Presence of the spiral ganglion cell bodies beyond the basal turn of the human cochlea

The purpose of this study was to obtain a better understanding of the number and distribution of spiral ganglion cell bodies (SGCB) in the central modiolus trunk of the human cochlea with normal hearing as well as with hearing loss due to various pathological conditions. A literature review was performed using the key words 'human spiral ganglion cell population', 'analysis of spiral ganglion cell population', 'survival of human spiral ganglion cells', 'human Rosenthal's canal', 'human ganglion cell counts', and 'distribution of human spiral ganglion cells' to identify articles published between 1968 and 2018. Articles were included if the number of SGCB in the four segments of the human cochlea and angular depth distribution of the SGCB were stated. Of 236 articles initially identified, 19 articles met the inclusion criteria. SGCB inside the Rosenthal's canal (RC) in the modiolus trunk extended to an angular depth of 630-680° which is near the end of the second turn of the cochlea. SGCBs in Segment IV of the cochlea account for approximately 25-30% of the entire SGCB population irrespective of the cochlear condition (normal vs. pathologic). In normal hearing subjects, the total number of SGCB ranged between 23,910 and 33,702 and in patients with hearing loss between 5733 and 28,220. This literature review elaborates on the current state of knowledge about the number and distribution of SGCB in the human cochlea.