Differential Diagnosis for Ventriculomegaly, Low B12, and Psychosis

Single Most Likely Diagnosis

• Vitamin B12 Deficiency: This condition can cause ventriculomegaly due to demyelination and axonal damage in the brain, leading to increased cerebrospinal fluid (CSF) space. Low B12 levels can also contribute to psychiatric symptoms, including psychosis. The combination of ventriculomegaly, low B12, and psychosis makes this diagnosis highly plausible.

Other Likely Diagnoses

• Normal Pressure Hydrocephalus (NPH): Characterized by ventriculomegaly, NPH can present with cognitive decline, gait disturbances, and urinary incontinence. While psychosis is not a primary symptom, it can occur, especially in the context of significant cognitive impairment. Low B12 might be coincidental or contribute to the neurological symptoms.
• Subacute Combined Degeneration (SCD) of the Spinal Cord: A complication of vitamin B12 deficiency, SCD can lead to neurological symptoms, including cognitive and psychiatric disturbances. However, the primary symptoms usually involve the spinal cord, with less direct implication on ventriculomegaly.

Do Not Miss Diagnoses

• Neurosyphilis: This condition can cause a wide range of neurological symptoms, including ventriculomegaly, psychiatric symptoms (such as psychosis), and can be associated with low B12 levels due to malabsorption or poor dietary intake in the context of significant illness. Missing this diagnosis could lead to severe and irreversible neurological damage if not treated promptly.
• CNS Lymphoma or Other Space-Occupying Lesions: Although less likely, these conditions can cause ventriculomegaly and, depending on their location, can lead to psychiatric symptoms. Low B12 could be a separate issue but warrants investigation due to the potential for significant morbidity if a space-occupying lesion is present.

Rare Diagnoses

• Leigh Syndrome: A rare genetic disorder that affects the central nervous system and can cause ventriculomegaly among other symptoms. Psychiatric symptoms can occur, and while B12 metabolism might not be directly implicated, the condition's rarity and severity warrant consideration in a differential diagnosis to ensure no stone is left unturned.
• Methylmalonic Acidemia: A disorder of amino acid metabolism that can lead to neurological symptoms, including those related to B12 deficiency (as B12 is a cofactor in the metabolism of methylmalonic acid). Ventriculomegaly and psychiatric symptoms could potentially occur in the context of this disorder, although it would be an uncommon presentation.