What are Prism Lenses?

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Prism Glasses for Double Vision (Diplopia)

Prism glasses correct diplopia or double vision. Double vision means someone sees two separate images of a single object. The prism in the glasses helps align the two images and correct the problem.

Normal, standard glasses typically only correct two types of vision conditions, including astigmatism (curved cornea) and focal distance (farsightedness and nearsightedness). Prism glasses also have a thicker base and thinner apex than standard prescription glasses.

Your eye doctor (optometrist) may prescribe prism glasses if you struggle with:

  • Eye strain
  • Frequent headaches
  • Strabismus (eye turn), which causes difficulty fusing two images together into a 3D view
  • Tiredness with close-up work, such as looking at a computer
  • Suppression

What Causes Double Vision?

Double vision has numerous causes. Sometimes these causes are easy to treat, while others can be symptoms of more serious eye problems. 


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Monocular diplopia means double vision that is still present with one eye closed. Making this distinction helps to figure out the cause of double vision. Binocular (true) diplopia generally has more serious causes than monocular diplopia.

double vision

Some of the most common causes of double vision include:

  • Dry eyes
  • Misalignment of lenses
  • Migraine headache
  • Squinting
  • Stroke
  • Thyroid problems
  • Diabetes
  • Cataracts
  • Multiple sclerosis
  • Keratoconus
  • Brain tumor

The most common cause of double vision for children is squinting. It occurs because the eyes are not properly aligned and they look in different directions. Over time, a child’s eye muscles weaken or become completely paralyzed. Vision corrections in childhood can lead to temporary double vision that eventually adjusts and goes away.

Double vision is a fairly common side effect of certain serious health conditions. Stroke patients sometimes develop temporary double vision because of damage to the blood vessels responsible for supplying the nerves. 

When a person has diabetes or a thyroid health issue, especially hyperthyroidism or Grave’s disease, double vision can occur. Diabetes and multiple sclerosis are also linked to double vision.

Eye health issues can lead to diplopia. The two most common vision conditions that cause double vision are cataracts and keratoconus.

Finally, something as simple as someone’s eyes not producing enough tears can cause diplopia. Inflammation and soreness are symptoms of dry eye, and that irritation can lead to double vision. Over-the-counter eye drops can help, but when the condition is this severe, it usually requires prescription eye drops designed to provide lubrication and hydration.

Hemianopia & Prism Lenses

Hemianopia is a cause of double vision. When someone experiences hemianopia, he or she is blind in half of the visual field. This condition is usually caused by brain tumors, stroke, and other trauma. In addition to partial blindness, hemianopia causes other symptoms, including:

  • Double vision
  • Vision distortion
  • Dimmed vision
  • Poor depth perception
  • Reduced night vision
  • Moving the head or body from the affected side
  • Difficulty comprehending what’s seen
  • Visual hallucinations

Symptoms tend to increase when someone’s vision is focused on a single object.

Causes of hemianopia include:

  • Stroke
  • Traumatic brain injury (TBI)
  • Brain tumor
  • Dementia
  • Alzheimer’s disease
  • Epilepsy
  • Multiple sclerosis
  • Lymphoma
  • Hydrocephalus
  • Shaken baby syndrome
  • Elevated pressure in the brain
  • Carotid artery aneurysms

Any type of damage to the optic nerve or brain can trigger hemianopia.

Treating hemianopia varies based on the cause. Sometimes the problem clears up on its own, especially when its cause is a head injury or stroke. Medication is an option for people with hemianopia due to a brain tumor. Double vision might also clear on its own if a person undergoes surgery or treatment to remove or shrink the tumor.

Sometimes the problem is incurable, in which treatment is used to correct the vision issue. For example, someone with permanent hemianopia might:

  • Wear corrective prismatic lenses
  • Learn vision compensatory training
  • Undergo vision restoration therapy

How Do Prism Correction Glasses Work?

Yes, prism glasses are one of the most common methods for dealing with double vision, regardless of the cause. Prism glasses make it easier to maintain binocular vision, which is the ability to maintain focus on an object with both eyes.

People who struggle with binocular vision see two images instead of a single image. The light they see falls in different places on their retinas, which causes distortion and doubling of what they are seeing.

When lenses feature a prism, the light is bent before it hits the retina. It is redirected so it falls correctly and the brain can interpret the message it’s getting in the usual manner, resulting in a single clear picture of what someone is seeing.

Side Effects of Prism Eyeglasses

Prism glasses are an effective tool for correcting double vision, but some people experience side effects when wearing these types of lenses. In most cases, side effects can be corrected with an adjustment to someone’s lens prescription, eyeglass frames, or medication. 

Side effects of prism glasses include:

  • Headaches
  • Pain with eye movement
  • Wandering eyes
  • Misaligned eyes
  • Nausea
  • Double vision

Alignment is extremely important when wearing prism lenses. Prism lenses may not work correctly if they are misaligned, which is easy to fix with a professional adjustment. If you experience any problems with new glasses or your old prism glasses stop working, contact your vision healthcare specialist to discuss the issue.

How Much Do Prism Lenses Cost?

The cost of prism lenses varies based on the frames you choose and if your lenses are only designed to treat double vision. 

Most temporary (Fresnel) prism glasses cost about $250 to $500 and permanent (ground) prism glasses cost about $600 to $1500.There are instances in which prism glasses cost several thousand dollars. 

Speak to your doctor about how much prism glasses cost and to discuss ways to reduce the out-of-pocket expenses.

How to Read a Prism Glasses Prescription

Prism eyeglass prescriptions have two values: prism and base. The prism measurement is a number between 0.5 and 5.0 that refers to the amount of prism correction needed to correct eye alignment issues. Base (the thickest edge) indicates the direction of the prism and can be BO: Base Out (towards the ear), BI: Base In (towards the nose), BU: Base Up, or BDn: Base Down.

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