Optic Nerve Decompression in Craniofacial Fibrous Dysplasia Involving Optic Canal: An Experience at AIIMS, New Delhi

Background: Gradual progression of craniofacial fibrous dysplasia frequently involves sphenoid and ethmoid bone results encasement of optic canal which leads to visual impairment to complete blindness Methods: Retrospective study was carried out at All India Institute of Medical Sciences on hospital records of 16 cases of craniofacial fibrous dysplasia operated for optic canal involvement with or without visual impairment with an objective to discuss about the indications, suitable timing and outcome of optic nerve decompression in cases of optic canal involvement. Results: Out of 16 cases, 56.25% were female with mean age 13.06 years. 15 patients had Idiopathic Fibrous dysplasia and 01 had Cranio-metaphyseal dysplasia. 62.5% were polyostotic and 37.5% were monostotic with 62.5% of optic neuropathy. Bilateral lesion occurred in 03 patients. Optic nerve decompression was done for curative treatment in 62.5% and for prophylactic in 37.5% cases. Post-operative visual improvement occurred in 90% patients and in no patient vision was deteriorated. Conclusion: Prophylactic decompression can be carried out in expert hand for involvement of optic canal in craniofacial fibrous dysplasia for prevention of future visual impairment.


Introduction
Fibrous dysplasia(FD) is a benign, slowly progressive bone disorder of unknown aetiology where normal bone is replaced by various degrees of fibrous tissue and immature woven bone 1 . It presents in childhood or early adolescent 2,3 in which progression typically arrests at puberty 4 . FD is caused by somatic activating mutations in the subunit of the stimulatory G protein encoded by the gene GNAS á 5,6 . It appears to arise from a perturbation in the mesenchymal precursor of bone, producing a defect in osteoblastic differentiation and subsequent maturation of bone 7 It is a relatively uncommon, nonfamilial congenital disorder of bone 3 and the disease usually manifest before 3 rd decade of life 8 . Patient may present with involvement of single bone (monostotic; MFD), multiple bones (polyostotic; PFD) or McCune-Albright Syndrome(MAS). MAS has been classically defined as a triad of PFD, café-au-lait skin macules and endocrinopathies (precocious puberty, hyperthyroidism or acromegaly) 9 . Malignant change may occur as osteosarcoma or other forms of sarcoma in less than 1% of cases of FD 10 .
Occasionally, in young children and prepubertal adolescents, the lesions may grow rapidly with cortical bone expansion and displacement of adjacent structures such as the eye and the teeth. It frequently affects anterior skull base and encase optic canal. Craniofacial ûbrous dysplasia may develop within bone adjacent to the optic canal, grow gradually, and compress the optic nerve leading to visual disturbances 3,11 . Optic canal involvement causes compression of optic nerve which results progressive venous congestion and subsequent intraneural oedema 13 .The cause of both acute and gradual loss of vision in patients with ûbrous dysplasia remains unclear and the pathological course is unknown.
A clinical staging had been suggested by Barrionuevo  There is different opinion regarding prophylactic decompression of the optic nerve. However, when visual impairment begins it tends to be progressive 1,12 and may continue after puberty 13 . Prophylactic decompression is sometimes performed based on the assumption that the risk of future optic neuropathy outweighs the risks of the operation which include postoperative blindness 13,14 . Use of high speed suction drill, continuous irrigation, ultrasonic bone curette has reduced the risk of postoperative visual loss recently.
There is inadequacy of study regarding the absolute indications, timing and outcome of surgery in cases of optic canal involvement in FD. The objective of the study is to discuss about the indications, suitable timing and outcome of optic nerve decompression in cases of optic canal involvement with or without features of optic nerve compression in fibrous dysplasia in light of our experience at All India Institute of Medical Sciences, New Delhi.

Methods
Retrospective study was carried out on hospital records at All India Institute of Medical Sciences, New Delhi operated for Craniofacial Fibrous Dysplasia between 1998 to 2015. 16 patients were included in this study who were clinically diagnosed and radiologically confirmed cases having involvement of optic canal. Patients who had no features of optic nerve compression but the lesion has involved the optic canal in CT scan were also included. Diagnosis was confirmed by histopathology in all cases. Skull base FD without involvement of optic canal, unfit for general anaesthesia and unwilling for operation were excluded from this study.
All cases were evaluated by neuroophthalmologist for visual analysis. Visual acuity by Snellen's chart test, visual field by Goldman perimeter in cases of visual impairment (equal to or better than 1/60) and colour vision by Ishihara colour plate were carried out. Fundoscopy were done in all cases of visual impairment. High resolution CT scan with minimum 2mm thickness for optic nerve and optic canal were carried out. All patient underwent testing of all relevant endocrine axes. Musculoskeletal radiology was considered only for suspected cases. Follow up carried out 3 monthly intervals in initial post-operative period, followed by yearly basis. During follow up craniofacial, necessary radiological and ophthalmological evaluation was recorded.
During surgery, high speed drill with suction and irrigation were used for prevention of traumatic or thermal injury to optic nerve. Debulking or shaving and optic nerve decompression through trans ethmoidal route was the mainstay of all forms of surgery. Moore's incision, Moore's with lynch extension, Lateral Rhinotomy and modification of these incisions were used in external and combined approaches according to the extension of disease.

Results
16 patients with Craniofacial FD with or without visual impairment was operated for optic canal involvement over past 18 years. In 16 patients 19 optic nerve were decompressed. Among those 09 (56.25%) were female and 07(43.75%) were male. Age ranges from 05 years to 35 years with mean age 13. 12 out of 16 (75%) were 20 years or below and 4(25%) were above 20 years of age. 15 patients had Idiopathic FD and 01 had Cranio-metaphyseal dysplasia.
In regards of bone involvement 10 were polyostotic(62.5%) and 06(37.5%) were monostotic. In PFD 01 was associated with cranio-metaphyseal dysplasia. All patient presented with mild to moderate headache, 10(62.5%) with orbital and or periorbital pain with gradual visual impairment. Clinically 10(62.5%) had decreased visual acuity, 08(50%) had features of optic neuropathy at Fundoscopy, 09(56.25%) had proptosis, 07(43.75%) had facial swelling or deformity. 01 presented with mouth breathing, widely separated eye, torsional nystagmus and nasal deformity. Presenting symptoms are given in Table-I. In HRCT scan, all cases had optic canal involvement but optic nerve compression were present in 13 eyes in 10 patients (62.5%). These all patients presented with gradual visual impairment from a duration of 12 weeks to 12 months. 06 patients (37.5%) had no optic nerve compression due to narrowing of optic canal in HRCT scan and they also had no clinical features of it but dysplastic foci involved the optic canal and had potential risk of optic nerve compression in future.
Optic nerves were decompressed in all cases. Endoscopic approach was followed in 05, External approach by Lateral Rhinotomy in 09 and combined approach in 02 cases. Image guided navigation was utilized in all 2 combined approaches. Per operative CSF leak occurred in 01 case which was immediately closed by soft tissue graft and fibrin glue. He was also managed by lumber drain, mannitol, antibiotic and Eptoin. During post-operative period vision improved in 11/13(84.6%) cases. In no cases vision was deteriorated in post-operative period. 01 patient had PL-ve in Lt eye preoperatively; who had no improvement of vision in post-operative period in Lt eye. In 02 patients vision was restricted up to hand movement ; they had little improvement of vision after surgery. All 06 cases who underwent prophylactic optic nerve decompression had no vision changes during post-operative period. Surgical outcome and indications are shown in Table III.

Discussion
FD is predominantly a disease of adolescent and early childhood. In MFD and PFD, progression of the lesions appears to taper off as the patient approaches to puberty. Although active disease is continued; symptoms into adulthood are uncommon 16 . Skull base FD can present from 4 years up to late age with an average age of 22 years and a female male ratio is 2:1. About 60% of which presents at below 20 years of age 16 . Our cases also presented at early age from 5 years and 75% of which presented below 20 years of age. There is no sex preference. MFD represents 70% of cases and most commonly affecting the ribs and femur, next common is craniofacial bones. PFD involve the long bones, skull, and cranial base. Of significance, in 50 to 100% of patients with the polyostotic form and in 10% with the monostotic variant craniofacial involvement is present 3 . Sex prevalence of Female 56.25% and male 43.75% of our series correlates with series of Ricalde P et al 3 but prevalence of MFD and PFD respectively 37.25% and 62.5% does not correlates; as only craniofacial FD with optic canal involvement are taken in our series.
Most commonly involved bones of skull base are ethmoid 70%: followed by sphenoid 43%, frontal 33% and Maxilla 29% 16 . Our patients are closer to same involvement of bones with 75% ethmoid, sphenoid 62.5%, frontal 25% and Maxilla 18.75%. FD appears to arise from a perturbation in the mesenchymal precursor of bone, producing a defect in osteoblastic differentiation and subsequent maturation of bone. Molecular biological findings have provided some insight of site of origin. One of the putative defects appears to involve a missense mutation, which gives rise to an anomaly of intracellular signalling that produces increased cell proliferation and inappropriate cell differentiation, resulting in a disorganized fibrotic bone matrix 1 . Additionally, increased interleukin-6 may also play a role in the development of fibrous dysplasia 7 .
Similar to other series, the most frequent ophthalmic sign was proptosis 62.5%, optic neuropathy 56.25%. It is recognised that compressive optic neuropathy does not always supervene, even with anatomical narrowing of the optic canal 17,18 . It usually occurs in a chronic and progressive manner or may also manifest in an acute and impressive fashion. Visual impairment has been ascribed to a multitude of underlying pathological processes. One of these processes appears to involve progressive diminution of optic nerve venous drainage and, ultimately, retinal ischemia that develops due to optic nerve compression resulting from fibrous dysplasia-related optic canal stenosis. The arrangement of fibres within the optic nerve at the optic canal is such that the peripheral fibres run circumferentially along the periphery of the nerve, central vision may be preserved in cases in which there are peripheral visual field defects 3 . Other causes of ophthalmic signs include exophthalmos-induced optic nerve traction, sinus mucocoele formation, with increased intraorbital pressure, spontaneous haemorrhage, bone cyst formation, or rare vascular events 11 .
CT scan accurately establishes the diagnosis and extent of bone involvement. Involvement of optic canals, orbital fissures, frontonasal ducts and osteomeatal complex can be best evaluated by CT scanning. CT characteristics of fibrous dysplasia, include expansion of the involved bone with heterogenous pattern of CT densities associated with scattered or confluent islands of bone formation. Exact evaluation of optic canal for canal dimension, site of foci, extent of nerve compression etc. can be made by CT scan. Furthermore, CT scan can differentiate fibrous dysplasia from other osteodystrophies of the skull base including osteogenesis imperfecta, Paget's disease and osteopetrosis.
Ocular complications have been classified into primary and secondary processes 2 . Primary complications include involvement of the frontal bone with proptosis; the skull base with extraocular muscle palsies and trigeminal neuralgia; the optic canal with visual loss and optic atrophy; the sphenoid bones with chiasmal compression; and the maxillary bone with epiphora. Secondary complications comprise malignant change, ossifying fibroma formation, and development of a mucocoele 20 .
Optic nerve involvement in FD of Sphenoid bone may be about 50% to 90% 11 . It may vary from partial to complete encasement. Up to 76% may cause bilateral involvement.
Of these bilateral cases 75% may have complete circumferential encasement and 25% partial encasement and all the canals may be patent 19 . In a series Cuttler and colleagues 23 shows 100% involvement optic nerve in 87 patients with more than 50% encasement in 83% cases. In our series, we found 4 patients (25%) of bilateral optic canal involvement. Optic nerve involvement is also similar to Lee et al 11 series of 62.5%.
The surgical treatment of fibrous dysplasia is based on two different approaches, conservative or radical. Conservative shaving or osseous contouring has been recommended by some authors. who maintained that periodic contouring could be performed until a static phase was reached, even if continued growth was observed after conservative treatment. However, interventions have become more aggressive as advances have been made in surgical techniques and now a day most authors are in favour of radical surgical therapy, which permits the complete removal of the lesion followed by immediate reconstruction 21 . Case-4 (Absolute visual field defect in preoperative period. In post operative period visual field improved after 3 months) may have breathing or feeding problems caused by narrow nasal passages. Sclerosis of the skull may lead to asymmetry of the mandible, as well as to cranial nerve compression, that may finally result in hearing loss and facial palsy25]. The x-rays of individuals with craniometaphyseal dysplasia show unusual shaped long bones, particularly the large bones in the legs. The metaphyses are wider and appear less dense in people with this condition.

Conclusion
Role of optic nerve decompression in craniofacial fibrous dysplasia is controversial but timely intervention can save and improve vision. Extent of optic nerve encasement does not correlate with extent of visual impairment. All compressive optic neuropathy can be decompressed until preoperative visual acuity becomes PL-ve. Use of modern equipment's; like high speed drill system, continuous irrigation and suction and navigating system can make precise, non-traumatic and safe decompression of optic nerve with adequate shaving and debulking of dysplastic bone for aesthetic improvement of facial deformity. Prophylactic surgical intervention by expert surgeon can prevent future impairment of vision.