HBs are not commonly encountered by neurosurgeons, however they are not rare, accounting for 2% to 15% of primary intramedullary spinal cord tumors in reported series. The reported male-to female ratio is 1.6:1 to 5.5:1 . Similar to the literature data, the majority of the HBs in our patients occurred in the cervical and dorsal spinal cord. A reason for this may be the predominant distribution of embryonic precursor cells in these areas of the spinal cord [11, 21, 22]. Overall, onset symptoms in our series recapitulate those commonly reported in other studies: sensory and motor deficits, pain, urinary dysfunction, and occasionally bulbar symptoms.
It is well known that the postoperative neurological outcome typically correlates with the preoperative functional status in patients with intramedullary tumors, and HBs are not an exception [14, 23]. Also large tumors, and those ventrally located have been associated with a worse neurological outcome , possibly because of a more ventral location of corticospinal fibers.
Postoperatively, 50% to 80% of patients experience a worsening of their neurological status . However, these deficits are usually transient, and patients recover to the baseline after some weeks/months. Almost one third of our patients experienced postoperative neurological worsening. Nonetheless, consistent with the reported trend, just 1 patient did not recover to the baseline at last follow up. Assessing the neurological status at the last follow up is therefore more interesting and reliable than the immediate postoperative outcome.
HBs outcome and preoperative neurological status
The importance of the neurological condition at admission has been already shown to have an impact on the final outcome [19, 23]. Although our study failed to find a clear statistical significance, our data confirm that preoperative McCormick score of I or II tend to be associated with a better outcome, compared to those patients presenting with more severely impaired neurological condition. This is particularly important in case of VHL-associated HBs. In these patients it is certainly wise to wait until spinal HB becomes symptomatic before indicating surgical removal in order to avoid unnecessary operations, but not too long in order to guarantee the best neurological outcome to the patient. There is a strong association between VHL disease and HBs, and especially spinal HBs. About one third of patients with spinal HBs are affected by VHL disease, and up to 40% of VHL-related CNS HBs develop in the spinal cord [9, 20]. The management of HBs in VHL patients can be challenging, due to the presence not only of other tumor types as pheochromocytoma, but also of multiple HBs. In 3 of our VHL-affected patients, 2 spinal HBs were resected in a single operation, and in one patient 3 spinal HBs were removed during the same surgery. These patients were admitted with severe neurological status (McCormick 3 and 4), and had a McCormick 3 at the last follow-up, confirming that the multifocality of VHL lesions can severely impact on postoperative outcome.
HBs and surgical approach
The association of surgical approach with neurological outcome in spinal surgery has been debated for a long time. Our study showed that laminotomy is significantly associated with a better outcome compared to laminectomy, both in univariate and multivariate logistic regression models. This is in contrast with some studies, which failed to show any correlation between surgical approach and neurological outcome . It is not easy to find a reason to interpret this result. However, we assessed outcome using McCormick scale that is a functional scale, not a grading of neurological motor or sensory functions per se. Therefore, while motor and sensitive functions depend mainly on the preservation of neural pathways, mobility and functional independence can be affected also by other factors such as paraspinal muscles and osteoligamentous conditions. We hypothesize that laminotomy may guarantee a better preservation of osteoligamentous and muscular connections, thus facilitating postoperative improvement.
HBs and IONM
The use of IONM is still considered controversial. Since HBs have normally a clear border separating them from the spinal cord, monitoring is considered by some authors not useful to increase safety in surgical resection . However, in the last 20 years the use of IONM during the resection of spinal HBs has become more and more common, with an increasing number of authors showing the importance of such method also in HBs removal, both for surgical strategy and for prognostic reasons [4, 6, 22, 25, 27, 15, 26].
In our results, the use of IONM was associated with optimal outcome. There are a number of reasons which may support this result, which is in line with previous literature . In our clinical setting, it is common practice to abandon surgery if D-wave amplitude decreases over 50% and does not recover with corrective measures such as irrigation, papaverine and hypertension, as it has been shown to predict persistent motor deficit . Another benefit may have been related to MEP monitoring. Although SEPs are generally considered useful for spinal cord preservation in scoliosis surgery, MEPs are more reliable in monitoring corticospinal tract function, particularly in the short term . Even if they do not appear to be correlated with long-term outcome, they may provide timely information of initial damage to part of the motor system since a decrease of MEPs during surgery normally anticipates a reduction in D-wave amplitude. This may be very useful to modify surgical strategy and timing accordingly. A decrease of MEPs amplitudes might suggest the need of making a pause during pial dissection, or at least move to a different side of the tumor, irrigating the spinal cord with warm saline solution to allow amplitudes to recover. This strategy might potentially be beneficial to the spinal cord, which is stressed not only by tumor compression but also by the even cautious microsurgical maneuvers. In any case, it is worth noting that the univariate analysis of our data in the second scenario revealed that the absence of IONM is not associated with a poorer outcome. However, although it is not possible to say that the absence of IONM increases the odds for a poor outcome, our analysis shows that their use may increase the chances for a postoperative improvement of neurological function in the long-term follow up.
VHL vs Sporadic HBs
We showed that surgery offers high chances to improve neurological condition or at least to avoid further deterioration due to the HB. Our data suggest that VHL-patients may have a poorer outcome compared to those with sporadic HBs. This is in contrast with some published series [2, 25], but in line with others [29, 31], and further research is certainly needed to clarify this issue. However, we expect poorer long-term outcome in VHL patients to be due to disease progression and cumulative functional morbidity rather than the single surgical operation per se, as the growth of multiple HBs may multiply the risk of preoperative neurological deficits and related sequelae from cumulative surgeries, as already pointed out in other studies.
Our study has several limitations. First, there are limitations due to the study design, which is retrospective. Second, neurological outcome was not consistently available for all patients due to losses at follow up. Third, this series is composed by patients who were operated during a long-spanned time period, when different surgical tools and devices were available. Moreover, this series includes patients treated in two different Institutions, with a possible bias due to potentially different surgical strategies.