Brain Metastasis from Melanoma is the Most Likely Diagnosis

Given the constellation of posterior auricular lymphadenopathy, a suspicious skin lesion behind the right ear, pulmonary findings suggesting metastatic disease, and multiple intracranial signs, this patient almost certainly has metastatic melanoma with brain involvement.

1. Brain Metastasis from Melanoma

Mechanism(s) of Disease

Hematogenous dissemination of melanoma cells to the CNS occurs because the brain lacks lymphatic drainage 1.
Melanoma has a high propensity for CNS metastasis, with clinical detection in 10-40% of patients and autopsy studies revealing up to 90% involvement 1.
Metastatic deposits disrupt the blood-brain barrier, cause vasogenic edema, and increase intracranial pressure, producing headache, focal neurologic deficits, and personality changes 1.
The posterior fossa (cerebellum) is a common site for metastases, explaining the ataxic gait and left-sided cerebellar signs in this case 1.

Key History Features + Risk Factors

Headache is the most common presenting symptom of brain metastases from melanoma 1.
Progressive morning headaches worse with Valsalva maneuver indicate elevated intracranial pressure 1.
New neurologic findings (personality changes, ataxia, visual field defects) should raise immediate suspicion for CNS metastases in any melanoma patient 1.
History of cutaneous melanoma, particularly with prior excision or known primary lesion 1.
The small lesion behind the right ear with posterior auricular lymphadenopathy strongly suggests a primary melanoma with regional nodal spread 1.
Weight loss and pulmonary findings (decreased breath sounds, crackles) indicate systemic metastatic disease 2.

Physical Exam Findings That Increase Likelihood

Posterior auricular lymphadenopathy adjacent to a suspicious skin lesion is highly suggestive of melanoma with nodal metastasis 1.
Ataxic gait with left-sided cerebellar signs (dysmetria, intention tremor, dysdiadochokinesia) localizes to posterior fossa involvement 1.
Bilateral temporal visual field loss suggests chiasmatic or suprasellar involvement, or bilateral occipital lobe metastases 1.
Personality and mood changes reflect frontal lobe or diffuse cerebral involvement 1.
Decreased breath sounds in the right middle lobe with diffuse crackles suggests pulmonary metastases 2.
Papilledema on fundoscopy (if present) would confirm elevated intracranial pressure 1.

Helpful Diagnostic Studies

First-line imaging:

MRI brain with gadolinium contrast is the best diagnostic technique for detecting CNS metastases from melanoma 1.
MRI is superior to CT for identifying small metastases, posterior fossa lesions, and leptomeningeal disease 1.
Melanoma metastases typically appear as multiple enhancing lesions with surrounding vasogenic edema; hemorrhage is common due to melanoma's vascular nature 1.

Confirmatory and staging studies:

Biopsy of the posterior auricular lymph node or skin lesion provides tissue diagnosis and confirms melanoma 1.
CT chest/abdomen/pelvis with contrast to identify the primary site and assess systemic metastatic burden 2.
Serum LDH is often elevated in metastatic melanoma and correlates with tumor burden 1.
Brain biopsy is rarely needed if systemic melanoma is confirmed and imaging is characteristic 1, 3.

Pre-test Probability for This Case

HIGH pre-test probability

Points FOR brain metastasis from melanoma:

Suspicious skin lesion behind the right ear with regional lymphadenopathy is pathognomonic for melanoma with nodal spread 1.
Melanoma has the highest propensity for brain metastasis among solid tumors, with cumulative 5-year incidence of 7.4% 2.
Multiple neurologic findings (cerebellar signs, visual field defects, personality changes) suggest multifocal intracranial disease, typical of metastases 1.
Pulmonary findings indicate systemic metastatic disease, making brain metastases highly likely 2.
Headache is the most common presenting symptom of melanoma brain metastases 1.

Points AGAINST:

None significant; all clinical features align with metastatic melanoma.

2. Glioblastoma (Primary High-Grade Glioma)

Mechanism(s) of Disease

Glioblastoma arises from malignant transformation of glial cells (astrocytes), leading to rapid proliferation, neovascularization, and infiltrative growth 3.
Median survival is less than 12 months despite aggressive treatment 3.
Tumor growth causes mass effect, vasogenic edema, and elevated intracranial pressure, producing headache, focal deficits, and personality changes 3.
Glioblastomas are typically solitary and unilateral, though they can cross the corpus callosum (butterfly glioma) 3.

Key History Features + Risk Factors

Progressive neurologic symptoms over weeks to months, including headache, personality changes, and focal deficits 3.
Glioblastoma typically presents in older adults (peak incidence 45-70 years), making this 47-year-old patient within the typical age range 3.
No clear environmental risk factors; ionizing radiation is the only established risk 3.
Seizures occur in 30-50% of patients with supratentorial glioblastoma 3.

Physical Exam Findings That Increase Likelihood

Focal neurologic deficits corresponding to tumor location (e.g., cerebellar signs if posterior fossa involvement) 3.
Personality and mood changes suggest frontal lobe involvement 3.
Papilledema indicates elevated intracranial pressure 3.

Helpful Diagnostic Studies

First-line imaging:

MRI brain with gadolinium contrast shows an irregularly enhancing mass with central necrosis, surrounding vasogenic edema, and mass effect 3.
Glioblastoma typically appears as a solitary, ring-enhancing lesion with irregular borders and infiltrative margins 3.

Confirmatory studies:

Surgical resection or stereotactic biopsy is required for definitive histopathologic diagnosis 3.
Histology shows hypercellularity, nuclear atypia, microvascular proliferation, and necrosis 3.
Molecular markers (IDH mutation, MGMT promoter methylation, 1p/19q codeletion) guide prognosis and treatment 3.

Pre-test Probability for This Case

LOW pre-test probability

Points FOR glioblastoma:

Age 47 years is within the typical range for glioblastoma 3.
Progressive neurologic symptoms (headache, personality changes, ataxia) are consistent with a primary brain tumor 3.

Points AGAINST:

Posterior auricular lymphadenopathy and skin lesion are not explained by glioblastoma, which does not metastasize outside the CNS 3.
Pulmonary findings (decreased breath sounds, crackles) are inconsistent with glioblastoma, which does not cause systemic disease 3.
Bilateral temporal visual field loss suggests multifocal or chiasmatic involvement, less typical of a solitary glioblastoma 3.
Weight loss is uncommon in isolated glioblastoma without systemic disease 3.
Glioblastoma is typically a solitary lesion, whereas this patient's multifocal neurologic findings suggest multiple intracranial lesions 3.

3. Chronic Subdural Hematoma

Mechanism(s) of Disease

Chronic subdural hematoma results from tearing of bridging veins between the cortex and dural sinuses, typically following minor head trauma 4.
Blood accumulates slowly in the subdural space over weeks to months, forming a liquefied hematoma with a fibrous capsule 4.
The hematoma exerts mass effect, causing headache, altered mental status, and focal neurologic deficits 4.
Elderly patients and those on anticoagulation are at highest risk due to brain atrophy and increased bridging vein tension 4.

Key History Features + Risk Factors

History of head trauma (often minor or remote) is present in most cases, though 30-50% of patients do not recall trauma 4.
Risk factors include advanced age (>65 years), chronic alcohol use, anticoagulation, and brain atrophy 4.
Symptoms develop insidiously over weeks to months, including headache, confusion, personality changes, and focal deficits 4.
This 47-year-old patient is younger than the typical chronic subdural hematoma population 4.

Physical Exam Findings That Increase Likelihood

Altered mental status, confusion, or personality changes are common 4.
Focal neurologic deficits (hemiparesis, ataxia) may occur depending on hematoma location and mass effect 4.
Papilledema may be present if intracranial pressure is elevated 4.

Helpful Diagnostic Studies

First-line imaging:

Non-contrast CT brain is the initial study of choice and shows a crescent-shaped hypodense or isodense collection along the cerebral convexity 4.
Chronic subdural hematomas appear hypodense (dark) on CT due to liquefaction of blood over time 4.
Contrast-enhanced CT or MRI brain is recommended if dural metastasis or tumor is suspected, as these can mimic chronic subdural hematoma 4.

Confirmatory studies:

Surgical evacuation (burr hole drainage or craniotomy) is both diagnostic and therapeutic 4.
Histopathology of the hematoma capsule can reveal unexpected findings such as dural metastasis 4.

Pre-test Probability for This Case

LOW pre-test probability

Points FOR chronic subdural hematoma:

Progressive headache and personality changes are consistent with chronic subdural hematoma 4.
Focal neurologic deficits (ataxia, cerebellar signs) can occur with mass effect 4.

Points AGAINST:

No history of head trauma is reported, though this does not exclude the diagnosis 4.
Posterior auricular lymphadenopathy, skin lesion, and pulmonary findings are not explained by chronic subdural hematoma, which is an isolated intracranial process 4.
Weight loss is not a feature of chronic subdural hematoma unless there is an underlying malignancy 4.
Bilateral temporal visual field loss suggests chiasmatic or multifocal involvement, atypical for a unilateral subdural hematoma 4.
Dural metastasis can mimic chronic subdural hematoma on imaging, and contrast-enhanced CT is recommended to differentiate the two 4.
This patient's age (47 years) is younger than the typical chronic subdural hematoma population 4.

Summary of Pre-test Probabilities

Diagnosis	Pre-test Probability	Key Discriminating Features
Brain metastasis from melanoma	HIGH	Posterior auricular lymphadenopathy + skin lesion + pulmonary findings + multifocal neurologic signs [1,2]
Glioblastoma	LOW	No explanation for lymphadenopathy, skin lesion, or pulmonary findings [3]
Chronic subdural hematoma	LOW	No explanation for lymphadenopathy, skin lesion, pulmonary findings, or weight loss [4]

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