What does "softening" refer to?

Context:

Stru¨ mpell’s testimony
Dr Adolf von Stru¨mpell an eminent neurologist and internist from Leipzig mentioned in his 1925 memoirs the medical consultation in Moscowjust briefly. However, sections from his diary were published in 1974 by
his daughters Dr Regina Stru¨mpell and Dr Anna Klapheck from Dusseldorf (Strumpell and Klapheck, 1974). There he noted the first medical consultation with the Russian professors Kramer and Kozhevnikov: March 20, 1923 – Endarteritis luetica with softening is very probable, although the diagnosis of lues is uncertain . The next day he examined Lenin in his apartment.Lenin extended his left hand to him in a friendly manner. Right hemiplegia and near-complete motor aphasia with right hemianopsia were diagnosed.
The same afternoon, the full team of consultants met and discussed endarteritis luetica with secondary softening as the highly likely diagnosis. However, this diagnosis remained uncertain as the CSF was normal and Wass-erman’s test was negative but in tertiary syphilis Wasserman’s test in the CSF is false negative in 34–90%, while as in blood tests only 5% are false negatives (Sepp et al., 1950; Lishman, 1987; Isselbacher et al., 1994; Stoudemire et al., 2000).