State-of-the-Art Transcranial Sonography for Neuropsychiatric and Dementia Disorders: A Narrative Review

Transcranial sonography (TCS) is widely acknowledged as a frontline imaging tool in movement disorder practice, particularly for separating idiopathic Parkinson's disease from its many mimics. In recent years, however, investigators have extended its reach, showing that the same portable probe can also capture structural and hemodynamic signatures of neuropsychiatric disorders and the major dementia syndromes. Across neuropsychiatry, a dim ("hypoechoic") median raphe emerges as the sonographic hallmark of serotonergic imbalance: it recurs in major depressive disorder, bipolar depression, and panic disorder, predicts better response to selective serotonin reuptake inhibitors, and even foreshadows post-stroke depression. Conversely, a bright, enlarged substantia nigra, the classic Parkinson marker, also surfaces in a subset of depressed patients and in some antipsychotic-treated individuals, implying latent dopaminergic stress unmasked by medication. In dementia research, TCS reliably tracks structural and hemodynamic changes that mirror, or sometimes precede, findings on MRI. Third-ventricle expansion correlates with falling Mini-Mental State Examination and Montreal Cognitive Assessment (MoCA) scores, while simple linear indices of the mesial temporal lobe (the medial temporal lobe-to-choroidal fissure ratio, the medial temporal lobe atrophy score in sonography (MTA-S), and the ventricle enlargement score in sonography (VES-S)) separate Alzheimer's disease from normal aging with MRI-like accuracy, but none of its costs or contraindications. Symmetric substantia nigra hyperechogenicity coupled with frontal-horn dilatation points to early dementia with Lewy bodies, whereas an asymmetric signal and a low "Onset Index" favor Parkinson's disease dementia. Vascular dementia, in turn, shows a sonographic triad of wide ventricles, sluggish middle-cerebral flow, and a stiff pulsatility profile, reflecting small-vessel disease. Taken together, these findings position TCS as a rapid, bedside window onto serotonergic tone, nigrostriatal integrity, mesial-temporal atrophy, and cerebrovascular health. Its portability and low running costs make it especially attractive when MRI is impractical, and its real-time nature suits longitudinal follow-up in cognitive and psychiatric clinics. Standardized acquisition protocols and large, multicenter validation studies are now needed to translate these promising markers into routine personalized care.