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What is Normal pressure hydrocephalus and explain in detail?
Normal-pressure hydrocephalus (NPH), also called malresorptive hydrocephalus, is a form of communicating hydrocephalus in which excess cerebrospinal fluid (CSF) occurs in the ventricles, and with normal or slightly elevated cerebrospinal fluid pressure. As the fluid builds up, it causes the ventricles to enlarge and the pressure inside the head to increase, compressing surrounding brain tissue and leading to neurological complications. The disease presents in a classic triad of symptoms, which are memory impairment, urinary frequency, and balance problens/gait deviations (note: this diagnosis method is obsolete). The disease was first described by Salomón Hakim and Adams in 1965.The treatment is surgical placement of a ventriculoperitoneal shunt to drain excess CSF into the lining of the abdomen where the CSF will eventually be absorbed. NPH is often misdiagnosed as other conditions including Parkinsons disease (due to gait) or Alzheimers disease (due to cognitive dysfunction).

Signs and symptoms
NPH exhibits a classic triad of clinical findings (known as the Adams triad or Hakims triad). The triad consists of walking difficulty, reduced attention span, and urinary frequency or incontinence. The triad is considered obsolete for diagnosis purposes and newer guidelines are available.Gait deviations are present in nearly all patients and usually the first symptom. This is caused by expansion of the lateral ventricles to impinge on the corticospinal tract motor fibers. The typical gait abnormality in NPH is a broad-based, slow, short-stepped, "stuck to the floor", or "magnetic" movement. The gait abnormalities in NPH may bear resemblance to a gait associated with Parkinsons disease. The gait deviation can be classified as mild, marked, or severe: "marked" is when the patient has difficulty walking because of considerable instability; "severe" is when it is not possible for the patient to walk without aids (such as a cane or a wheeled walker).Dementia presents as progressive cognitive impairment which is present in 60% of patients at time of treatment. This is caused by distortions predominantly at the frontal lobe and the subcortex. Initial deficits involve planning, organization, attention, and concentration. Further deficits include difficulty managing finances, taking medications, driving, keeping track of appointments, daytime sleeping, short-term memory impairments, and psychomotor slowing. Late stage features include apathy, reduced drive, slowed thinking, and reduced speech.
Urinary incontinence appears late in the illness, and is present in 50% of patients at time of treatment. Urinary dysfunction begins as increased frequency often at night, and progresses to urge incontinence and permanent incontinence.

Pathogenesis
Every day, the body makes roughly 600–700 ml of CSF, and about the same amount is reabsorbed into the bloodstream. Hydrocephalus is due to an imbalance between the amount of fluid produced and its absorption rate. Enlarged ventricles put increased pressure on the adjacent cortical tissue and cause myriad effects in the patient, including distortion of the fibers in the corona radiata. This leads to an increase in intracranial pressure (ICP). The ICP gradually falls, but still remains slightly elevated, and the CSF pressure reaches a high normal level of 150 to 200 mm H2O. Measurements of ICP, therefore, are not usually elevated. Because of this, patients do not exhibit the classic signs that accompany increased intracranial pressure such as headache, nausea, vomiting, or altered consciousness, although some studies have shown pressure elevations to occur intermittently.The exact pathogenesis is unknown, but consensus on some mechanisms include:
An imbalance exists between production and resorption of CSF.
The resistance to CSF outflow is often elevated.
The disease is not caused by overproduction of CSF or obstruction of CSF flow at the ventricles.The syndrome is often divided into two groups, primary (also called idiopathic) and secondary, based on cause. The underlying etiology of primary NPH has not yet been identified. Primary NPH affects adults age 40 years or older, most commonly affecting the elderly. Secondary NPH can affect persons of any age and occurs due to conditions such as subarachnoid hemorrhage, meningitis, brain surgery, brain radiation, or traumatic brain injury.

Diagnosis
Patients with suspected NPH should have typical symptoms in addition to ventricular enlargement on neuroimaging. The international evidenced-based diagnostic criteria for primary NPH are:
Gradual onset after age 40 years, symptoms duration of ≥ 3–6 months, clinical evidence of gait or balance impairment, and impairment of cognition or urinary incontinence
Imaging from magnetic resonance imaging (MRI) or computed tomography (CT) is needed to demonstrate enlarged ventricles and no macroscopic obstruction to cerebrospinal fluid flow.  Imaging should show an enlargement to at least one of the temporal horns of lateral ventricles, and impingement against the falx cerebri resulting in a callosal angle ≤ 90° on the coronal view, showing evidence of altered brain water content, or normal active flow (which is referred to as "flow void") at the cerebral aqueduct and fourth ventricle.MRI scans are the preferred imaging. The distinction between normal and enlarged ventricular size by cerebral atrophy is difficult to ascertain. Up to 80% of cases are unrecognized and untreated due to difficulty of diagnosis. Imaging should also reveal the absence of any cerebral mass lesions or any signs of obstructions. Although all patients with NPH have enlarged ventricles, not all elderly patients with enlarged ventricles have primary NPH. Cerebral atrophy can cause enlarged ventricles, as well, and is referred to as hydrocephalus ex vacuo.
The Miller Fisher test  involves a high-volume lumbar puncture (LP) with removal of 30–50 ml of CSF. Gait and cognitive function are typically tested just before and within 2–3 hours after the LP to assess for signs of symptomatic improvement. The CSF infusion test and the lumbar test are similar tests to Miller Fisher test. The tests have a positive predictive value over 90%, but a negative predictive value less than 50%. The LP should show normal or mildly elevated CSF pressure. CSF should have normal cell contents, glucose levels, and protein levels.

Treatment
Ventriculoperitoneal shunts
For suspected cases of NPH, CSF shunting is the first-line treatment. The most common type used to treat NPH is ventriculoperitoneal (VP) shunts, which drain CSF fluid to the peritoneal cavity. Adjustable valves allow fine-tuning of CSF drainage