Tiny lines visible on Hillary Clinton’s glasses when she appeared before the Senate this week to give evidence on Benghazi attack indicate she may still be experiencing the after-effects following the blood clot she suffered last month.

It is thought Hillary Clinton, 65, was wearing a Fresnel prism on her spectacles – a stick-on panel often used to treat double vision.

The faint vertical lines are visible on the left lens of Hillary Clinton’s glasses in photographs taken during her testimony about the terrorist attack on the U.S. Consulate in Benghazi, Libya, on September 11, 2012, in which four Americans were killed.

Eye experts told the New York Daily News it is likely Hillary Clinton has had an adhesive Fresnel prism placed on her glasses.

“If she’s wearing a Fresnel prism, then she has double vision without it,” said Dr. Mark Fromer, medical director at Fromer Eye Centers.

He said the adhesive panel, which also aids muscle weakness, “helps bring images into focus”.

Hillary Clinton’s spokesperson confirmed she was wearing glasses as a result of the fall and concussion she experienced last month, but did not elaborate further.

The Secretary of State returned to work earlier this month following a month long absence, after initially falling ill with a stomach virus.

While at home recuperating from the bug, which forced her to cancel a planned visit to North Africa and the Middle East, Hillary Clinton suffered concussion after fainting and falling.

Doctors later discovered a blood clot between her skull and her brain during a follow-up exam on December 30, and she was admitted to New York-Presbyterian for treatment with blood thinners.

Hillary Clinton attended back-to-back hearings on the Benghazi attack before the Senate and the House on Wednesday.

It appears that Hillary Clinton was wearing a Fresnel prism on her spectacles during Benghazi attack hearing indicating she may still be experiencing the after-effects following the blood clot

What are Fresnel prisms?

Fresnel prisms are wafer-thin, transparent sheets of adhesive plastic.

One side is fixed to the lens of the wearer’s glasses, while the other has special grooves that alter the way light enters the eye.

The prisms, the strength of which can be adjusted depending on the patient’s needs, are often used to treat double vision.

The stick-on panels may be worn for several months. If they prove particularly successful, patients can have glasses made with built-in prisms.

Double vision can occur in one or both eyes. It can be caused by an irregularly shaped cornea or abnormalities of the cornea, dry eye, or abnormalities of the lens or retina.

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