Cortical blindness is among the rare neurological conditions resulting in binocular vision loss due to insult to the occipital cortex. Anton-Babinski syndrome (Anton syndrome or ABS) is visual anosognosia (denial of loss of vision) associated with confabulation (defined as the emergence of memories of events and experiences which never took place) in the setting of obvious visual loss and cortical blindness.
It is essentially neurological visual impairment/disturbance resulting from abnormality or damage in the brain rather than due to eye abnormalities.
The first description of ABS dates back to the Roman era when Seneca, a Roman philosopher, and politician, described the case of Harpaste, a slave who acutely became blind. She used to deny her illness and argue irrationally about room darkness and constantly ask her attendants to change quarters. This case demonstrates the main symptoms of ABS, including acutely acquired blindness and anosognosia in the presence of relatively well-preserved cognition.
The next documented description of visual anosognosia was made by the French Renaissance writer Michel de Montaigne (1533 - 1592), who described a nobleman who denied his own blindness. In 1895, the Austrian psychiatrist and neurologist Gabriel Anton (1858 - 1933) described the case of Juliane Hochriehser, a 69-year-old dairymaid who had anosognosia with cortical deafness due to a lesion on her both temporal lobes. He also outlined other cases of patients with objective blindness and deafness who denied their deficits. In 1914, the French-Polish neurologist Joseph François Babinski (1857 - 1932) used for the first time the term “anosognosia” to describe the unawareness of the deficit in patients with hemiplegia.
In 1920 Meyer first reported occipital lobe infarction and postulated compression of branches of the posterior cerebral artery as the causal factor for ABS.
Anton-Babinski syndrome is a very rare condition with only 28 cases published between 1965 and 2016.
Pathophysiology of blindness:
The ability to recognize visually presented objects depends on the integrity of the following:
1. Visual pathways
2. The primary visual area of the cerebral cortex (Brodmann area 17)
3. The secondary visual cortex, lying just anterior to area 17 (Brodmann areas 18 and 19) of the occipital lobe
4. The angular gyrus of the dominant hemisphere (Brodmann area 39) - visual association area.
Patterns of visual deficits in ABS:
1. Blindness: Although the anterior visual tracts are intact, bilateral occipital brain damage results in blindness.
2. Gunbarrel vision: Tiny islands of vision sometimes persist, and in such cases, the patient may report that vision fluctuates as images are captured in the preserved portions; in rare instances, only peripheral vision is lost, and central vision is spared, resulting in gun-barrel vision
3. Perception of movements alone: Movement of objects may be perceived, either consciously (Riddoch's syndrome) or unconsciously (blindsight)
4. Motion blindness: Patients will be able to see objects but will not be able to appreciate their motion. This may be explained by the presence of projections from the lateral geniculate nucleus, to both the primary visual cortex via the optic radiations and the motion-selective middle temporal area (MT or V5)
5. Charles Bonnet syndrome: In this condition, the patient will have a loss of vision (due to any reason), but insight will be preserved. Such patients may experience visual hallucinations containing images of unfamiliar objects and people.
Pathophysiology of anosognosia:
Theories to explain the unawareness of deficit on the Anton-Babinsky syndrome:
a. Conscious awareness system (CAS): This function of this system is to monitor the inputs from all the sensory organs and is located in the bilateral parietal lobes. Another system integrating the inputs is located within the bilateral frontal lobes and this is connected with the former system. This interconnected system helps in performing complex cognitive tasks. In Anton-Babinski syndrome, damage of association pathways between visual cortex and CAS would be responsible for the lack of awareness of the visual deficit.
b. Disconnection of the visual area from language area: This makes the patient fabulate answers as he/she will be not able to describe the visual stimulus.
c. Overaction of the secondary visual pathway: A secondary visual system is located in the superior colliculus, pulvinar, and temporoparietal regions. This system will usually send impulses to the visual association cortex (visual monitor). If there is no transmission of impulse in the geniculo-calcarine pathway, the secondary visual pathway will gain predominance and results in fabulation in blind patients.
2. Neuropsychological mechanisms:
a. Defective visual monitor: This leads to incorrect interpretation of images.
b. The presence of false-positive feedback from visual monitor: This leads to a perception of false images which in turn leads the speech areas to come out with an abnormal response.
Pathophysiology of confabulation:
Patients start to confabulate to fill in the missing sensory input. It was suggested by Anton that the functioning speech and language areas will be disconnected from the damaged visual pathways. If there are no proper sensory visual inputs, the functioning speech areas may confabulate a response. In this, patients adamantly claim that they are capable of seeing and/ or experiencing strange visual hallucinatory episodes, consequently resulting in confabulation.
Criteria for diagnosis of cortical blindness:
The classical description of clinical features of ABS:
Though the patient is blind, he will behave and talk as if he has normal vision. Attention is aroused however when the patient is found to collide with pieces of furniture, to fall over objects, and to experience difficulty in finding his way around. They may try to walk through a wall or through a closed door on his way from one room to another. Suspicion is still further alerted when they begin to describe people and objects around them who do not exist. Mental confusion may also be seen. Patients with ABS may give excuses for their blindness. They may explain that their inability to see is because of lack of proper lighting and they may try their best to prove that they are not blind thereby putting themselves in danger.
A proper history and physical examination should be followed by:
Treatment of ABS is based on the etiology. If due to stroke, the usual treatment is with antiplatelet agents. Any cardiovascular condition causing stroke should be ruled out.
ABS syndrome needs to be differentiated from:
Recovery of visual function has occurred in conditions causing Anton's syndrome such as hypertensive encephalopathy and cortical hypoperfusion. In these conditions, correction of the causative factor may lead to resolution of symptoms. The outcome of cortical blindness depends on the age of patient, etiology, severity, duration, initial recovery time, medical history.
Patients suffering from ABS are prone to trauma as they pretend to have a vision and try to keep moving around.
Patient counseling can be done based on etiology. But confabulation may be difficult to treat with counseling alone.
Before diagnosing a patient as having ABS, proper ophthalmological evaluation should be done. Discussion regarding the etiology of ABS should be done with neuroradiologist, neurologist, and clinician. A team of specialty trained nurses and clinicians is necessary to evaluate and provide management of this condition for the best outcomes.
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