Clinical Pathways in Neuro-ophthalmology : An ... - E-Lib FK UWKS

206 Clinical Pathways in Neuro-Ophthalmology, second edition
Zerr, 1998). In other studies, the sensitivity was 94 to 97% and the specificity 74 to
87% (Lemstra, 2000; Zerr, 2000a,b). False negatives have been documented (Chapman,
2000) and false positives have been noted with herpes simplex encephalitis,
meningoencephalitis, stroke, hypoxic brain injury, carcinomatous meningitis, vascular
dementia, Hashimoto’s encephalopathy, intracerebral metastasis, frontotemporal
dementia, dementia with Lewy bodies, and Alzheimer’s disease (Burkhard, 2001;
Chapman, 2000; Hernandez Echebarria, 2000; Lemstra, 2000; Zerr, 1998). The 14-3-3
protein assay may be positive in paraneoplastic neurologic disorders that may mimic
Jakob-Creutzfeldt disease, but the immunoblasting pattern of this protein distinguishes
most patients with paraneoplastic disorders from those with Jakob-
Creutzfeldt disease (Saiz, 1999).
2. Some patients with Alzheimer’s disease or Lewy body disease may develop a
homonymous field defect (Bashir, 1998; Brazis, 2000; Trick, 1995). MRI may be
normal or show only diffuse atrophy, and the EEG is normal or shows only mild
diffuse slowing. This diagnosis is suspected in patients with a slowly progressive
dementia without other ‘‘focal’’ neurologic findings, and the dementia usually far
outweighs the visual field impairment.
3. Most patients with field defects from cerebral infarction or hypoxia demonstrate MRI
changes compatible with ischemia. However, Moster et al described two patients,
one with bilateral homonymous congruous hemianopic central scotomata after
carbon monoxide poisoning and the other with bilateral congruous inferior visual
scotomata after global hypoxia, who were initially diagnosed with ‘‘functional’’
visual loss (Moster, 1996). Neither CT nor MRI adequately demonstrated the source
of the visual dysfunction, but single photon emission computed tomography
(SPECT) in one patient and positron emission tomography (PET) imaging in the
other confirmed the organic substrate of the visual impairment. Wang et al also
reported two patients with organophosphate intoxication associated with cortical
visual loss who had normal MRI but abnormal hypometabolism of the visual cortex
demonstrated on PET scanning (Wang, 1999). Brazis et al also presented a patient
with a homonymous field defect secondary to cerebral infarction with normal MRI
(Brazis, 2000). Functional imaging techniques, such as SPECT or PET, should thus be
considered in patients with suspected cortical visual loss and normal CT or MRI
studies. Functional MRI is also a promising method for the objective detection of
abnormalities in the afferent visual system (Miki, 1996).
4. Transient homonymous hemianopia with normal CT imaging has rarely been
reported with nonketotic hyperglycemia (Brazis, 2000; Harden, 1991). These patients
had other positive visual phenomena associated with a homonymous hemianopia.
Thus, nonketotic hyperglycemia may present with positive visual phenomena
associated with a homonymous field defect and normal neuroimaging.
5. Functional (nonorganic) hemianopias are associated with normal imaging studies
(Keane, 1998; Martin, 1998; Thompson, 1996). One method of determining if a field
defect is nonorganic is to test saccadic eye movements into the supposedly absent
portion of the field, with the patient assuming that eye movements and not visual
fields are being tested. Demonstrating ‘‘hemianopic’’ defects with both eyes open is
often useful (Keane, 1998). Another method is to place a 30-diopter Fresnel prism into
the upper quadrants of a trial frame (Carlow, 1995). After visual fields are obtained
without the prism, the prism is placed first base-out and then base-in and with each
change the fields are repeated. Patients with pathologic hemianopsias shift their