Visual Field Examinations Week 1

Perimetry The science of measuring the peripheral vision in order to determine the visual field of the patient.

Visual Field The portion of visual space which is visible to an individual at any given moment. There are normal parameters for a full visual field on a healthy patient. This may change daily, weekly, monthly or yearly if any pathology is present.

Normal visual field Superior=60 degrees Inferior=75 degrees Temporal=100 degrees nasally= 60 degrees

Island of vision The island is seen vision, while everything outside of the island is the “sea of darkness” or not seen vision. The highest point of the island is the sharpest or best vision achieved by the macula. The “Island” gradually slopes to the sea and this represents the boundaries of your visual field and the less sensitive vision.

Physiological blind spot Corresponds to the location of the optic nerve head in the retina. This should show up on any visual field test. If this does not show up on the test, the patient is not fixating. The blind spot is centered slightly below the horizontal midline and 15.5 degrees temporal relative to fixation. ( macula ) The blind spot is 5.5 degrees wide and 7.5 degrees high. Every eye has a blind spot. This is where the nerve exits the eye and blood vessels enter and exit. There are no retinal photoreceptors in this area, therefore, no vision.

Physiologic blind spot Note that the blind spot appears on opposite sides of each eye. To discern which eye you are looking at, it is as if you are looking out of your own eyes. The right blind spot will be temporal to the center cross and the left blind spot is also on the temporal side of the central cross.

Kinetic vs. static perimetry Kinetic perimetry Static perimetry A type of visual field in which the boundaries of the visual field are determined by a moving test object of fixed size and intensity while the patient’s fixation is held steady. This can only be done on a goldmann visual field. A type of visual field in which the boundaries are determined by using a test object of fixed size and increasing the intensity until it is seen. This is a stationary target. This is done on the Goldmann,Humphrey, or other automatic visual fields.

Isopter Lines that mark boundaries of visual field areas of equal or greater sensitivity. Used only on manual (Goldmann ) Perimetry. Each isopter is labled with the size and intesity of the test object.

Important vocabulary words Threshold A given point as that stimulus intensity which has a 50% probability of being seen. This test compares what has the probability of being seen with the possible inaccuracy of the test subjects. This test takes longer as it is comparing results and data. This can only be done on an automated visual field machine. Apostilb (asb) A unit of light per a unit of area. Intensity value of the stimulus. Used in static (automated) perimetry. Smaller asb = greater retinal sensitivity Decibel (dB) Used on manual VF. Larger dB= greater retinal sensitivity

Scotoma visual loss in an area. Absolute no vision Relative loss of vision (depressed)

Nerve fibers Axons connect the retina to the visual cortex. Nerve fibers pass from the retina to the optic disc and enter the optic chiasm. Fibers cross at the chiasm and go to the optic tract, into the lateral geniculate body, to the optic radiations and final end at the occipital cortex. The nerve fiber layer stretches across the retina to capture light images and then passes through the axon and the visual pathway to the brain

Nerve fiber layer

Nerve fiber layer If there is damage or pathology to the nerve fiber layer, most likely from glaucoma, it will show as a defect, or scotoma on the visual field. A defect will show up opposite on the visual field. If the damage is actually on the inferior portion of the nerve fiber layer, it will show superior on the visual field.

Retina Rods photorecptors in retina. Fewer rods in the peripheral retina. More concentrated in the central retina, which explains more acute vision centrally. Cones Photoreceptors found in the macula. These photoreceptors give us color vision. Rods and cones are light sensitive, capturing the image and converting the image into electromagnetic signals that travel through the nerve fibers. Axons Are nerve fibers that connect to the ganglion cells to form the nerve fiber layer.

Optic chiasm This is where nerve fibers from each eye cross to give you a complete picture when you see. Axons that originate nasally to the fovea cross to the opposite side, while the temporal side continue through the pathway without crossing. Nerve fibers after the chiasm consist of images from the temporal field of the same eye and nasal fibers from the opposite eye.

DO NOT PANIC This is only the first class. This is a lot of info, but we will be going over this every week so it will be etched into your brains! It is very important that you learn the visual pathway order, otherwise you will be lost later in this class. Also, you must remember that the nasal fiber cross at the chiasm and the temporal fibers stay on the same side ( same eye). This will save you when you have to identify different scotomas.