Pedretti Ch. 24 - Assessment of Visual Deficits Flashcards
Role of Vision in the Adaptation Process
According to Ayres, the brain is responsible for filtering, organizing and integrating sensory information in order to generate an adaptive response to a particular context.
Vision dominates other senses because it “takes us farther into the environment” and is faster
Vision helps us to anticipate developing situations and create a plan to handle them
Successful adaptation depends on
Successful adaptation depends on the ability to anticipate situations and context by utilizing sensory context.
eg. “It looks like rain, I’d better take an umbrella.”
We use vision in the decision-making process to:
- avoid objects
- “size up” situations (e.g. “that looks delicious”)
- Social communication (“reading” body language)
The speed of information processing through vision allows us to:
The speed of information processing through vision allows us to successfully adapt to dynamic environments. Monitoring our own movement against movement of other objects around us.
-This is why reintegrating someone with a visual impairment into the community can be so difficult.
Visual impairment can occur secondary to:
Visual impairment can occur secondary to: disease, trauma and aging. Often a combination of two of these.
Impairment can:
Impairment can alter the quality, quantity and speed of visual info sent to CNS OR how the CNS processes and responds to this info.
Impairment has the potential to change a client’s interaction with all aspects of the environment and people in it
Why are effects of visual impairment are often attributed to other causes
Effects of visual impairment are often attributed to other causes because visual impairment is “a hidden disability”
Visual Perception
Raw visual information (light) must be transformed into images and compared with stored memories, other sensory input and knowledge in order to be used for decision making. This is known as Visual Perception
Route of visual information
Information follows a circular route from the retina in anterior of the brain to the occipital lobe in posterior brain and back again
Along the way, visual info is sorted, fine-tuned, combined and repackaged to create a product for adaptation.=
Visual perception process
- Light enters eye and passes through cornea & lens to focus on the retina
- Information conveyed over optic nerve and tract to the lateral geniculate nucleus (LGN) of the thalamus
because of the optic chiasm, the LGN receives info from both eyes - Information then travels over the geniculocalcarine tracts (GCT) to V1 of the visual cortex
- Visual cortex sorts info, sharpens and fine-tunes features and then disperses info for cortical processing
- Info processed by temporal and parietal circuitry
- Info combined/integrated with other incoming sensory info to establish relationships between the body and the environment
- Info sent to prefrontal circuitry for cognitive processing to make decisions and plans
- This area, in conjunction with premotor circuitry, is responsible for planning skilled body movements (including eye movements)
- Important structures in this region are the frontal eye fields: responsible for voluntary visual search of space on the contralateral side of body based on expectation where visual info will be found in environment (e.g. light switches on walls, not ceilings)
Parallel-distributed sensory processing
To integrate, info sent to prefrontal area over two routes, a process known as “Parallel-distributed sensory processing”
Northern route
“Northern route” through posterior parietal circuitry:
- Parietal lobe is a synthesizer of sensory info, creating internal sensory maps to orient the body in space
- Info used to tune the CNS to presence of objects surrounding the body and spatial relationships of the objects to the body and to each other
- In order to do this, the visual info is integrated with other sensory info
- The “map” created is body centered and dynamic, changing in shape and content as the body moves through space
- Each hemisphere contains a map of the contralateral side of the body
- Map is not detailed, a general impression of objects in space on that side of body
- CNS relies on visual info from peripheral areas of retinal fields to create & maintain maps
Southern route
“Southern route” through posterior temporal circuitry:
- info combined with language and auditory input
- processed for visual object info and recognition
- Purpose is to identify objects and classify them.
- Uses precise input from the macular-foveal area to tune into visual details of objects
- Critical to distinguish discrete features of objects (e.g. facial features)
Does all info travel over GCTs?
Not all info travels over GCTs. Many pathways leave optic nerve & tract to subcortical areas including hypothalamus & brainstem
Brainstem contains
Superior colliculi (located in midbrain), the primary brainstem processing centers for visual input. Responsible for detecting moving visual stimuli in peripheral visual fields. When motion is detected, automatically initiates eye movement toward that direction. Thus, serves as an early warning system.
Nuclei of cranial nerves III, IV and VI which control the extraocular muscles of the eyes.
Basic visual functions such as light (pupillary) reflex and the accommodation reflex
Hierarchic Model of Visual Perceptual Processing
- Visual perceptual function can be conceptualized as a hierarchy of processes that interact with each other in a unified system, where each process is supported by the one that precedes (bottom-up) it and can’t properly function without integration of the lower-level process.
- Ability to adapt to vision is a result of each processes working in synergy. Although discrete processes can be identified, they do not work independently of each other.
Hierarchic Model of Visual Perceptual Processing: 3. Explanation of hierarchy: Visual Cognition
- Explanation of hierarchy:
Visual Cognition: ability to use vision to complete cognitive processing (manipulate and integrate visual input with other sensory info to gain knowledge, solve problems, make plans and decisions). - begins in childhood
- e.g. if we see a 12 inch adult, by applying size constancy, we assume the adult must be a good distance away
- Serves as a foundation for all academic endeavors
- Can’t occur without visual memory
Explanation of hierarchy: Visual Memory
Visual Memory: mental manipulation of visual stimuli requires ability to create and retain a picture of the object in the mind’s eye while visual analysis is completed. Must also be able to store and retrieve images from long-term memory.
Explanation of hierarchy: Pattern Recognition
Pattern recognition:
Identifying the salient features of an object and using them to distinguish the object from its surroundings.
1. Salient features: one that distinguishes a particular object from another. E.g. the salient feature that distinguishes an “E” from an “F”
2. Involves two abilities:
- identify the configural and holistic aspects of an object (general shape, construe and features)
- ability to identify specific features of an object (color, shading, texture)
Explanation of hierarchy: Visual Scanning
Visual Scanning: organized and thorough scanning of the visual array through use of saccadic eye movements (eye moves toward an object of interest)
1. Most important details re-examined multiple times through a cyclic saccades to ensure correct identification
Explanation of hierarchy: Visual Attention
Visual Attention: saccadic movements reflect engagement of visual attention shifting from object to object.
- Occurs on two levels:
- Automatic/reflexive level controlled mainly by brainstem. Engaged by novel stimulus occurring in the peripheral visual field (e.g. flash of light). Serves as protection from intrusions.
- Voluntary level driven by cortical processes of cognition. Completed for purpose of gathering info, driven by desire to locate an object in the environment or obtain particular info (e.g. looking for keys OR locating exit). - How a person attends to an object determines if that visual input is analyzed by the CNS
- A critical prerequisite for visual cognitive processing
- Type of visual attention engaged by CNS depends on type of visual analysis needed → Global awareness (e.g. awareness chair is in a room) vs. selective visual attention (e.g. identify style of chair). It is necessary to use more than one type of visual attention simultaneously.
- Visual attention requires large amount of neural processing and therefore can easily be disrupted by brain injury, BUT is highly resilient.
All levels of hierarchy can’t occur without visual input from visual functions of:
- oculomotor control: enables eye movements to be completed quickly and accurately and ensures perceptual stability
- visual fields: register the visual scene and ensure CNS receives complete information
- visual acuity: ensures visual information sent to CNS is accurate.
Deficits in the hierarchy
Brain injury or disease can disrupt visual processing at any level in the hierarchy.
Because of the way the hierarchy is structured, a client can appear to have a deficit in higher level process, even though the deficit is actually at a lower level.
Intervention: ophthalmologist
Opthamologist - medical doctor who completed residency in ophthalmology. Primarily responsible for diagnosing and treating medical condition that cause visual impairment. Neuro-ophthalmologists are board certified and treat the largest number of persons with visual impairments from brain injury.
Intervention: optometrist
Optometrist - independent healthcare provider with a doctorate in optometry from a postgraduate university program. Specialize in variety of areas: neurorehabilitative, developmental and behavioral optometry. NOT medical doctors. Also treat and diagnose medical conditions resulting in vision loss. Provide nearly ⅔ of primary eye care in US.
*Which specialty the team uses depends primarily on availability and reimbursement
OT Eval
To develop an intervention plan, OT must link limitations in occupation and performance to the presence of a visual impairment
Assessments are utilized by the OT to establish this link. Also known as establishing “medical necessity” → prerequisite for reimbursement for OT services.
Ophthalmologist or optometrist are rarely part of the rehab team. Process is time-consuming and difficult.
OT practitioner must convince physician/case manager that a deficit exists and is limiting occupational performance and that a referral to specialists is needed. Requires OT to be well versed in basic visual assessments such as: acuity, contrast sensitivity function and visual field.
One example of assessment by textbook author: Brain Injury Visual Assessment Battery for Adults (biVABA). Designed specifically for OTs in developing intervention plans. Consists of 17 subtests including tools used by ophthalmologists and optometrists.
Occupational Therapy Intervention: Main focus
Main focus is to change outcome in categories of visual disability and visual handicap
Occupational Therapy Intervention: two approaches
Remediation: attempts to establish or restore client’s ability to complete visual processing
Compensatory: emphasis is on changing the context of the environment or task to enable client to successfully use current level of visual processing
- Approaches may be used alone or together depending on client needs
- Education of client and family is always used in conjunction with the two approaches
Visual acuity
Visual acuity: the ability to see small visual details, contributes to CNS recognizing objects by providing clear and precise visual information.
Greater quality visual input → More precise image created by CNS processing → faster and more accurate CNS in recognizing and discriminating objects → faster and more accurate information processing to facilitate decision making
Acuity process
- Light rays enter eye through pupil and are focused precisely on the retina by the cornea, lens and optic media.
- Retina processes the light and records a “picture” that’s relayed to the CNS via the optic nerve and pathway
- Process requires: precise focus of light on retina, maintaining sharp focus over various focal distances, sufficient illumination of retina to capture a quality image, optic nerve transmitting image through CNS for processing.
- Any compromise of these structures results in degradation of the image and reduced acuity
Measuring visual acuity
Most commonly measured by reading progressively smaller optotypes on a chart. Common measurement: Snellen fraction (e.g. 20/20 vision) which is a ratio of distance to the size of the optotype.
- 20/50 = Standing at a distance of 20 feet, person can see a letter that a person with normal vision would see at 50 feet.
Visual acuity is a continuum
Visual acuity is a continuum of visual function ranging from detection of high-contrast features to detection of low-contrast features
Low-contrast acuity AKA contrast sensitivity function (CSF):
Low-contrast acuity AKA contrast sensitivity function (CSF): ability to detect borders of object as they decrease in contrast from background
Because environment is made of low-contrast features (curbs and steps, faces), CSF is critical to negotiating an environment safely.
CSF can be impaired even when high-contrast acuity is intact.
Two forms of high-contrast acuity:
Distance acuity and reading acuity
Distance acuity:
Distance acuity: ability to see objects at a distance
Reading acuity:
Reading acuity: ability to see objects clearly as they come closer to the eye. Near vision range → 3-16 inches from eye. Measured by reading progressively smaller print. Dependent on brainstem neural processing of accommodation.
Accommodation
Accommodation: enables eye to maintain clear focus on object as it comes closer. As object approaches eye, point of focus on retina is pushed farther back, causing it to go out of focus. CNS adjusts through three step process of accommodation:
- Eyes converge to ensure light rays stay parallel and in focus
- Crystalline lens thickens to refract light rays more strongly and shorten focal distance
- Pupil restricts to reduce light ray scattering
Controlled by cranial nerve III (oculomotor). Injuries that affect brainstem (where nuclei of CN III is located) or this nerve, can disrupt accommodation.
Presbyopia
Accommodation works effectively until 4th decade of life. At age 50 lens becomes less flexible, creating presbyopia
Why do both forms of acuity need to be measured
Because of the influence of accommodation, both forms of acuity need to be measured to gain accurate assessment of acuity function
Deficits in Visual Acuity
Most deficiencies are due to defects in optical system which cause image to be poorly focused on retina
Myopia
Myopia (nearsightedness): image of object is focused on a point in front of the retina and is blurred once it reaches retina. Corrected via a concave lens in front of retina.
Hyperopia
Hyperopia (farsightedness): image comes into focus behind the retina causing image to remain out of focus on retina. Corrected via a convex lens in front of retina.
Astigmatism
Astigmatism: light is focused differently by two meridians 90 degrees apart. Caused by cornea that are not perfectly smooth and spherical (spoon shaped or dimpled) which blurs image because both meridians can’t be focused on the retina. Corrected via cylindrical lens in front of eye.
Optical defects primarily occur because of impairments in 3 areas of processing
- Disruption of the ability to focus light onto retina
- Inability of retina to accurately process the image
- Inability of optic nerve to transmit info to the rest of CNS
Impairments may occur from:
brain injury or disease
Disruption of the Ability to Focus an Image on the Retina
Focusing an image depends on the transparency of structures between outside of the eye and the retina and their ability to focus light rays
Light passes through four transparent media: Cornea Aqueous humor Crystalline lens Vitreous Humor
Disruption of the Ability to Focus an Image on the Retina
Conditions that can occur with head trauma: Corneal scarring
Corneal scarring - may result from direct trauma to eye. Inner layers of cornea are damaged and scar as they heal, creating an irregular surface that refracts light unevenly. Experiences blurred vision similar to astigmatism.
Disruption of the Ability to Focus an Image on the Retina
Conditions that can occur with head trauma: trauma-induced cataract
Trauma-induced cataract - caused by trauma to crystalline lens which may displace or develop a cataract that clouds the lens and reduces acuity.
Disruption of the Ability to Focus an Image on the Retina
Conditions that can occur with head trauma: vitreous hemorrhage
Vitreous hemorrhage - trauma to eye can result in bleeding to the vitreous humor. Since blood is opaque, light cannot pass through and client experiences floaters, shadows and episodes of darkness as blood passes in front of retina.
- only vitreous hemorrhage is temporary and resolves w/o treatment
Disruption of the Ability of the Retina to Process the Image
Health and integrity of retina affects quality of image sent to CNS for processing. Receptor cells of retina can be damaged by injury or disease
Diseases:
Macular degeneration
Diabetic retinopathy
–Both associated with age and increase in incidence in the 7th and 8th decade of life
Retina damage can reduce high- and low-contrast acuity.
Approx. 1 in 4 persons older than 80 has a visual impairment that affects retina
Too often, visual loss resulting from disease is overlooked or misdiagnosed as an impairment in attention or cognition associated with CVA
Disruption of the Ability of the Optic Nerve to Send the Retinal Image
Can occur from penetrating injury to the nerve
Can also occur from indirect trauma from optic canal fracture from facial or blunt forehead fractures. Most common in children and young adults. Usually results in unilateral injury.
Closed head injuries can cause stretching or tearing of optic nerve. Results in significant and most often bilateral damage.
Compression of nerves from intracranial swelling or hematoma resulting in bilateral damage
Glaucoma damages nerve fibers carrying peripheral visual field input. Can also affect central visual field and reduce visual acuity.
Multiple Sclerosis causes plaques to develop along nerve resulting in optic neuritis causing reduced acuity, sensitivity to light
Occupational Limitations Caused by Reduced Visual Acuity
Severity of limitation
Severity of limitation depends on extent of loss of acuity and whether there is a loss of central acuity, peripheral acuity or both.
Occupational Limitations Caused by Reduced Visual Acuity
Central acuity loss
Central acuity loss: inability to discriminate small visual details and to distinguish contrast and color.
- Activities dependent on reading, writing, fine motor coordination are affected (reading recipes, dialing, identifying money, makeup/shaving)
Occupational Limitations Caused by Reduced Visual Acuity
Peripheral acuity loss
Peripheral acuity loss: mobility is affected. Unable to identify landmarks/obstacles in path, unable to accurately detect motion, may impair ability to safely maintain orientation in environment.
Assessment of Performance Skills: 1st step
First begin with observation of client performance in daily activities
Clients with deficits in visual acuity often complain of:
Clients with deficits in visual acuity often complain of: difficulty reading print, print being too small, faint or distorted, parts of words missing or words running together and swirling
Clients with CSF deficits often complain of:
clients complain of inability to see faces clearly or distinguishing between similar colors or low contrast substances like spilled water on floor
Both high-contrast and low-contrast acuity measured. High contrast acuity measured for:
High-contrast acuity measured for distance and for reading via a Snellen chart.
- Needs to be illuminated and at an exact viewing distance (remember snellen measurement is a ratio with distance) to get an accurate measurement.
- Acuity level is determined by the smallest line that can be read with majority accuracy
- Standard charts measure primarily in a range of acuity that can be compensated for with glasses, nothing below a 20/200. Below this level, needs to be referred out.
- Test charts for measuring low range visual acuity: LeaNumbers Low Vision Test Chart and the Warren Test Card from the biVABA
Assessing CSF
CSF also assessed via similar charts, but size of letters remains the same and contrast diminishes down each line
Assessments include: LeaNumbers Low Contrast Screener and Tests & LeaSymbols Low Contrast Screener and Tests
Must also be illuminated properly & at specific distance
If OT recognizes deficit:
Remember, OT’s role is to link the presence of a deficiency to a limitation in occupational performance NOT diagnosing cause. That is the role of the ophthalmologist or optometrist who then determines how to manage condition.
When OT determines an acuity deficiency, that information is used to modify activities and environment
Visual Field
Visual field: the external world that can be seen when a person looks straight ahead.
Extends 60 degrees superiorly, 75 degrees inferiorly, 60 degrees to the nasal side, 100 degrees to the temporal side
Most of the field is binocular and is seen by both eyes except small portion of peripheral temporal field which is monocular because of occlusion from the nose
