FUNCTIONAL APPROACHES:   
conservative surgery may be performed, purely depending upon the size of tumor,either via a “middle cranial fossa approach ”( 1 - 8)  or a” suboccipital approach ”( 9 - 16 ). 
The middle fossa route or the variant called “enlarged middle fossa” is limited, for reasons of space availability, to tumors confined within the internal auditory canal or those who have grown into the angle for a few millimeters, without impinging on neural structures or major vessels. 
The suboccipital  and the newest retrosigmoid transmeatal lateral approach (R.S.T.M.L.), on the other hand, are suitably employed in a wide range of tumor sizes for tumors of all sizes (9-16; 23-25; 27-31, 33, 36-42).  
The suboccipital approach is best performed for large lesions impinging on brainstem and posterior fossa vascular structures.  
The retrosigmoid trans meatal lateral approach is preferably carried out in cases of small or medium sized tumors, not extending in cerebello pontine angle more than 2 to 2,5 cm.  
It is the surgical access most preferred by the neurosurgical community nowadays.  
It allows full visualization of tumor and cerebello pontine angle, the opening of internal auditory canal up to the fundus and preservation of cochlear nerve without even touching inner ear sound-generator structures (26, 32, 34, 35). 
To allow this aim a proper operative patient positioning is required (semisitting; ”amaca”; lateral decubitus position) and graded orientation of the patient head to easily approach with the drill the internal auditory canal. This approach is the preferred one in functional surgery for hearing preservation.  
     
DESTRUCTIVE, ABLATIVE APPROACHES:    
translabirinthine (17- 2O)   and trancochlear (21, 22)   approaches show the disadvantage to destroy  the hearing supporting structures (Fig.1) during the operative approach to the tumor,in particular to that part of the tumor which terminates at or close to the fundus of internal auditory canal.  
These approaches, in the personal view of the Author, should be reserved to cases where hearing is not at all present preoperatively or its level is close to  anacusis.  
Both approaches can be used for small or medium sized tumors,with more difficulties in cases of large space occupying neurinomas.  
For these large neurinomas impinging on neural structures and vessels of the posterior fossa the enlarged translabyrinthine approach or transcochlear are better used.  
All the approaches through the temporal bone are tipically of otological interest.  
Easiness to reach the fundus of the canal and to look there for a bony structure(the Bill's bar), just indicating the presence of the facial nerve at the beginning of the labyrinthine tract by using the translabyrinthine approach, has at times renowned interest of the scientific community in this surgical approach, making cooperation between neurosurgeons and otologists not interested in the issue of hearing preservation, feasible.     

ARGUMENTS IN FAVOUR OF PRESERVING WHATEVER PREOPERATIVE HEARING  

Postoperative preservation of hearing means integrity, both anatomical and functional, of  cochlear nerve fibers spared by surgery. 
Tone perception treshold not lower than 5OdB and discrimination of speech higher than 5O% are commomly cosidered as limits posed upon preoperative hearing function, a key and opening gate leading to functional surgery with respect to cochlear nerve sparing.   
A wider concept and the availability of sound microneurosurgical skills, togheter with  experienced attitute to save whatever neural function can be mantained, would induce to try hearing preservation even at the level of detectable pure tone perception aside from discriminative scores, which are considered as essential requisite of a useful heraing. 
Capacity to detect pure tones without discrimination of words or phrases does reflect a neural function at very simple level, but it deserves consideration for a series of reasons. 
Single detection of discrimination scores,preoperatively determined , is possibly not representative of the whole patient capability to discriminate and understand words. Words-phrases and language perception are comprehensive  of many more several aspects of the whole speech processing capability.  
Among these ,discrimination of language in a noisy environment is to be considered, togheter with sound source localization, the interaction existing between discrimination capability and lip reading interaction and the effects of language  redundancy.  
Moreover, discrimination scores, when  lower than 8O-9O% do show a wide intrinsic variance: for a given score of  5O% obtained  with a 25 words list, confidence limits of 95% are situated between 28 and 69%, according to the binomial distribution of discrimination scores.  
Therefore it seems wise to the Author to consider, as the unique prerequisite for a functional attempt at hearing preservation, the simple preoperative detection of a pure tone threshold even if not accompanied by satisfactory discrimination scores.  
Whatsoever preselected audiological criterion, togheter with what is believed to be its  value to pose a candidacy to preservation surgery may lead to  unselection of patients: with regard to preservation of an existing hearing patrimony, even if at very simple levels, but may possibly exclude some of them from hearing amelioration after pressure effects of the tumor impinging on cochlear fibers is taken away. 
Hearing which I also consider worthwhile to be saved is imperfect indeed, but why destroy it definitely and un-necessarily?  
Such an imperfect hearing  may not be useful in  verbal perception: mean level of  spoken language is indeed around 6OdB.   
On the other hand that imperfect hearing may become useful in mantaining perception of an acoustic space all around and for localizing environmental loud stimuli,greatly exceeding 6O dB.  
Low frequencies of the tonal curve seem particularly  worth of preservation, since well recognized as most relevant in language acquisition process.   

INDICATIONS FOR SELECTING FUNCTIONAL APPROACHES  

Theoretically, dimension of tumor does not constitute predefined limit to preservation surgery, as already suggested by the Author himself.  
Objective limitations or impossibility to preserve cochlear function can only be represented by intraoperative findings, direct under the light of the microscope at high magnification. During surgery, in the case where only part of cochlear nerve fibers do show a cleavage plan with respect to tumor capsule whereas others do not,it seems essential at least to try  maintenance of this contingent.  
It is also essential to mantain integrity of vessels directed to the cochlea (inner ear end organ) and small vessels derived from AICA (anterior inferior cerebellar artery) supplying nerve bundles themselves.The most proximal part of the acoustic nerve, called the Root entry zone into the brainstem is trhe most delicate, since  entirely constituted by central myelin. Cochlear fibers must be cut only if there is intraoperative evidence of infiltration by tumor tissue.  
Only in that given case where the entire nerve do enter the mass of the tumor, the wisest thing to do is to cut the nerve in its entirety.  
This case occurring,even an imperfect  hearing can not be saved at all. In the case the tumor is strictly limited to the internal auditory canal, even in the case where it grows a little bit out of the porus reaching the angle space,the middle cranial fossa approach (or the enlarged middle cranial fossa approach) can be carried out (1-8) (Fig.2,3).  
In all other cases,especially when the tumor does reach neural structures (nerves, cerebellar peduncle, pons or even medulla oblongata) the up to date approach is the retrosigmoid trasmeatal (25-29, 30-32, 34, 36-42) (Fig.4,5), preceded by deliquoration of the posterior fossa CSF content and slight elevation of the cerebellum.  
The opening of the occipital bone is limited to 3-4 cm (Fig.6), the sigmoid sinus fully exposed,possibly slightly retracted so as to minimize cerebellar displacement. 
Through this approach, slight cerebellar elevation allows exposure of nerves,vessels and tumor bulk within the angle (Fig.7,8). Medial surface of petrous pyramid until the meatal area can be inspected.  
Skepticism regarding the difficulty to total removal of tumor from the inside of  internal autitory canal,is connected  to the fact that reaching the fundus is not an easy task,that appropraite surgical position of the head  has to be mantained,with possibility to vary the visual angle to the deepest part of the canal (Fig.9). 
Surgery by the retrosigmoid approach can bear the possibility to open ,while drilling the posterior wall of IAC,the posterior semicircular canal or the vestibule, thus almost for sure causing loss of function.  
Should such an occasion verify,prompt closure with bony pate' and fibrin glue or bone wax must be carried out promptly and hearing possibly saved. Inner ear anatomical preservation and possibility to open the canal from inside the skull and not coming from the labyrinthine block, therefore the real possibility to preserve neural function of the cochlear nerve by having saved the labyrinth, make this approach the preferred one by the vast majority of neurosurgeons.  
The retrosigmoid approach to the cerebello pontine angle is therefore a functional way to deal with such tumors, trying to preserve all possible function of neural structures. 
The step of opening the internal auditory canal is required to debulk the far lateral extension of the neuroma, which can at times reach the fundus or even the cochlea, rendering, in this latter case,preservation of any hearing impossible.  
Incision of the dura, moreover, before beginning the drilling of the posterior canal wall, should be carried out anterior to the bulging of the saccus endolymphaticus, so that its integrity is mantained, and hopefully also its function in draining endolymphatic fluid. Surgical experience shows that opening of the internal auditory canal is necessary not only in order to preserve fibers of cochlear nerve exiting the modiulus in the cochlear fossette, but also to the aim of sparing under direct vision the course and integrity of the facial nerve.  
Facial nerve in fact may be tightly adherent to the tumor just at the fundus, where its vascular supply is particularly rich, since it encounters at this level the vascularity of the labyrinthine portion of the nerve itself. In the case the the action of blunted dissection cuses traction effects on the neural structures endangering what we try to save,it is better to use cutting devices,away from vessels.  
Nowaday knowledge says that no evidence exist that minimal tumor remnants on cochlear fibers do regrowth in time, thus transforming every our effort in an endlessy surgical dream, without concrete and realistic result.  
In those patient, very selected indeed, where hearing residuals must be mantained on the site of operation, it seems wise to leave what is supposed to be  an infiltrated nerve, still functioning (see for example in these cases the usefulness of intraoperative recordings of Acoustically evoked brainstem potentials), instead of sacrificing the nerve as a whole, with the expectation of cochlear or direct brainstem nucei prostheses. 
Our criteria are indeed very broad to select patients, so that  the author trend is to limit the translabirinthine (destructive) approach to whatever tumor of   whatever size, but in which hearing is already lost or at the end of the tonal scale.    
                                               
CONCLUSIVE THINKING ON HEARING PRESERVATION  

The basic concept and the operative phylosophy is to do every possible effort to save existing neural function at whatever level it may be found preoperatively.  
Data in favour to mantain a simple and defective hearing, defective since it does not produce any intelligibility of speech, are not at disposal.  
On the other hand nothing has been demonstrated on the issue that better thing to do is to destroy instead of preserving such a simple and imperfect neural capability. 
It seems wise to underline that the capability and possibility to convey simple tones to the brain can and should be tempted, supported by a nerve still capable to attain tone perception.The author strongly support the opportunity to mantain both highly  qualified as far as simple acoustic function. 
The latter would subserve emergency situations and favour  the patient efficiency in terms of having an”alarm” hearing on one side,subserving loud stimulation of the auditory pathways. 
No patient operated upon for hearing preservation purposes has never preferred, postoperatively, not to have that simple hearing still active. It does not interfere negatively with  the really functioning ear.  
The imperfect hearing can be better appreciated in the presence of a not perfectly functioning ear on the other side.  
For example,an imperfect hearing on the operated side, may allow the placement of a cochlear implant on the contralateral ear in the case of ensuing anacusis for whatever reason during life.