Key points
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The frontal sinus is the most anatomically complex of the paranasal sinuses and subject to a great degree of anatomic variations, enough variations, in fact, that they can be used to distinguish 1 monozygotic twin from the other.
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When surgery is indicated, the approach of choice is usually endoscopic. However, even in the age of modern endoscopy and stereotactic image guidance, endoscopic frontal sinus surgery is challenging.
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Recent advances in frontal sinus management include the use of intraoperative stereotactic image guidance, as well as the availability of minimally invasive office-based balloon catheter dilation procedures.
| ABRS | Acute bacterial rhinosinusitis |
| AFRS | Allergic fungal rhinosinusitis |
| AIFRS | Acute invasive fungal rhinosinusitis |
| CRS | Chronic rhinosinusitis |
| CSF | Cerebrospinal fluid |
| ESS | Endoscopic sinus surgery |
| FSDP | Frontal sinus drainage pathway |
Introduction
The frontal sinuses are the most complex of all paranasal sinuses. Their intrinsic variability and their anatomic location have historically made it difficult to access them surgically. Even today, despite the existence of sophisticated endoscopy systems, specialized instrumentation, and stereotactic navigation, the frontal sinuses still command a great deal of respect. This article presents an overview of frontal sinus anatomy, pathology, and principles of management for the major pathologic entities affecting them.
Introduction
The frontal sinuses are the most complex of all paranasal sinuses. Their intrinsic variability and their anatomic location have historically made it difficult to access them surgically. Even today, despite the existence of sophisticated endoscopy systems, specialized instrumentation, and stereotactic navigation, the frontal sinuses still command a great deal of respect. This article presents an overview of frontal sinus anatomy, pathology, and principles of management for the major pathologic entities affecting them.
Anatomy of the frontal sinus
The frontal sinuses are absent at birth; they are visible radiographically as early as age 4. Paralleling general craniofacial growth, frontal sinus expansion peaks during adolescence and stops by around age 19. Unilateral or bilateral aplasia is seen in about 5% of people.
Frontal sinus anatomy is highly variable: in fact, it is unique to each individual, such that even monozygotic twins can be distinguished from each another on the basis of their frontal sinus configuration alone. Indeed, frontal sinus anatomy is regarded as a reliable means of personal identification in the forensic sciences.
The frontal sinuses are paired asymmetric structures separated by a bony septum (the intersinus septum). The greater part of their volume lies within the anterior aspect of the frontal bone and is bounded by anterior and posterior tables. Patients with marked pneumatization may have a pronounced lateral recess, which can be difficult to reach endoscopically. Inferiorly, each sinus narrows into an obliquely oriented transition point known as the frontal ostium, which, in the sagittal plane, is bounded by the nasofrontal buttress (also referred to as nasal or frontal beak) anteriorly and by the skull base posteriorly. The frontal ostium marks the superior limit of what is known as the frontal recess. Although this term is in common usage, it is somewhat imprecise and difficult to define anatomically, and some have argued for a more inclusive and accurate alternative, the frontal sinus drainage pathway (FSDP).
The FSDP can be divided into superior and inferior compartments. The superior compartment lies between the anteroinferior frontal bone and the anterosuperior ethmoid bone (ie, slightly above the agger nasi cell and ethmoid bulla), and it may or may not contain air cells. The inferior compartment is either the ethmoid infundibulum (if the uncinate process attaches to the skull base) or the middle meatus (if the uncinate attaches to the medial orbital wall). A variety of anatomic structures and factors can influence the patency of either compartment; these include: agger nasi cell (presence and degree of pneumatization); supraorbital ethmoid cells; frontal cells (types 1 through 4); suprabullar cells; and interfrontal sinus septal cells ( Fig. 1 ). Surgical anatomy is reviewed in greater detail by Folbe AJ, Svider PF, Eloy JA: Anatomic Considerations in Frontal Sinus Surgery , later in this issue.
Overview of frontal sinus pathology and management
This section presents a brief overview of the main pathologic processes that affect the frontal sinus, with an emphasis on management principles. Most of these entities affect the frontal sinuses less commonly than the remainder of the sinonasal tract; in such cases, the topics are discussed broadly, since pathophysiology, workup, and management do not differ much.
Acute Rhinosinusitis
Acute rhinosinusitis is defined clinically as the presence of up to 4 weeks of purulent rhinorrhea accompanied by either nasal obstruction, facial pain/pressure/fullness, or both. In the early phase of the disease, the etiology is presumed to be viral; however, if there is failure to improve within 10 days, or if there is worsening of symptoms after an initial improvement, then a diagnosis of acute bacterial rhinosinusitis (ABRS) is made. Acute frontal sinusitis is considerably less common than sinusitis of the maxillary and ethmoid sinuses; however, it is difficult to evaluate this given that radiographic imaging is not routinely indicated in the management of this disorder. Frontal sinus involvement is most common in adolescent boys and young men, presumably due to the peak vascularity and development, which occurs between ages 7 and 20 years; the reason for the apparent gender predilection remains unclear.
The treatment of uncomplicated ABRS (defined as ABRS without evidence of extension outside of the sinonasal tract) is pharmacologic. Initial antibiotic selection is empiric, aimed at covering the most common bacterial pathogens, namely, Streptococcus pneumoniae , Hemophilus influenzae , and Moraxella catarrhalis . Complicated ABRS is often polymicrobial in etiology. Treatment is with broad-spectrum intravenous antibiotic therapy and often surgical intervention to address the complication and/or the sinonasal tract pathology. Recently, the Streptococcus anginosus group (also known as the S milleri group or group F streptococci) has emerged as an important group of pathogens responsible for severe suppurative complications of rhinosinusitis. In one study, S milleri was isolated in 67% of cases of sinogenic intracranial abscess in children. Medical management of ABRS is discussed in further detail by Sohal M, Tessema B, Brown SM: Medical Management of Frontal Sinusitis , later in this issue.
Chronic rhinosinusitis
Chronic rhinosinusitis (CRS) is defined as inflammation of the sinonasal tract lasting more than 12 weeks. Although this definition has a certain practical utility, it fails to capture the complexity of CRS. Most forms of CRS fall into one of two categories: CRS with polyposis or CRS without polyposis. However, CRS may also represent a common endpoint for a variety of systemic disorders. In discussions of CRS management, the concept of maximal medical therapy is often cited as the threshold beyond which surgical management is indicated, but a universal definition of this concept does not exist. In a 2007 survey of 388 American Rhinologic Society members, Dubin and colleagues found that oral antibiotics (a 3–4-week course) and nasal steroids were the only 2 modalities that greater than 90% of respondents considered part of their standard regimens. Oral steroids, saline irrigation, and allergy testing were employed less consistently (50%–90%).
Clinical and radiographic assessments after completion of medical therapy determine the persistence or resolution of paranasal sinus disease. Just as with all sinonasal disease, high-resolution computed tomography (CT) is essential to the evaluation of frontal sinus disease. CT may serve a diagnostic purpose in revealing unfavorable FSDP or frontal sinus anatomy, and it also serves as a roadmap. In some cases, frontal sinus disease may be attributable to outflow pathway obstruction at the level of the anterior ethmoid sinuses, and management of the anterior ethmoid air cells can be sufficient. In other cases, cannulations of the FSDP and balloon dilation are sufficient to manage limited disease. However, in many cases, surgical management of the frontal sinuses requires a formal frontal sinusotomy or a more extended dissection (such as a Draf III procedure). Evaluation and decision making are discussed in greater detail by Saini AT and Govindaraj S: Evaluation and Decision Making in Frontal Sinus Surgery , in this issue; medical management is reviewed by Sohal M, Tessema B, Brown SM: Medical Management of Frontal Sinusitis , later in this issue.
Complications of Frontal Sinusitis
Complications of rhinosinusitis are uncommon, occurring in only 1% to 3% of all cases. In the preantibiotic era, postseptal orbital complications were associated with a high incidence of vision loss (as high as 20%) and intracranial complications with high rates of mortality (up to 17%).
Orbital complications of rhinosinusitis may arise by direct extension or via retrograde thrombophlebitis. Several modifications have been made to the original classification scheme proposed by Chandler and colleagues in 1970. In spite of key differences, they share at least 2 common features: preseptal involvement represents the least severe of the complications, and cavernous sinus thrombosis (CST) represents the most severe. In the Groote Schuur Hospital classification, CST is considered an intracranial complication. The rest are organized thus, in approximate order of severity:
- 1.
Preseptal disease (either cellulitis or, less commonly, eyelid abscess)
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Postseptal extraconal involvement (either subperiosteal phlegmon/cellulitis or abscess) ( Fig. 2 )
Fig. 2
Contrast-enhanced coronal CT showing a right-sided acute bacterial rhinosinusitis (frontal, ethmoid and maxillary) complicated by an orbital subperiosteal abscess ( arrow ).
- 3.
Postseptal intraconal involvement (either cellulitis or abscess)
- 4.
Orbital abscess (either localized or diffuse)
Preseptal cellulitis generally responds quickly to systemic broad-spectrum antibiotics. Theoretically, a diagnosis of preseptal cellulitis can be made on purely clinical grounds, therefore obviating the need for CT scanning; however, in practice, it is common to obtain CT scans with any suspected orbital complication, and patients will often have had imaging by the time ophthalmologic consultation is requested. Surgical therapy in preseptal cellulitis is considered only in cases of antibiotic failure or overt disease progression.
By contrast, postseptal complications often require surgical intervention. Surgical indications include abscess formation, signs of severe orbital disease (eg, vision loss or ophthalmoplegia), and failure to respond to appropriate antibiotic therapy. When indicated, surgery should address orbital and sinonasal disease simultaneously. Depending on the specific complication, orbital disease may be approached via open orbitotomy, or, in some cases, via a transnasal endoscopic approach (eg, in the case of a medially located subperiosteal abscess). Historically, sinonasal disease was approached via external frontoethmoidectomy or frontal sinus trephination. The treatment of choice today is endoscopic sinus surgery (ESS). However, it should be noted that acute sinonasal inflammation may make endoscopic frontal sinus surgery quite difficult, and open techniques remain an option.
Intracranial complications of rhinosinusitis can lead to severe morbidity or even death. These include meningitis, epidural abscess, subdural empyema ( Fig. 3 ), brain abscess, cavernous sinus thrombosis, superior sagittal sinus thrombosis, and frontal bone osteomyelitis. In a patient with rhinosinusitis, the presence of fever and acute or progressive headache may herald the onset of an intracranial complication; however, in some cases, it may not be clinically evident until more severe symptoms (such as neurologic deficits or changes in mental status) arise. In addition to broad-spectrum antibiotics, management of intracranial complications usually involves surgical intervention. The sinonasal infection and intracranial complication should be addressed simultaneously. Ideally, sinonasal surgery should clear all infectious material from the frontal sinus and establish adequate/safe outflow. However, given a choice between the 2 goals, priority should be given to drainage; hence, trephination remains a viable temporizing option if formal frontal sinusotomy is not feasible.
