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A 35 year old woman reports new vision loss. The examination is normal except that she consistently fails to identify any visual targets presented alone in her left hemifield, whereas she always identifies them when they are presented in her right hemifield. Formal visual field testing discloses these visual field abnormalities. Brain MRI is certifiably normal.
Where is the lesion?
Incorrect
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Correct!
Although this seems a relatively evasive answer, it is correct. You are looking at a complete homonymous hemianopia identified on confrontation and formal
visual field testing. When a homonymous hemianopia is COMPLETE, you can localize it to the retrochiasmal visual pathway, but not to a specific region
within that pathway!
As the MRI is certifiably normal, you will wonder if the defect is of psychogenic origin. That presumption is difficult to disprove clinically. Some clinicians have developed maneuvers to detect such defects, such as having the patient rapidly switch fixation back and forth between single targets displayed in opposite hemifields, and seeing if the movement into the putatively defective field is inaccurate. But I cannot make anything work reliably to exclude psychogenic homonymous hemianopia.
If you were depending on MRI here, be careful! There are several retrochiasmal lesions that are either invisible on MRI or so subtle that they are easily overlooked. Among them: 1) Recent hypoxic-ischemic injury owing to sudden systemic hypotension (MRI visual cortex volume loss may develop months later); 2) Creutzfeldt-Jakob disease, a prion protein disorder that may show subtle relevant MRI abnormalities only months later (positron emission tomography would earlier show a metabolically low visual cortex region); 3) focal encephalitis, also called “cerebritis” (small areas of high T2/FLAIR or cortical enhancement may be overlooked or dismissed);
4) Alzheimer disease with predominant volume loss in occipital and parietal
lobes (relatively more MRI volume loss on the side opposite to the hemianopia may be overlooked)
5) focal
metabolic encephalopathy in nonketotic hyperglycemia or mitochondrial cytopathy (which can be evanescent); and 6) optic tract lesion (because it is a
narrow structure wedged between midbrain and temporal lobe, lesions here are often overlooked).
This patient had signal abnormalities in the right optic tract
and in the cervical spinal cord consistent with multiple sclerosis.
As the MRI is certifiably normal, you will wonder if the defect is of psychogenic origin. That presumption is difficult to disprove clinically. Some clinicians have developed maneuvers to detect such defects, such as having the patient rapidly switch fixation back and forth between single targets displayed in opposite hemifields, and seeing if the movement into the putatively defective field is inaccurate. But I cannot make anything work reliably to exclude psychogenic homonymous hemianopia.
If you were depending on MRI here, be careful! There are several retrochiasmal lesions that are either invisible on MRI or so subtle that they are easily overlooked. Among them: 1) Recent hypoxic-ischemic injury owing to sudden systemic hypotension (MRI visual cortex volume loss may develop months later); 2) Creutzfeldt-Jakob disease, a prion protein disorder that may show subtle relevant MRI abnormalities only months later (positron emission tomography would earlier show a metabolically low visual cortex region); 3) focal encephalitis, also called “cerebritis” (small areas of high T2/FLAIR or cortical enhancement may be overlooked or dismissed);
This patient had signal abnormalities in the right optic tract