Curable (reversible) forms of paralysis must be distinguished from incurable (irreversible) forms. Irreversibility occurs after about 18 months after the onset of paralysis without signs of improvement. At this point, the facial muscles can no longer be "reanimated" as they transform into fatty tissue in the absence of neuronal input. Complete facial paralysis must be distinguished from incomplete facial paralysis (partial loss of muscle activity). Facial paralysis can be categorized as acute versus chronic by their progression, as well as spastic and flaccid by their muscle tone. Synkinesis results from an abbherant nerve regeneration of the facial nerve in newly sprouting nerve fibres (axons).