Except for lesions confined to the optic canal, computerized tomography can demonstrate enlargement of the orbital optic nerve in the majority of cases. 97–99 % of ONSMs were demonstrated on CT scans [15]. Tram-tracking, a radiographic sign in which the denser and thickened optic nerve sheath outlines a central lucency representing the residual optic nerve, is a characteristic finding suggestive of an ONSM.

Magnetic resonance imaging provides excellent visualization of ONSMs even in the intracanalicular segment of the optic nerve. It shows a thickening of the nerve with isointense or slightly hyperintense signal intensity compared to normal nerve on T1- and T2-weighted images. Fat suppression MRI images provide improved tissue contrast of orbital structures and accurately represent anatomic borders of the optic nerve, both in the orbit and in the optic canal [15, 25]. When used with contrast administration, fat suppression T1-weighted images give superior delineation of tumour surface adjacent to orbital fat.

In our series on CT scan, intraosseous extension of ONSM has been seen either into the medial wall of the orbital cavity extending therefore into the ethmoidal cells (one patient) or to the lateral wall producing hyperostosis of the sphenoid ridge (two patients). Calcification occurred in ten patients (42 %).

On MRI, an inhomogeneous contrast enhancement particularly evident with fat saturation techniques has been seen in ten patients (42 %) with significant larger tumour volumes. The presence of a non-enhancing optic nerve in the middle of an enhancing tumour giving evidence of the classical “tram-track sign” has been seen in four patients accounting for 20 % (4/20) since the introduction of fat suppression techniques in magnetic resonance tomography.

The selection of an adequate treatment option for ONSMs still remains a matter of debate.

Current management of ONSM includes observation, surgical excision and/or radiotherapy, depending on the extent of the tumour and the degree and rate of decline in visual impairment [6, 7, 14, 24, 26, 27, 31, 34, 40, 41, 43, 46, 47].

The natural history of this disease is progressive loss of visual acuity, as evidenced by a cohort of patients who underwent observation in several series [6, 15, 16, 22, 32]. Furthermore, if untreated, ONSM can eventually extend intracranially, resulting in seizure, hemiparesis [11, 20, 22] or bilateral blindness, within a few years.

The current literature provides few studies detailing the results from surgical resection.

Some treatment protocols indicate surgery of ONSM mainly in blind patients with disfiguring proptosis, patients with intracranial extension or young patients, where a presumed higher biological activity of the tumour tissue determines the need for immediate surgical intervention [48]. Only few cases of visual improvement after microsurgical resection of ONSMs have been reported, but the majority describe a decrease of vision through surgery [18, 20, 48] and an association with a high rate of complications, including blindness resulting from central retinal artery occlusion [2, 4, 5, 11, 20, 22, 38] and high rates of local recurrence (>38 %) [23]. In reviews of surgical patients treated by Ito et al. [20] and Cristante [13], only 2 of 21 patients who underwent resection had improved vision, although 30 % respectively 62 % remained with functional visual acuity. Both patients with improved vision had anteriorly located tumours which tend to grow extradurally and were more amenable to resection. Single case reports describe surgery of ONSM with benefit only if the meningioma is located anteriorly, close to the globe of the eye bulb [11, 20, 22, 42, 46, 49]. In Kennerdell’s series, seven patients with functional acuity preoperatively remained stable for 2–10 years after undergoing subtotal resection, although his group experienced loss of vision in the majority of cases [22]. On the other hand, Sibony et al. described in 1984 that six out of eight patients improved in either acuity or visual field or both. Three of them experienced a decline in visual function a year after surgery but 38 % of patients maintained stable acuity over a period of 5 years [42]. Delfini et al. support surgical intervention in patients with symptoms in progress, despite their results with >80 % decline in vision after surgery [14].

Some other reports presented a more favourable outcome after surgery of ONSMs. Verheggen et al. picture an improvement of visual acuity in intracanalicular and intraorbital meningiomas in 89 % of their surgically treated cases. They conclude that early surgery improves the chance of preserving visual acuity in cases of ONSM with extradural extension, although they found high risk of injury of vessels, cranial nerves and ocular muscles in the orbital funnel [46].

In a large series presented by Schick and co-workers in 2004, the postoperative visual outcome did not differ significantly from the preoperative vision in 73 surgically treated patients with ONSM. Furthermore, most patients remained stable during a long follow-up period. The authors recommended decompression of the optic canal and resection of intracranial tumour parts prior to adjuvant radiotherapy [40].

In our surgical series, we cannot entirely underline that preservation of vision is only achievable in apical ONSMs. We did not experience a correlation between tumour extension and visual loss before and after operation. Rather the visual results were most closely related to duration of visual symptoms, in a way that visual improvement occurred in patients with rapid decline of vision only. This finding is in concordance to Rosenberg et al. who evaluated 20 microsurgically treated patients with ONSM and found a 69 % stable or even improved vision postoperatively [36]. However, microsurgical excision of the ONSM with preservation of vascular supply and its pial surroundings of the optic nerve does not imply preservation of vision in all cases as visual loss might be dependent on tissue consistency, producing various amounts of pressure when encasing the optic nerve, difficile pial blood supply and manipulation to the optic nerve producing vasospasm of the optic nerves vasa nervorum.

Regardless of the position of the ONSM in the orbital cavity, standard osteoplastic fronto-lateral or pterional craniotomies attaining the supraorbital margin of the orbit were performed. Epidural dissection of the orbital roof and removal of the lateral orbital wall are followed by stepwise drilling of the optic canal, after defining the anatomic limits of bone resection as the lower rim of the optic foramen and superior orbital fissure downwards. For types Ib or Ic tumour, debulking of the orbital cavity can be performed first. Dural resection borders are the orbital apex anteriorly, in particular the annulus of Zinn at its whole length and the optic nerve sheath. The decompression of the optic nerve in the optic canal is mandatory in cases of intracanalicular ONSMs.

If protrusion of the tuberculum above the line connecting the surface of the nerves as they enter the optic canals is evident, it has to be removed in order to enlarge the transfrontal operative field.

In all cases with intracranial extension of the ONSM, an intradural approach is necessary with opening of the Sylvian fissure as the first procedure. The ipsilateral optic nerve and carotid cisterns are identified. In this step, preservation of the small feeding vessels between the carotid artery and the optic nerve is important, justifying the fact that we favour irrigation with isothermal Ringer’s solution instead of coagulation.

Careful resection of tumour tissue around the optic nerve is enforced as long as tumour/nerve borders are visible. The dura of the region of the tuberculum sellae and chiasmatic sulcus is carefully inspected for evidence of meningeal tumour infiltration.

If the ONSM infiltrates the optic nerve, resection is limited to the exophytic part. However, in careful selected cases of type Ia tumours, epineural decompression might be necessary to relieve pressure of the optic nerve itself. In blind patients with disfiguring painful proptosis, the nerve can be transected and the intraorbital portion is removed.