Shadow Near Temporal Lobe on CT Scan
Immediate Diagnostic Approach
The finding of a "shadow" near the temporal lobe on CT requires immediate clarification of its characteristics—specifically whether it represents a hypodense lesion, mass effect, vascular abnormality, or artifact—followed by high-resolution temporal bone CT without contrast if vascular/temporal bone pathology is suspected, or MRI with contrast if parenchymal pathology is the concern. 1, 2
The term "shadow" is non-specific and requires systematic characterization based on:
Critical Features to Identify
Location specificity: Determine if the shadow involves the temporal bone structures (middle ear, mastoid, petrous bone), the temporal lobe parenchyma itself, or vascular structures adjacent to the temporal region 2
Density characteristics: Assess whether this represents hypodensity (suggesting edema, infarction, or demyelination), hyperdensity (suggesting hemorrhage or calcification), or a mass lesion with specific attenuation values 3, 4
Associated findings: Look for mass effect, midline shift, temporal horn asymmetry, or bone erosion, as these significantly narrow the differential diagnosis 4, 5
Differential Diagnosis by Location
Temporal Bone/Vascular Structures
High-resolution CT temporal bone without contrast is the definitive next step if the shadow appears related to temporal bone anatomy, as it can identify vascular variants (high-riding jugular bulb, jugular bulb dehiscence), sigmoid sinus abnormalities, and bony erosions 1, 2
Glomus tumors (tympanicum or jugulare) appear as soft tissue masses with characteristic bone erosion patterns—glomus tympanicum shows promontory involvement without jugular foramen erosion, while glomus jugulare demonstrates jugular foramen and carotid canal bone destruction 6
Vascular malformations including dural arteriovenous fistulas (dAVF) and arteriovenous malformations may appear as subtle density changes; CTA head and neck with IV contrast is indicated if vascular pathology is suspected, with 90% combined sensitivity and specificity for dAVF 7
Temporal Lobe Parenchyma
MRI with contrast is superior to CT for evaluating temporal lobe parenchymal lesions, demonstrating higher sensitivity for subtle structural abnormalities, tumors, and inflammatory processes 5
Temporal lobe epilepsy-related changes including mesial temporal sclerosis, cytoarchitectural abnormalities, and gliosis are often missed on CT but readily identified on MRI, particularly with coronal sequences 3, 4, 5
Tumors (low-grade gliomas, gangliogliomas, dysembryoplastic neuroepithelial tumors) are common in the temporal lobe and require MRI with contrast for characterization; CT detected only 12/50 temporal lobe lesions compared to 16/50 by MRI in one comparative study 5
Infectious/inflammatory processes including herpes simplex encephalitis characteristically affect the temporal lobes and show progressive abnormalities—initial CT may be negative in 20-75% of cases, with MRI being far more sensitive 7
Management Algorithm
Step 1: Characterize the CT Finding
Review the CT systematically: Assess bone windows for temporal bone abnormalities, soft tissue windows for parenchymal lesions, and vascular structures for asymmetry 2
Measure temporal horn size bilaterally: Asymmetric enlargement may indicate ipsilateral volume loss, though this is unreliable for microscopic pathology 4
Step 2: Select Appropriate Advanced Imaging
If vascular/temporal bone etiology suspected: Order high-resolution CT temporal bone without contrast first, followed by CTA head and neck if vascular pathology remains a concern 1, 2
If parenchymal lesion suspected: Proceed directly to MRI brain with and without contrast, using coronal sequences through the temporal lobes 5
If infectious encephalitis suspected: MRI is mandatory even if initial CT is negative, as 25-80% of HSV encephalitis cases show normal initial CT 7
Step 3: Clinical Correlation
Pulsatile tinnitus or vestibular symptoms: Strongly suggests vascular etiology (high-riding jugular bulb, glomus tumor, dAVF); pursue temporal bone CT and CTA 1, 6
Seizures: Temporal lobe epilepsy is likely; MRI with coronal temporal lobe sequences is essential, as CT misses most epileptogenic lesions 3, 4, 5
Altered mental status, fever, focal neurological deficits: Consider encephalitis or abscess; do not delay lumbar puncture for CT if no contraindications exist (papilledema, focal signs suggesting mass effect, new seizures, Glasgow Coma Score ≤10) 7
Critical Pitfalls to Avoid
Never biopsy a suspected vascular lesion without definitive imaging characterization—inadvertent biopsy of glomus tumors or vascular variants causes catastrophic hemorrhage 6
Do not rely on CT alone for temporal lobe pathology: MRI is superior for virtually all parenchymal processes, with CT missing the majority of epileptogenic lesions and early encephalitis 3, 4, 5
Avoid unnecessary delays: If clinical suspicion for encephalitis exists, perform lumbar puncture based on clinical criteria rather than waiting for CT in patients without contraindications 7
Do not dismiss temporal horn asymmetry: While unreliable for microscopic pathology, significant asymmetry warrants MRI evaluation for structural lesions 4
Recognize that normal initial imaging does not exclude pathology: Repeat imaging at 24-48 hours is often necessary for HSV encephalitis and evolving lesions 7