nasal entities from encephaloceles. Nearly all are present at birth and are slow-growing masses at the nasal root. However, there are many differences among congenital midline masses that allow the correct diagnosis to be made before radiographic confirmation.

Nasal dermoid cysts are the most common midline nasal mass. They consist of ectodermal and mesodermal elements. Dermoids usually present at birth but are often not diagnosed until later in childhood. The hallmark of a nasal dermoid is a pit or punctum with a single hair located on the nasal dorsum. They can often become infected and drain sebaceous material. Intracranial connection is rare but cannot be ruled out on clinical examination. Therefore imaging is required to evaluate for intracranial extension.

A nasal glioma presents as a firm rubbery mass with a bluish or reddish appearance. They are composed of neurogenic material consisting of glial cells in a connective tissue matrix. The overlying skin usually contains capillary telengectasias and the mass often extends intranasally through the nasal bones. There is no cerebral spinal fluid (CSF) surrounding these masses as they do not connect with the subarachnoid space. Therefore, they are not pulsatile and do not transilluminate. Intracranial extension is uncommon but may occur through the cribiform plate or at the frontonasal suture.

Less common entities that may occur at the nasal midline include hemangioma, vascular malformation, teratoma, sebaceous cyst, neurofibromata, ganglioneuromata, nasal fibroma, adenoma, carcinoma, and chondroma. A thorough history and physical examination is required for the correct diagnosis. Imaging is used to confirm the diagnosis and to rule out intracranial extension. Biopsies before complete work-up are discouraged.

Surgical management

There are several principles in frontoethmoid encephalocele reconstruction. First, when present, any open skin defects must be debrided and closed immediately to prevent infection and/or desiccation of exposed brain tissue. Second, all nonfunctioning extracranial brain tissue herniating through the skull base must be debrided and/or reduced intracranially. Third, the dura should be repaired to provide a watertight closure around the viable cerebral contents. Fourth, the skull base defect is repaired to prevent future herniation and encephalocele recurrence. Fifth, craniofacial, nasal, and medial canthal reconstruction is performed to restore the appropriate premorbid anatomy.

Preoperative Assessment

A multidisciplinary craniofacial team is necessary for the appropriate management of these patients. Members of this team typically include a craniofacial surgeon, neurosurgeon, otolarynoglogist, geneticist, and general pediatrician. Often the diagnosis is clearly evident on physical examination and biopsy should be avoided. A fundoscopic examination should be performed to rule out papilledema caused by increased intracranial pressure as a result of hydrocephalus.

Computed tomography (CT) is the most useful imaging modality for assessing this abnormality. CT images should include both brain and bone windows in the axial, coronal, and sagittal planes as well as three-dimensional reconstructions of the cranium and skull base defect. Magnetic resonance imaging is also useful to evaluate nasal masses as it provides more detailed soft tissue information. Ultrasound may be used to assess ventricular size to rule out hydrocephalus, but is often redundant if a CT scan can be performed as part of the initial evaluation.

Surgical Positioning

The operation is performed in the supine position. A Mayfield headrest is used to secure the cranium. The pins of the headrest are positioned posteriorly so that they do not interfere with the coronal flap or parietal bone graft harvest. The endotracheal tube is secured using a circum-mandibular 28-gauge dental wire. The entire head and face are prepared into the surgical field. A 6-0 silk tarsorraphy suture is used for eyelid closure and corneal protection.

Coronal Exposure

The transcranial portion of the case is performed via a coronal incision ( Fig. 2 ). A wavy-line pattern is used to break up the scar, making it less noticeable when the hair is wet or short. Before incision the scalp is infiltrated with 0.5% to 1% lidocaine with 1:200,000 epinephrine. A #15 blade scalpel is used to make the skin incision. Dissection using a Colorado needle tip cautery is done deep to the dermis. This protects the hair follicles and alleviates the need for hemostasis clips on the coronal flap skin edges. The dissection proceeds through the pericranium to the cranial bones and the coronal flap is elevated anteriorly. A pericranial flap based on the supraorbital vessels should be preserved during the coronal flap elevation. This flap may be necessary to assist in dural closure after encephalocele resection. If the pericranial flap is necessary for dural closure, we prefer to raise it secondarily from the underside of the coronal skin flap.

Jan 23, 2017 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Reconstruction of Frontoethmoidal Encephalocele Defects
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