FDG PET Scan in Late-Life First Episode Psychosis
FDG PET scanning is not routinely indicated for an 89-year-old patient with first episode psychosis, and should only be considered when there is continued diagnostic uncertainty after initial structural imaging and comprehensive workup. 1
Initial Diagnostic Approach for First Episode Psychosis in Elderly
Primary Evaluation
- First episode psychosis in an elderly patient (89 years old) should begin with structural brain imaging, preferably MRI, to rule out secondary causes of psychosis 1
- The American Psychiatric Society guidelines suggest that brain MRI is preferred over CT for patients with new onset psychosis, particularly when the clinical picture is unclear or the presentation is atypical 1
- Common organic causes of late-life psychosis that should be ruled out include tumors, infarcts (particularly in the temporal lobe), encephalitis, multiple sclerosis, and neurodegenerative conditions 1
When to Consider FDG PET
- FDG PET should only be considered after structural imaging (MRI or CT) has been performed and interpreted, and diagnostic uncertainty remains 1
- The Alzheimer's Association guidelines specify that FDG PET may be considered when "the etiological diagnosis is equivocal, when there is only an intermediate level of diagnostic confidence, or when having very high confidence in the etiological diagnosis is needed" 1
- FDG PET is not a marker of specific molecular pathology but rather indicates cellular dysfunction (particularly synaptic dysfunction) with topographic patterns that may suggest particular neurodegenerative conditions 1
Clinical Scenarios When FDG PET May Be Warranted
Suspected Neurodegenerative Conditions
- FDG PET can help differentiate between Alzheimer's disease, frontotemporal lobar degeneration, and Lewy body dementia when these conditions are suspected as underlying causes of psychosis 1, 2
- For differentiating Alzheimer's disease versus frontotemporal dementia, FDG PET has shown 80-99% sensitivity and 63-98% specificity 1
- For Alzheimer's disease versus Lewy body dementia, FDG PET demonstrates 70-92% sensitivity and 74-100% specificity 1
Atypical Presentations
- FDG PET may be helpful when the presentation is atypical or when multiple pathologies are suspected 1
- In very early clinical stages (early mild cognitive impairment) with psychotic features, FDG PET might provide additional diagnostic information 1
- The American College of Radiology notes that FDG PET is not recommended for routine use in new onset psychosis without neurologic deficits 1
Important Limitations and Considerations
Age-Related Considerations
- In patients with severe stage dementia with global impairments (common in the very elderly), FDG PET is not helpful as it will likely show diffuse and global hypometabolism regardless of underlying cause 1
- The interpretation of FDG PET requires clinical-radiographic correlation and should incorporate pretest probability based on the clinical presentation and risk profile of the patient 1
Alternative Approaches
- If diagnostic uncertainty remains after structural imaging, cerebrospinal fluid (CSF) analysis for amyloid beta and tau/phosphorylated tau profiles may be more appropriate than FDG PET for evaluating Alzheimer's disease neuropathologic changes 1
- When FDG PET is not available, a single photon emission computed tomography (SPECT) scan may be considered as an alternative 1
Clinical Pitfalls to Avoid
- Ordering FDG PET as a first-line investigation before appropriate structural imaging and comprehensive medical workup is not recommended 1
- Failing to recognize delirium (which can present with psychotic features) as a medical emergency that requires prompt identification and management 3
- Missing hypoactive delirium, which is commonly overlooked but may present with psychotic features, especially in elderly patients 3
- Attributing symptoms solely to a primary psychiatric disorder without thoroughly investigating potential medical causes in elderly patients with new-onset psychosis 3, 4