Perceptual visual dysfunction in children: Indian perspective
Pehere NK and Dutton GN have very vividly voiced their views on "Perceptual visual dysfunction in children" in an editorial published in Indian Journal of Ophthalmology.
According to the authors, Perceptual visual dysfunction (PVD) comprises a group of vision disorders resulting from anomalies of higher visual functions, affecting the higher visual centres in the posterior parietal and temporal lobes. Typically, parents notice these children being unable to perform certain everyday visual tasks, yet to be functioning well in other areas of activity. When tested in the clinic, many can have normal or near‑normal primary visual functions like visual acuity and visual fields, nevertheless they can be significantly visually disabled.
Higher Visual Functions
The term higher visual function, includes visual perception, visual cognition, visual attention and visual guidance of movement. Visual perception refers to becoming aware of something, through seeing.
Visual cognition concerns the mental process of interpreting incoming visual information for recognition and thinking about its significance, relating this information to known the imagery in memory. Visual attention refers to the capacity to attend visually to objects of interest within the visual scene. Visual guidance of movement encompasses mapping of visual information in the mind and using this to guide movement of the limbs and body. Disorders of these functions are called, cognitive or PVDs or higher functioning cerebral visual impairments. These can exist in the presence or absence of issues with primary processing of vision i.e. visual acuity, visual field, color vision and contrast sensitivity.
Higher Visual Processing
There are two broad functional divisions in our visual brains, each one with a specific task. These are called the dorsal and the ventral streams. Primary processing of the incoming image data for visual acuity, visual fields, color vision and contrast sensitivity takes place within the occipital lobes. After this initial processing, the visual information is carried from the occipital lobes by a connecting pathway, called the 'dorsal stream' (also known as the 'vision for action' pathway), to the intraparietal sulcus (IPS) in the posterior parietal lobes, and another connecting pathway, called the 'ventral stream' (also known as the 'vision for perception' pathway), to the inferior temporal lobes.
Processing in the posterior parietal lobes
The posterior parietal lobes serve the following functions:
a. Non‑conscious moment‑to‑moment synthesis of the structure of the visual scene, and passing this framework to the frontal lobes to accord attention to an object of interest: While alert, we are bombarded with a lot of visual information. Our posterior parietal lobes, without our knowing, map the overall visual scene and send this information via the superior longitudinal fasciculus to the frontal lobes, which choose a corresponding element of interest in the scene (provided by the image analysis system in the temporal lobes) to attend to at any one time.
b. Creating an up to date mental virtual three‑dimensional map of the external surrounding world: The posterior parietal lobes create a three dimensional map of the external world in our minds, which is refreshed from moment‑to‑moment. This helps us perform all our visually guided actions with precision and allows us to move efficiently through our surroundings without bumping into people or obstacles. Similar processing also takes place for sound, the direction of which is mapped in nearby areas.
c. Visual guidance of body movements: As the posterior parietal lobes give us information about the locations, dimensions and distances of objects in relation to our body, these measures are used to facilitate our movement through the environment including calibration of the gap between our fingers and thumb as we reach with precision to pick up an object. Brain cells and their neurones in the posterior parietal lobes are clustered in separate regions, with each having a specialist function. For example 'visuomotor function' (action based on visual input) is located in the 'lateral intraparietal area' (LIP) linked to the frontal eye fields that direct fast eye movement to the target of interest, and the 'parietal reach region' (PRR) that calibrates reaching out and the 'anterior intraparietal area' (AIP), which brings about accurate grasp.
Extensive damage to the posterior parietal lobes would therefore be expected to give rise to the following:
1. Inability to process multiple visual objects in the surroundings - This is called 'simultanagnostic vision' (simultan‑ at the same time, agnosia‑ not knowing)
2. Impaired visual guidance of movement‑ This is called 'optic ataxia' (a term coined by Hungarian Neurologist Rudolph Balint‑ optische ataxie).
3. Inability to give attention to hearing and vision at the same time‑ Such children tend to look away from the person talking during conversations
4. Inability to locate the direction of sound‑ If someone calls, the child cannot make out, from where the person is calling.
5. Inability to move the eyes from one object to another on request, despite normal eye movements. This is called apraxia of gaze. Here the eyes cannot be moved to an item specified by another person because it cannot be seen owing to the simultanagnosia.
6. Lower visual field impairment is a common accompaniment because upper visual pathways in the superior optic radiations serving the lower visual field pass through the posterior parietal lobes. This may lead to collision with low objects while moving around.
The pathway between the occipital and posterior parietal lobes is referred to functionally as the dorsal stream and anatomically as the superior longitudinal fasciculus. The superior longitudinal fasciculus (SLF) is a bidirectional occipito‑parietal‑frontal tract connecting the occipital and the prefrontal cortices (serving visual attention) via the posterior parietal cortex. Thinning and decreased fractional anisotropy within the SLF has been shown to be a biomarker for PVD in children with cerebral palsy.
Processing by the inferior temporal lobes
The inferior temporal lobes provide a storehouse for the images acquired over a lifetime. The incoming image data provided by the occipital lobes is compared with this image store and when a match takes place, an item is 'recognized'. This is how faces, objects, shapes and routes are recognized by seeing.
Specific neurons in the inferior temporal lobes are highly selective for the kind of visual elements that they respond to. There are specialized centres for processing faces (the fusiform face area‑ FFA), everyday objects (the lateral occipital area‑ LO area), routes (the parahippocampal place area‑ PPA) for example. Most of these areas are clustered together on the underside of the temporal lobes near their junction with the occipital lobes.
Injury to left temporal lobe may lead to symptoms of difficulty in recognizing shapes, objects, and letters. Injury to the right temporal lobe may lead to difficulty in recognizing faces (prosopagnosia‑ from Greek prosopon, meaning face) and the language of facial expressions, and topographic agnosia (inability to retain where things are and routes). If both temporal lobes are injured, one may experience difficulty in analyzing line length and orientation, estimation of object size, and impairment of visual memory.
The pathway between the occipital and temporal lobes is referred to functionally as the ventral stream and anatomically as the inferior longitudinal fasciculus.
Motion perception Motion perception is served anterior to the occipital lobes in the middle temporal lobes, referred to as area MT (earlier called area V5). This area primarily receives inputs from the more peripheral visual fields. Such motion detection and analysis is largely a subconscious function. Bilateral damage to this area leads to inability to see moving targets, called akinetopsia, which is rare or inability to see fast moving objects or to perceive and interpret biological movement, which is now being increasingly recognized in children with prematurity and periventricular leukomalacia and is called dyskinetopsia
When to suspect PVD in a child?
One should suspect PVD when a child:
1. Seems to see well sometimes but other times not so well
2. Has a limited span of visual attention
3. Is making additional use of the other senses like hearing and touch, for various tasks
4. Needs more time to learn the details of an object, especially if there are multiple things around.
5. Needs more time to find familiar people in a busy environment, such as during a birthday party.
6. Has problems with eye‑hand coordination e.g., placing a ring on a ring tower, putting a coin in a piggy bank.
7. Has difficulty in recognizing people by seeing
8. Has difficulty in understanding the language of facial expressions
9. Has difficulty seeing moving things like a ball or traffic
10. Has difficulty judging distances and depth: A child may find it hard to learn to climb stairs. She may prefer to sit to go down stairs, rather than walk.
11. Has difficulty getting orientated to new surroundings like a wedding hall
In a child with any of the above symptoms, one should seek a history of perinatal brain injury or any other neurological issues, look for the presence of optic disc pallor or cupping, seek the presence any lesions on MRI brain and the presence of any neurological visual field defects. If any of these are present, the chances of the child having PVD are greater. One then needs to probe more deeply into the child's visual functioning in different areas, using a structured history taking approach as described below.
Importance of structured history taking
The clinical manifestations of PVD may go unrecognized for years despite visiting eye care professionals and parents being highly educated, because the visual origin of the child's symptoms may not have been considered.
Practical tips on the use of the CVI inventory
1. One should remember that it is not a diagnostic or a screening tool for PVD, but it is an inventory to profile the child's visual functioning in different spheres of day‑to‑day life, to identify problem areas and to come up with strategies to help with these issues.
2. For time management, one may take help from support staff to administer the inventory, but it is important for the ophthalmologist to confirm the positive observations made.
3. While interviewing parents, it is important not to guide them towards an answer but to just explain the questions and choices.
4. One may find multiple problem areas for a child, which may need to be prioritized and tackled one by one.
5. The strategies we choose for each difficulty need to be appropriate for the family's socio‑economic background and available support system.
6. It is important to communicate these issues with all those working with the child for various issues including the parents, teachers, and all therapists, in language they all can understand.
7. Most families can cope with only 2 or 3 new strategies at once, so when many are needed, they are introduced gradually.
Through a comprehensive eye examination, one must identify any treatable ophthalmic conditions like refractive errors, amblyopia, and any large angle constant squint needing surgery. In addition to regular examination, one should look for the following:
Tests for cognitive visual functions
- 1.Lea puzzle
- 2.The Lea mailbox test
- 3.Object sorting test
- 4.Tests for motion perception
- 5.Tests for simultanagnosia
Neuropsychological evaluation is commonly carried out in some European countries as a part of the work‑up for PVD. The evaluation comprises the intelligence quotient (IQ) test for both performance and verbal IQ, and tests for attention and memory. A verbal IQ better than the performance IQ, is disorders. These include autism spectrum disorder (ASD), attention deficit hyperactivity disorder (ADHD), specific learning disability (sLD) and intellectual disability.
The diagnosis of PVD should be suspected in any child in whom the visual behaviors are not explained by the ophthalmic examination. The presence of a known neurological issue, such as an eventful perinatal history, lesions found on neuroimaging, the presence of optic disc pallor or cupping, a visual acuity crowding ratio >2, raises the index of suspicion for the diagnosis of PVD. One should then use an inventory to take a systematic history about each of the domains of visual functioning in the child's day‑to‑day life.
The two purposes of the inventory are to elicit features consistent with the diagnosis of CVI and to profile the child's visual difficulties in order to plan management. To further substantiate the diagnosis, additional clinical tests for higher visual dysfunctions should be administered, remembering that although specific, they have low sensitivity, so cannot be used to 'rule out' the diagnosis of PVD.
Videos recorded by parents are often useful to understand how their child functions in familiar environments. OCT of the retina provides a quick diagnostic tool in a busy clinic, again recognizing that only positive tests are informative. Neuropsychological evaluation is ideal, but often not available. There are testing tools like the DTVP‑3 and MVPT‑4 that an eye care professional can use after training. It is important to be aware of the close differential diagnoses like ASD, sLD, ADHD and mild intellectual impairment and to make appropriate referral if needed, yet recognizing that PVD can coexist.
"A key component in helping children with PVD is to educate parents, carers and teachers about the visual basis of symptoms and explaining simple, doable strategies appropriate for the family's socio‑economic profile. In our experience such simple strategies often revolutionize the affected child's world and expedite learning and development."
Source: Pehere NK, Dutton GN. Perceptual visual dysfunction in children - An Indian perspective. Indian J Ophthalmol 2021;69:2004-11.