Medical News Today: What is a slit lamp exam?

The slit lamp exam is a standard diagnostic procedure, which is also known as biomicroscopy. A slit lamp combines a microscope with a very bright light.

The slit lamp exam usually forms part of a comprehensive eye exam. The individual will sit in a chair facing the slit lamp with their chin and forehead resting on a support.

The doctor can use this instrument to observe the eyes in detail and determine whether or not there are any abnormalities. They will be able to discuss the results with the person immediately.

What does a slit lamp exam show?

Slit lamp exam
A slit lamp exam enables a doctor to see the structures within the eyes.

Doctors use the slit lamp as part of a complete eye exam to get a better look at the structures within a person’s eyes. These include the following:

  • Conjunctiva: The conjunctiva is a thin, clear membrane that covers the white part of the eye. It also includes the membranous surface of the inner eyelids.
  • Cornea: The cornea is the transparent covering of the iris and pupil. It protects the eye and also helps to send light through the pupil to the retina at the back of the eye.
  • Eyelids: The eyelids help to protect the eyeball from debris or injury. Blinking helps to lubricate the eye and prevent it from drying out.
  • Iris: The iris is the colored part of the eye. It controls the amount of light going into the eye by constricting and dilating the pupil.
  • Pupil: The pupil is the black dot in the middle of the eye. It allows light to enter the eye and travel to the retina.
  • Lens: The lens sits behind the iris and focuses the light onto the retina.
  • Sclera: The sclera is the white part of the eye. It consists of relatively tough fibrous tissue that helps to provide structure and protection for the rest of the eye.
  • Retina: The retina is the eye tissue containing the cells that sense light. These cells link to nerves that eventually join to form the optic nerve.


Procedure

After an initial look at the eyes, the doctor may apply a special dye called fluorescein to them to make the exam easier. They will administer this as an eye drop or on a small, thin paper strip that touches the white of the eye.

The doctor will then administer a series of eye drops that will dilate the pupils. The dilation will make it easier for the doctor to see the other structures in the eye. It takes about 20 minutes for the drops to work.

Once the individual has dilated pupils, the doctor will repeat the eye exam. This time they will hold a particular lens close to the eye.

The procedure does not hurt, although there may be some brief stinging during the application of the eye drops.

Dilated pupils become very large, which can make the eyes sensitive to light. This can make driving or spending time outside uncomfortable. However, the eye drops should wear off within a couple of hours, and wearing sunglasses should help during this period.


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Preparation

There is no special preparation for this test. If the doctor plans to dilate the pupils, the individual may wish to bring sunglasses and arrange a ride home after the test.


Interpreting the results

Slit lamp exam can detect a range of eye issues
A doctor can use a slit lamp exam to detect issues such as cataracts, corneal injuries, and damage to the sclera.

A doctor will be able to detect a range of conditions by carrying out a slit lamp exam. Among many others, they may observe abnormalities such as:

  • cataracts, which is an opacity or cloudiness of the lens
  • corneal injury or disease
  • damage to the sclera
  • a detachment of the retina
  • damage to the retina or the blood vessels that supply it
  • macular degeneration, an eye disease that destroys the central vision
  • disease or swelling of the middle layer of the eye
  • diseases of the optic nerve, such as glaucoma
  • bleeding in the eye
  • presence of a foreign body in the eye

Risks

A slit lamp exam is generally very safe, although the drugs that dilate the pupils come with a few risks. They can increase eye pressure, which causes nausea and eye pain.

Anyone experiencing these symptoms should let a doctor know immediately.


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Other types of eye exam

Other common eye exams include:

Wood’s lamp examination

A Wood’s lamp shines UV light onto the eye to reveal any abrasions or scratches on the cornea. Doctors may use this if a slit lamp is not available.

Fundus examination

Fundus can be used instead of a slit lamp exam
To perform a fundus exam, a doctor will use an ophthalmoscope.

During a fundus exam, the doctor will use an ophthalmoscope to look into the eye.

Some will use a direct ophthalmoscope, which is a small, handheld instrument with a light on it. However, most doctors will use an indirect ophthalmoscope as they can wear this on their head, and it gives them a wider field of vision for the examination.

They will ask the patient to stare off into the distance while they use the device to examine the inner structures of the eye.

Gonioscopy

For this procedure, the doctor will first administer numbing eye drops. The individual will sit with their head resting in the slit lamp microscope, and the doctor will place a special contact lens directly onto the eyeball.

The test should reveal any signs of increased eye pressure, which may indicate glaucoma.

Outlook

A slit lamp eye exam is a straightforward and painless procedure that doctors use to look closely at the inner structures of a person’s eye.

A doctor will decide whether or not a person needs the test, and for what purpose.

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