2.2 Management strategies
This study series was divided into 3 groups based on management strategies, namely, the observation group, the surgery group, and the optic nerve decompression group.
The observation group included 9 patients with asymptomatic MFD, and adopted the method of clinical observation and regular fellow-up. The patients received a paranasal sinus CT scan periodically. The clinical data were shown in Table 1. Three of the nine patients had chronic rhinosinusitis, who received functional endoscopic sinus surgery (FESS) only for their chronic rhinosinusitis. The other six patients were found with FD on physical examination. Two of them received a biopsy through an endoscopic endonasal approach.
The surgery group included 10 patients with symptomatic MFD. The clinical data were shown in Table 2. Five of the patients underwent FESS for their chronic rhinosinusitis simultaneously. FD was relatively hard and was removed by an electric drill (Medtronic IPC, USA)and Kerrison Rongeurs. Seven of the cases were excised with the aid of image-guided navigation (Fusion, Medtronic ENT, USA) (Fig. 3C, 3D).
The optic nerve decompression group included 4 patients with vision loss, who underwent endonasal endoscopic optic nerve decompression with the aid of image-guided navigation. The clinical data were shown in Table 3. One of the patients received bilateral endoscopic optic nerve decompression simultaneously. The MAS patient accepted the first surgery of left endoscopic optic nerve decompression in February 2016 and again in July 2017 (Fig. 3A and 3B).
Result
The patients in the observation group were followed up regularly. For example, in case one in table 1 the paranasal sinus CT scan in June 2008 showed ivory high-density pattern sphenoid bone FD, then the patient received endonasal endoscopic biopsy and was diagnosed as FD by histopathological examination. He was asymptomatic and followed up regularly. The paranasal sinus CT taken in 2008 was compared with the paranasal sinus CT reviewed in 2016, which revealed that the FD was in a static state (Fig.1).
In the surgery group, five patients with ethmoid bone FD underwent total resection, while three patients with sphenoid bone FD underwent subtotal resection and two patients underwent partial resection through an endonasal endoscopic approach. Five of the patients underwent FESS for their chronic rhinosinusitis simultaneously. The symptoms of the patients were relieved after surgery, and there were no intracranial or intraorbital complications. The patients in this group were followed up for 1-13 years, and there was no recurrence.
During the operation, we found that although some MFD lesions were large, neither the germinal center nor the pedicle was large. A thin layer of fibrous tissue separated FD from normal bone, and it was relatively safe to dissect along this layer of fibrous tissue. For example, case three in Table 2 is a two-year-old boy. The patients’ paranasal sinuses CT scan showed that the left ethmoid sinus and nasal cavity were filled with a large (40.5mm×6.3mm×29.9mm) and highly dense mass (Fig. 2). The mass originated from the left cribriform plate, compressed the left lamina papyracea, and extended to the bottom of the nasal cavity. The frontal sinuses and sphenoid sinuses were undeveloped. The thin layer of fibrous tissue was found between the FD and the partial cribriform plate and lamina papyracea during the surgery (Fig. 2). This phenomenon was also seen in the resection of sphenoid bone FD (Fig. 3C).
In addition, MFD may have degenerative changes with age, as shown in case 4 and case 5 (Fig. 3D) in table 2. The paranasal sinus CT of case 5 revealed frosted glass changes of the left sphenoid pterygoid process and great wing, with a 2.3× 1.0 cm soft tissue shadow. The pterygoid process and great wing change was diagnosed as FD and the ‘soft tissue shadow’ change was determined to be hyaline degeneration by pathological examination.
In the optic nerve decompression group, the effects of optic nerve decompression were shown in Table 3. The youngest patient in this group, a 10-year-old girl with MAS, showed left visual loss again one and half a years after her first left optic nerve decompression. The patient received left optic nerve decompression again, and the left visual acuity remained stable so far. However, the CT showed that the right optic nerve was encased by FD, but the patient’s right eye vision remained stable all the time (Fig.3A and 3B). Case 1 in Table 3 underwent bilateral optic nerve decompression due to cystic degeneration from FD (Fig. 3E). The CT scan of 2 years before surgery revealed that the left optic nerve was completely encased and the right optic nerve was partially encased by FD (Fig. 3F, G, H), but the vision was normal until 1 month before the surgery.
Discussion
In view of the rarity, complex and varied natural history, and clinical manifestation of FD, it is difficult for clinicians to develop a standardized treatment plan. As a consequence, no treatment guidelines currently exist 1. Although there was not an international guideline for the management of FD/MAS, an international workshop has been set up to focus on improving FD/MAS management and understanding the importance of a multidisciplinary team for the lifetime management of FD/MAS 7.
FD treatment includes clinical observation, medical therapy and surgery. Medical treatment for FD is limited to relieving a patient’s symptoms. One of these medications is bisphosphonate, which may help to improve the function, relieve the pain, and reduce the fracture risk. However, a recent randomized, double blind, placebo-controlled trial disputed these effects 8,9. Radiotherapy is contraindicated owing to high prevalence of malignant transformation. Therefore, clinical observation and surgery are currently the main treatment strategies for FD.
Clinical observation is adopted for asymptomatic FD patients, but the patients should be required to have periodic radiologic evaluations. As FD is a slow-growing lesion, tends to be stable after puberty, and has a low malignancy potential, most of the specialists recommend clinical observation if there are no symptoms, but periodic imaging should be performed to confirm that there is no progression or regrowth in follow-up 1. Comparing craniofacial FD management between 1980 and 2002 with management between 2003 and 2013 showed that observation has replaced surgery as the most used method. Watching carefully and attentively was indicated in cases of stable lesions, and it was the best therapeutic option, if possible 4. In France, with the establishment of National Reference Centers, 57 specific recommendations have been provided for the diagnosis, prognosis, and follow-up of patients with FD/MAS. If the skull and/or facial bones are involved, a skull CT is recommended to accurately evaluate the risk of neurological compromise due to alterations of the foramina. MRI should be viewed as a second-line imaging study. Radiographic monitoring is recommended every 2-3 years for follow-up patients 10. It was demonstrated that the characteristics of FD on CT and the natural radiographic progression may vary from a “ground-glass” or homogenous appearance to a mixed radio-dense/radio-lucent lesion as the patient ages 2. Based on preoperative radiology, the sensitivity and specificity to correctly detect FD were 54.6% and 96.9%, respectively.
Lesions with classic ground glass appearance or mixed pattern on CT should not warrant diagnostic biopsy, especially in asymptomatic patients. However, if radiologic diagnosis is uncertain, biopsy may be warranted to arrive at the definitive diagnosis based on radiographic and histologic correlation 11. Biopsy of a lesion does not specifically induce growth of FD, but if the lesion is asymptomatic, and/or in the cranial base, a biopsy may not be necessary2.
Surgery is indicated in symptomatic patients. Although application of surgical treatment for craniofacial FD is controversial, many publications have provided views on the surgical treatment of FD12-14. Comparing craniofacial FD management between 1980 and 2002 with management between 2003 and 2013 revealed that radical resection (if possible) of FD was the only technique to obtain resolution of the disease 4. The advantages of surgical treatment of FD should be appropriately weighed against possible complications 1. The aim of the surgical treatment for craniofacial FD is to remove the bulk of the lesion, reserve the cranial nerve compression, and resolve the aesthetic problem6. The extent of the resection should be based on the location of the pathological bone and its proximity to important structures, as radical or complete resection may not be necessary or possible 5. Treatment protocols should be tailored to individual patient’s needs, with the aim to achieve the best possible esthetic and functional outcome with the least postoperative morbidity15-16. Therefore, surgical treatment planning must take into account several factors (i.e., natural history of the disease, presence of symptoms, site of the lesion, and the relationship with critical anatomic structures) 1.
Early surgery is necessary for the patient with MFD when the lesion was limited to ethmoid or sphenoid bone with obvious symptoms. It is because the growth and expansion of the FD may lead to the obstruction of the sinus ostia and result in rhinosinusitis and mucocele. Furthermore, it has been reported that the lesion may oppress the adjacent bone, such as the lamina papyracea and the anterior skull base, extend into the orbit and intracranial cavity, and ultimately lead to epiphora, diplopia, proptosis, impairment of visual acuity, meningitis, cerebrospinal fluid leak, and so on 17. Five patients with ethmoid bone FD and three with sphenoid bone FD in the surgery group of this study series underwent total resection or subtotal resection through an endonasal endoscopic approach respectively. Only two patients with sphenoid bone FD underwent partial resection through an endonasal endoscopic approach. And five surgery group patients underwent FESS for their chronic rhinosinusitis simultaneously. Meanwhile, what we found in these procedures was that in the process of FD expansion toward the cavity of a paranasal sinus, the epithelial tissue in the sinus cavity may be compressed into a thin layer of fibrous tissue, which separated FD from normal bone, and it was relatively safe to dissect along this layer of fibrous tissue. Combined with use of intraoperative image-guided navigation and advanced surgical instruments, these surgeries were performed successfully without intracranial or intraorbital complications.
For FD patients with impaired vision, it was recommended to have optic nerve decompression as soon as possible. However, if FD was encroaching on the optic nerve without impaired vision, prophylactic decompression is not recommended. This is the current consensus1,18-20. A retrospective analysis in 91 patients with craniofacial FD involving the optic nerves showed that 17% of nerves were less than 50% encased, 22% were 50-99% encased, and 61% were 100% encased. Yet optic nerve decompressions were performed in only 13 patients (6 prophylactic and 7 therapeutic) since the authors regarded that the majority of optic nerves encased with FD did not present symptoms of optic neuropathy and appeared to be stable over time18. Satoh K, et al. held that current strategies should focus on esthetic improvement, with careful observation carried out to assess for optic canal encroachment without prophylactic decompression 19. In another study, in asymptomatic patients, stable vision occurred in 76% of patients receiving decompression and 95% of patients not undergoing surgery (P< 0.001). Vision impairment may be associated with the concomitant presence of cystic lesions (i.e., mucocele, hemorrhage, and aneurysmal bone cyst) 20. Holl DC, et al. reported a case of FD in which the patient developed an aneurysmal bone cyst leading to left optic nerve compression with an acute visual loss. An emergency optic nerve decompression resulted in complete restoration of vision. 21. In this study, one patient had acute bilateral visual loss due to cystic degeneration formation. Another patient with MAS underwent left optic nerve decompression twice, but in fact, although the CT showed that the right optic nerve was encased by FD, the right eye vision of the patient remained stable all the time.
Navigation plays an important role in paranasal sinus and adjacent skull base FD resection, and in optic nerve decompression. Stereotactic navigation was recommended, as the FD/MAS process often distorted normal intranasal landmarks used in sinus surgery 5. Navigation assisted, endonasal endoscopic optic nerve decompression was usually effective for the treatment of nontraumatic optic neuropathy22.
Conclusion
Clinical observation is adopted for asymptomatic patients with FD, but the patients should be required to undergo periodic radiologic evaluations. Surgery is indicated in symptomatic patients. In FD patients with visual change or vision loss, an optic nerve decompression was recommended as soon as possible; however, if FD is encroaching on the optic nerve without impaired vision, prophylactic decompression is not recommended. Navigation plays an important role in paranasal sinus and adjacent skull base FD resection and optic nerve decompression.