Negative- and low-pressure hydrocephalus (NegPH/LPH) is an uncommon and frequently misdiagnosed condition lacking standardised diagnostic and management protocols. We conducted a systematic review of published studies from 1980 to 2024 to synthesise evidence on aetiology, pathophysiology, and treatment. Thirty studies reporting 215 patients were identified, with antecedents including traumatic brain injury, aneurysmal subarachnoid haemorrhage, normal-pressure hydrocephalus, postoperative cases, and spontaneous presentations. Four interacting models were commonly described: viscoelastic brain change with reduced compliance, transmantle pressure gradients, poroelastic or glymphatic dysfunction, and impaired cerebrospinal fluid pulsatility. Management strategies centred on restoring compliance and cerebrospinal fluid pathways through subatmospheric external ventricular drainage, low-pressure or valveless shunting, targeted leak repair, and endoscopic third ventriculostomy. Outcomes were variable, but early recognition and carefully titrated drainage were repeatedly associated with clinical improvement, whereas delayed intervention, infection, and irreversible white-matter injury predicted poor recovery. This review highlights NegPH/LPH as a distinct neurosurgical entity and proposes a diagnostic and management framework, while emphasising the urgent need for multicentre registries with standardised definitions and outcome measures to guide evidence-based care.