When she arrived at the Capitol last week after a more than two-month absence recovering from shingles, Senator Dianne Feinstein, Democrat of California, 89, appeared shockingly diminished.

Using a wheelchair, with the left side of her face frozen and one eye nearly shut, she seemed disoriented as an aide steered her through the marble corridors of the Senate, complaining audibly that something was stuck in her eye.

Ms. Feinstein’s frail appearance was a result of several complications after she was hospitalized for shingles in February, some of which she has not publicly disclosed. The shingles spread to her face and neck, causing vision and balance impairments and facial paralysis known as Ramsay Hunt syndrome. The virus also brought on a previously unreported case of encephalitis, a rare but potentially debilitating complication of shingles, according to two people familiar with the senator’s diagnosis who spoke on the condition of anonymity to describe it.

Characterized by swelling of the brain, post-shingles encephalitis can leave patients with lasting memory or language problems, sleep disorders, bouts of confusion, mood disorders, headaches and difficulties walking. Older patients tend to have the most trouble recovering. And even before this latest illness, Ms. Feinstein had already suffered substantial memory issues that had raised questions about her mental capacity.