The evolving management of optic nerve sheath meningiomas

The management of optic nerve sheath meningiomas (ONSMs) has been a controversial and sometimes contentious issue for a number of reasons. In the first place, these tumours are not associated with any mortality or significant neurological morbidity. In the second place, although they theoretically can spread across the planum sphenoidale to affect the opposite optic nerve, they rarely do so and, thus, almost never cause loss of vision in the opposite eye. The major manifestation of ONSMs, therefore, is monocular visual loss, and their management should reflect these issues.

Because primary ONSMs arise from the arachnoid cap cells surrounding the intracanalicular or intraorbital portions of the nerve, they almost always are intimately associated with the nerve and tend to surround the nerve by the time they become clinically apparent. Similarly, secondary ONSMs extend from the planum sphenoidale into the subdural or subarachnoid spaces (or both) surrounding the nerve within the optic canal and, ultimately, within the orbit. As if that were not enough, many ONSMs involve the portion of the optic nerve at the apex of the orbit within the annulus of Zinn, an area not amenable to safe and comprehensive surgical exploration. Thus, despite a few reports indicating preservation of vision following removal of an ONSM, attempts to cure this tumour surgically almost always produce blindness in the affected eye—precisely what one is trying to avoid in the first place—as well as ophthalmoparesis from damage to ocular motor nerves and/or extraocular muscles. If, on the other hand, the optic nerve is removed along with the tumour, the patient may be “cured,” but the procedure will, of course, cause blindness in an eye that may have had useful vision and that may have retained that level of visual function for several years without intervention and, in addition, there will be at least temporary ophthalmoparesis.

Although the potential for hormone therapy for meningiomas, the cells of which usually possess oestrogen and/or progesterone receptors, has received considerable attention, clinical studies using tamoxifen (an anti-oestrogen agent) and mefipristone (an antiprogesterone agent), have been disappointing. Thus, until recently, the most popular management options for a patient with a presumed ONSM have been observation without intervention and surgical excision of the tumour along with the optic nerve.

Earlier this year, Turbin et al¹ provided substantial data indicating that conventional fractionated radiation therapy is a better treatment for ONSMs than either surgery or no intervention. Although other authors had reported similar results,^2,3 the study by Turbin et al is the most comprehensive to date. Over a mean follow up period of 10 years, the patients in their study who received conventional fractionated radiation therapy alone had a substantially better visual outcome than did patients who were observed without intervention or patients who underwent surgery, and this was the case regardless of the level of vision when treatment was initiated. Thus, it is clear that radiation therapy should be considered for most patients with an ONSM.

Unfortunately, radiation therapy for ONSMs is not without potential side effects. Although these are often mild and transient (for example, local erythema and local alopecia), the potential for severe visual loss from radiation induced retinopathy, optic neuropathy, or even damage to the optic chiasm and/or contralateral optic nerve cannot be ignored. Thus, in recommending radiation therapy to a patient with an ONSM, one must consider the potential side effects that could damage the vision that one is attempting to preserve or improve, and any method that could prevent or reduce the frequency and severity of these side effects would be welcome. In this issue of the BJO (p 12651265), Pitz et al report the results of treatment of 15 patients with ONSMs using a more precise form of fractionated radiation therapy—stereotactic fractionated conformal irradiation. This technique should not be confused with stereotactic radiosurgery. The latter techinique consists of the delivery of a single, large dose of focused radiation (for example, 40–50 Gy) (or, occasionally, several large daily doses) to the lesion, whereas the technique described by the authors is basically the same as conventional fractionated radiation therapy (for example, 1.8–2 Gy per day to a total dose of about 50 Gy), except that much more precise delivery of radiation is achieved through use of a three dimensional stereotactic system, thus reducing the extent of spread of radiation and, therefore, the potential for damage to surrounding structures, particularly the globe, the optic chiasm, and the contralateral optic nerve. In their study, Pitz et al found that over a mean follow up period of 37 months after completion of treatment, there was no evidence of tumour growth or extension, none of the treated seeing eyes experienced deterioration of vision, and several eyes improved with respect to visual acuity, visual field, or both. Perhaps, most importantly, none of the radiated eyes developed radiation retinopathy or optic neuropathy, and none of the contralateral eyes showed evidence of radiation injury. Although the follow up period may have been too short to observe spread of an inadequately treated tumour or to observe any major ocular complications of the radiation, the results of Pitz et al. suggest that stereotactic fractionated radiation therapy for ONSM may produce a better long term visual result with fewer vision threatening side effects than conventional fractionated radiation therapy. If this is truly the case, one would have little reason to delay treatment of ONSMs in patients with good visual sensory function because of fear of radiation side effects or inadequate tumour treatment. It is thus to be hoped that the results reported by Pitz et al will stand up over the next 10–20 years of follow up. In the meantime, I would urge all physicians to consider using stereotactic (three dimensional conformal) fractionated irradiation rather than conventional fractionated irradiation for their patients with presumed ONSMs.