Spinal Schwannoma Classification
Preoperative planning remains crucial for successful Spinal Schwannoma treatment and relies to a great extent on proper tumor classification. The literature includes multiple classification systems for spinal schwannomas, each of which is associated with both positive and negative ramifications for preoperative planning 1) 2) 3). 4).
Consequently, there is a lack of consensus concerning the optimal system of classification for schwannomas 5).
The literature includes numerous schwannoma classification systems. Jinnai and Koyama 6) classified schwannomas into five groups based on the relationship between the tumor and the dura mater and/or intervertebral foramen. This classification system is useful, as it takes into consideration tumor localization relative to the dura, but it does not take into account volume, which is important for preoperative surgical planning.
Sridhar classification
Sridhar 7) was the first, in 2001, to suggest a classification system of benign spinal schwannoma including giant and invasive spinal schwannomas (type I to V).
Park et al. 8) reported the use of a new classification system, and Type VI and Type VII were added. But the classification system as defined by Park et al. were inadequate because both the figures and the tumors were not clearly described in their manuscript.
A case could not be classified based on Sridhar’s spinal schwannoma classification system. Thus, as shown in a case, of Kotil type VIII must be added to the modified Sridhar classification (Kotil classification) system of benign spinal schwannomas 9).
Sun and Pamir however, think classification of seven distinct types of schwannomas using Sridhar et al.’s system is not practical because the characteristics of seven tumors types are difficult to remember. Another drawback of their system is that tumor volume is only considered for dumbbell-shaped tumors, and craniocaudal dimension is not a consideration, which limit the diagnostic value and consistency of the classification system 10).
Asazuma Classification
Asazuma et al. 11) devised a schwannoma classification system for cervical dumbbell- shaped tumors that consisted of nine categories. An important drawback of their classification system is that it cannot be used for thoracic or lumbar schwannomas, which are as common as cervical schwannomas.
Asazuma et al. classification system for dumbbell spinal schwannoma:
Type 1 intradural extradural restricted to the spinal canal. The constriction occurs at the dura.
Type II are all extradural, and are subclassified as:
IIa do not expand beyond the neural foramen.
IIb inside spinal canal + paravertebral.
IIc foraminal + paravertebral.
Type IIIa are intradural and extradural foraminal, IIIb are intradural and extradural paravertebral.
Type IV are extradural and intravertebral.
Type V are extradural and extralaminar with laminar invasion.
Type VI show multidirectional bone erosion.
Craniocaudal spread: IF & TF designate the number of intervertebral foramina and transverse foramina involved, respectively (e.g. IF stage 2 = 2 foramens).
Schwannomas involving C1 & C2: May involve vertebral arteries and require additional caution.
Sun and Pamir Classification
It is based on consideration of tumor volume and localization relative to the dura and spinal canal. For approximate calculation of tumor volume, spinal schwannomas were considered ellipsoid bodies, and tumor volume was calculated using the following formula:
Tumor volume = 4 / 3 π × (craniocaudal length / 2) × (transverse diameter / 2)2 .
Tumors were then assigned to 1–3 volume groups (group A, B, and C) and designated as 1 of 4 types (type I, II, III, and IV) accord- ing to localization (i.e., group B type II tumor). Tumor volume <2 cm3 was considered group A, 2–4 cm3 group B, and >4 cm3 group C. Tumor typing was as follows: localized exclusively intra- durally: type I; intradural localization with extradural extension to the nerve root foramina, but restricted to the spinal canal: type II; intradural dumbbell-shaped tumor in the spinal canal extending to the extraforaminal region: type III; and localized completely outside the root foramina: type IV
Sridhar et al.’s 12) classification system is arguably the most similar of the previously reported systems to the novel classification system described by Sun and Pamir however, they think classification of seven distinct types of schwannomas using Sridhar et al.’s system is not practical because the characteristics of seven tumors types are difficult to remember. Another drawback of their system is that tumor volume is only considered for dumbbell-shaped tumors, and craniocaudal dimension is not a consideration, which limit the diagnostic value and consistency of the classification system 13).
Based on the findings, Sun and Pamir think that all schwannomas should be classified according to localization and volume, so as to achieve the desired benefit of classification—ease and reliability of preoperative decision making and preparation. In addition, this classification system makes tumor localization easier to understand, as compared to other systems, and is suitable for all schwannoma types.
It is a simple and effective tool that shows extremely helpful for avoiding unnecessary surgical approaches and complications. Due to the system’s simplicity of having only three tumor groups and its reliability—indicated by the associated low postoperative side effect rate, use of this novel classification system should be considered by any surgical department that seeks a standardized schwannoma surgery protocol. 14).