Anton Syndrome

Etiology

Causes include[3][4][5][6][7][8][9][10][11]:

1. An ischemic stroke involving bilateral occipital lobes due to the involvement of posterior cerebral arteries (most common cause)
2. Cardiac surgery
3. Cerebral angiography
4. MELAS (mitochondrial myopathy, encephalomyopathy, lactic acidosis, and stroke-like episodes)
5. Preeclampsia
6. Obstetric hemorrhage
7. Head trauma
8. Adrenoleukodystrophy
9. Hypertensive encephalopathy
10. Angiitis (autoimmune) of the central nervous system
11. Progressive multifocal leukoencephalopathy, in the setting of human immunodeficiency virus infection
12. Multiple sclerosis
13. Posterior reversible encephalopathy syndrome (PRES) secondary to SARS-CoV-2 (COVID-19) pneumonia
14. Fat embolism syndrome
15. Trousseau syndrome

Epidemiology

Anton-Babinski syndrome is a very rare condition with only 28 cases published between 1965 and 2016.[12]

Pathophysiology

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 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[13]:

Theories to explain the unawareness of deficit on the Anton-Babinsky syndrome:

1. Disconnection phenomena:

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 the 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[1]:

Patients start to confabulate to fill in the missing sensory input. Anton suggested that 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.

In short, ABS can be caused by lesions along the following structures:

• Visual cortices
• Bilateral lateral geniculate bodies
• Posterior limbs of the internal capsules
• Optic radiations
• Corpus callosum

History and Physical

Criteria for diagnosis of cortical blindness:

1. Loss of all visual sensations, including the perception of light and dark
2. Loss of menace reflex
3. Preservation of light and accommodation pupillary reflexes
4. Normal fundoscopic examination, and
5. Preservation of ocular movements

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 a closed door on their 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 the lack of proper lighting, and they may try their best to prove that they are not blind, thereby putting themselves in danger.

Evaluation

A proper history and physical examination should be followed by:

• Detailed neuroophthalmological evaluation including testing for visual acuity and the field of vision
• Magnetic resonance imaging of the brain
• Echocardiogram and carotid doppler to rule out the cardiovascular cause of stroke
• Visual evoked potential in confusing cases and pediatric patients

Treatment / Management

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.

Differential Diagnosis

ABS syndrome needs to be differentiated from:

• Cerebral visual impairment
• Delayed visual development
• Homonymous hemianopia
• Lack of facial recognition
• Visual agnosia
• Visual neglect
• Visual perceptual disorders

Prognosis

Recovery of visual function has occurred in conditions causing ABS, such as hypertensive encephalopathy and cortical hypoperfusion. In these conditions, correction of the causative factor may lead to the resolution of symptoms. The outcome of cortical blindness depends on the age of the patient, etiology, severity, duration, initial recovery time, medical history.

Complications

Patients suffering from ABS are prone to trauma as they pretend to have a vision and try to keep moving around.