Today, the gold standard for OPG diagnosis is MRI. To maximize diagnostic yield, MR protocols should be planned with the assistance of an experienced neuroradiologist and should include orbit-directed scanning with fat suppression and contrast injection.

Tractography, the use of diffusion tensor imaging (DTI) MR pulse sequence for the assessment of white matter tracts, is a promising tool [6]. This method was utilized in two studies to assess tumor-induced changes in OPG. In these studies, the OPG caused fibers to either end abruptly within the tumor mass, or induce abnormalities in the arrangement of the visual white matter tracts [29, 62]. Identification of abnormally displaced fibers using this method may also aid in presurgical planning and assist in preserving visual function. The relevance of these changes to the individual patient is still not clear.

Tumors may differ in MR appearance depending on their locations, but they are usually isointense on T1, hyperintense on T2, receive variable enhancement, and have cystic as well as solid nonenhancing components. Tumors of the ON usually do not have cystic changes, appearing as gross thickening of the nerve itself, with or without nerve sheath enlargement. The differential diagnosis for ON enlargement is broad, including meningioma, neuroma, hemangioblastoma, and lymphoma [44]. Optic nerve sheath enlargement is found in nonneoplastic diseases such as increased intracranial pressure, optic neuritis, Grave’s disease, sarcoidosis, toxoplasmosis, central vein occlusion, idiopathic intracranial hypertension, and tuberculosis [76, 87]. Posterior tumors may appear as minimal chiasmal thickening, or as large masses protruding into the third ventricle with apparent mass effect. Cystic changes are common and are a part of the natural history of the tumor. Neoplastic changes posterior to the lateral geniculate nucleus are rare and are difficult to differentiate from NF1 T2 hyperintensities. Unfortunately, initial radiologic appearance does not correlate with visual prognosis. We have found deteriorating vision in patients with tumors that seem to be stable anatomically, as well as stable vision in patients whose tumors demonstrated structural progression. Recently, it has been suggested that dynamic contrast enhancement may correlate with progression, with larger mean permeability values in aggressive tumors [50].

Due to the long follow-up required in these patients and the importance of early detection of changes in the tumor bulk or in its internal components, we recommend the use of volumetric measurements. These measurements, although time-consuming, may improve patient care by enabling more accurate decision making [89, 97]. Figure 4 demonstrates a volumetric follow-up of one of our patients; some of the corresponding MR images are presented in Fig. 5.

Neuro-ophthalmology is crucial for both diagnosis and follow-up of OPG, as visual decline is the potentially worrisome consequence of the tumor. Neuro-ophthalmological evaluation is often difficult, especially in young patients where cooperation is limited [8]. A decline in visual acuity is often present at diagnosis and sometimes may be the reason for initial testing, even in the very young (e.g., an infant that starts bumping into objects or sitting closer to the television). In addition, color vision, visual field, eye movements, relative afferent pupillary defect, pupil size, and fundus should all be evaluated. Any progressive change should be considered seriously as a reason to initiate therapy. In very young children, a normal exam does not rule out visual impairment from an OPG. Optical coherence tomography (OCT) has been shown to detect loss of retinal nerve fiber layer in children with OPG (Fig. 6). This may prove to be an auxiliary tool in the diagnosis of visual damage in young children, as well as providing evidence regarding visual reserve and need for treatment [10, 17].

Endocrine assessment should be done in any child with chiasmatic/hypothalamic glioma. Endocrinological abnormalities such as central precocious puberty and growth hormone deficiency are detected in approximately 20 % of OPG patients. Endocrine symptoms may even precede neuro-ophthalmological signs [92]. It should be noted that growth hormone deficiency is present in approximately 2.5 % of NF1 patients even without OPG [19].

For tumors with characteristic MRI appearance that are epicentered on the optic pathway, especially in an NF1 patient, no biopsy is required [60]. Biopsies are indicated if the tumor has an atypical appearance on MR, for an unusual age group (older than 10 years old or younger than 1 year old), or with unusual clinical characteristics (rapidly progressing or severe neurological deficits other than vision loss). When biopsy is indicated, tumor topography dictates the preferred technical method. When the ventricular system is expended, an endoscopic approach can be used to combine multiple procedures, including biopsy, septum pellucidotomy, and accurate shunt placement [21, 22]. Otherwise, a stereotactic needle biopsy can be performed for larger lesions, either with a frame or by frameless techniques [74]. Smaller lesions must occasionally be approached openly through standard microsurgical techniques, or by basal transsphenoidal techniques when the tumor fills the sella turcica. It has been argued that the mere knowledge that an OPG has pilomyxoid characteristics is important for early treatment decisions. The value of a biopsy of an OPG for molecular diagnosis is still controversial. The example of the bRAF mutations, provided previously, may represent only the beginning of a long awaited breakthrough. Biopsies may be useful for molecular diagnosis of tumor characteristics that may have prognostic or therapeutic consequences [41, 75, 95]. These analyses, although not yet included in routine clinical practice, are proving valuable and we expect them to be essential for therapeutic decisions and estimating prognosis in the near future.

Clinical series describing surgical results for OPG are usually comprised of small patient numbers, with vague inclusion criteria, no control groups, and poor follow-up. The current situation includes wide variations between different centers and groups with regard to the threshold for open surgery.

Tumors confined to the optic nerve are considered for resection if they show progressive proptosis or intractable pain in a blind eye. These tumors can be approached via unilateral frontal craniotomy and orbitotomy or through the eye, especially when enucleation of the entire eye is warranted. Globe-sparing-resection is also possible. It should be noted that sectioning the nerve close to the chiasm risks damage to the advancing nasal fibers (Wilbrand’s knee) from the contralateral eye.

Surgical management of chiasmatic/hypothalamic tumors is even more controversial. Different microsurgical techniques have been proposed for the removal of large OPG. These different approaches depend primarily on the individual topography of the tumor. Radical resection in a patient with viable eyesight is usually not recommended due to the susceptibility of the surrounding neural structures (hypothalamus and brainstem). In addition, there is a high risk of damage to the optic apparatus and vascular structures [98]. Subtotal resection of tumors that grow outside the visual system, such as the third ventricle, anterior, and lateral subarachnoid spaces, can be performed for a subset of large suprasellar lesions [30]. In addition, subtotal resection may have a beneficial effect on visual outcome via selective decompression of the optic apparatus [90] and may even cause long-term tumor control in selected patients and lessen the hypothalamic dysfunction that is usually associated with more radical resection [43, 98].

Chiasmatic/hypothalamic lesions may be approached via interhemispheric transcallosal, subfrontal pterional, subtemporal, or bifrontal interhemispheric translamina terminalis routes, depending on the direction and the extent of the tumor. Combination approaches may also be used for more extensive lesions. In a surgical series presented by Goodden et al. in 2010, a transcallosal approach was safely used in nine patients with large exophytic tumors that were either causing obstructive hydrocephalus or progressing on serial scanning. Significant debulking (>50 %) was achieved in ~70 % of patients with no incidence of visual deterioration [36]. Image-guidance facilitates accurate navigation and avoidance of damage to critical structures. Hydrocephalus may respond to decompression alone, but in many cases may also require shunt placement, which is usually the preferred method [82]. Ascites secondary to ventriculoperitoneal shunting in chiasmatic tumors is common and may require diverting CSF to the atrium rather than to the abdomen [35]. Large cystic components containing mucinous fluid are common and may require multiple resections and drainages. Shunts placed within these cysts tend to malfunction after a short period of time; Ommaya reservoirs may be used in these situations for intermittent tapping and drainage.

Chemotherapy is considered the first line of treatment in most cases of children with progressive OPG.

“Gentle Chemotherapy” with vincristine and carboplatin was introduced in 1987 by Packer et al. and is now an accepted first-line treatment [71]. This regimen reported PFS rates of 75 % at 2 years and 68 % at 3 years for chiasmatic tumors [71]. Carboplatin alone may also be effective in OPG treatment, with a short-term PFS rate of 83 % and disease stabilization in 85 % [4]. Weekly vinblastine is also frequently used as first line or in patients with carboplatin allergies [58]. Recently, a new protocol of bevacizumab and irinotecan has shown preliminary effectiveness in the treatment of recurrent low-grade gliomas [72]. Temozolomide was suggested as a possible option for second- or third-line treatment with an overall disease stabilization rate of 54 % in patients with progressive OPG [40].

Adverse effects of chemotherapy vary according the regimen and drugs used. Among the side effects of the vincristine and carboplatin protocol are carboplatin-associated allergies occurring in ~10 % of treated patients [73], and peripheral neuropathy. Hematological side effects such as neutropenia and thrombocytopenia may occur with this protocol, but are usually associated with nitrosourea-based regimens [78]. Temozolomide was shown to cause grade 2–4 thrombocytopenia and neutropenia in ~23 % of treated pediatric OPG patients in addition to intratumor hemorrhage in ~3 % [40].