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. Primary visual area of the cerebral cortex (Brodmann area 17)
3. 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 can see objects but cannot perceive the motion of these objects. 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: Patients with visual loss from any cause may experience hallucinations, often very elaborate, with images of unfamiliar people or buildings, and so on, although with preservation of insight.
Pathophysiology of anosognosia:
Theories to explain the unawareness of deficit on the Anton-Babinsky syndrome:
a. The first theory describes a conscious awareness system (CAS) located on the parietal lobes, which monitors all the information received from the senses. This system connects with other, located on the frontal lobes, which integrates the information, in order to perform 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. The disconnection of the visual areas from other, such as language areas, makes the patients unable to describe the visual stimuli, and, because of that, fabulate an answer.
c. The signal transmission to the visual monitor (located on the visual association cortex) from a secondary visual system, located on the superior colliculus, pulvinar, and temporoparietal regions. In the absence of transmission on the geniculocalcarine pathway, this secondary visual pathway would allow blind patients to fabulate.
2. Neuropsychological mechanisms:
a. The monitor of visual stimuli is defective and is incorrectly interpreting images.
b. The presence of false feedback from another visual system. In this regard, the superior colliculus, pulvinar and temporo-parietal regions may transmit signals to the monitor when the geniculocalcarine system fails. In the absence of visual input, this false internal imagery may convince the monitor or speech areas to come out with a response.
Pathophysiology of confabulation:
Patients start to confabulate to fill in the missing sensory input. Anton suggested that damaged visual areas are effectively disconnected from functioning areas, such as speech and language areas. In the absence of input, functioning speech areas often 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 Anton syndrome often find excuses for their symptoms such as there is not enough light to see or they may also put themselves in danger to prove their intact vision. These patients act as though they could see, and in attempting to walk, collide with objects, even to the point of injury. They may offer excuses for the difficulties—”I lost my glasses,” “The light is dim”—or may only evince indifference to loss of sight.
Proper history and physical examination should be followed by:
Detailed neuro-ophthalmological evaluation including testing for visual acuity, field of vision
Magnetic Resonance Imaging of brain
Echocardiogram and carotid doppler to rule out cardiovascular cause of stroke
Visual evoked potential in confusing cases and pediatric patients
Treatment of ABS is based on the etiology. If due to stroke, the usual tretment is with antiplatelet agents. Any cardiovascualr condition causing stroke should be ruled out.
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 vision and try to keep moving around.
Patient counselling can be done based on the etiology. But confabulation may be difficult to treat with counselling 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 Physician.
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