The onset is gradual and uniform. The pathological findings of subacute combined degeneration consist of patchy losses of
myelin in the dorsal and lateral columns. Patients present with weakness of the legs, arms, and trunk, and tingling and numbness that progressively worsens. Vision changes and change of mental state may also be present. Bilateral spastic
paresis may develop and pressure, vibration, and touch sense are diminished. A positive
Babinski sign may be seen. Prolonged deficiency of
vitamin B12 leads to irreversible nervous system damage. HIV-associated vacuolar myelopathy can present with a similar pattern of dorsal column and
corticospinal tract demyelination. It has been thought that if someone is deficient in vitamin B12 and
folic acid, the
vitamin B12 deficiency must be treated first. However, the basis for this has been challenged, although due to ethical considerations it is no longer able to be tested if "neuropathy is made more severe as a result of giving folic acid to vitamin B12- deficient individuals". And that if this were the case, then the mechanism remains unclear. Administration of
nitrous oxide anesthesia can precipitate subacute combined degeneration in people with subclinical vitamin B12 deficiency, while chronic nitrous oxide exposure can cause it even in persons with normal B12 levels. Posterior column dysfunction decreases vibratory sensation and
proprioception (joint sense). Lateral corticospinal tract dysfunction produces
spasticity and dorsal
spinocerebellar tract dysfunction causes
ataxia. ==Cause==