Post-traumatic syringomyelia.

Some adults with the clinical triad of gait disturbances, mental deterioration, and urinary incontinence associated with chronic hydrodynamic hydrocephalus will improve after a CSF diversion procedure, even when the CSF pressure is normal at lumbar puncture. ' 2 This intriguing paradox was designated normal pressure hydrocephalus (NPH) by Hakim et al,' 2 and has continued to excite both clinicians and researchers seeking pathophysiological mechanisms in CSF hydrodynamics and both the clinical predictors and appropriate tests that would facilitate the accurate selection of candidates for a shunt. Experiments in chronic hydrocephalic animals and hydrodynamic studies in humans have shown that impairment of CSF flow and episodes of slightly raised CSF pressure are the basic defects in NPH.34 Our understanding of persistent or even increasing ventricular enlargement and clinical deterioration despite a "normal" CSF pressure is still incomplete, however. Periods of increased intraventricular pressure, including short lasting B waves,4-8 persistence of a slight pressure gradient between the ventricles and the cerebral convexity,9 a "waterhammer effect" due to increased CSF pulse pressure waves on the ventricular walls,'011 and altered viscoelastic properties of the ventricular wall'2 have been described as possible contributory mechanisms. It is still unclear whether there is a cause and effect relation between these phenomena and NPH or not. The site of impaired CSF flow may be situated within the ventricles or in the subarachnoid space. "3-'8 In noncommunicating NPH, partial obstruction to CSF flow within the ventricular system is mainly due to nontumoural aqueduct stenosis. '9 In communicating NPH, impairment of CSF flow is distal to the 4th ventricle, most often at the level of the basal cisterns ("cisternal block"). About 50% of communicating hydrocephalus is idiopathic, the other 50% secondary to subarachnoid haemorrhage, meningitis, cranial trauma, or intracranial surgery.271314 There are neither physiological nor pathological arguments to maintain the myth that NPH may be due to defective CSF absorption through the arachnoid villi. This would not lead to ventricular enlargement, as there is no pressure gradient between the ventricles and the convexity.202' Clinical deterioration is probably due to diminished periventricular blood flow, possibly caused by hydrocephalic compression and stretching of the periventricular arterioles and venules, as shown in experimental hydrocephalus22 and in studies on cerebral blood flow in NPH.23-26 This would lead to a state of "misery perfusion 27; with periventricular blood flow reduced sufficiently to result in axonal dysfunction but not enough to cause irreversible loss of cellular integrity. Narrowed periventricular arterioles due to hyaline vessel wall degeneration in hypertensive patients might be another precipitating factor and would explain why the prevalence of vascular risk factors in NPH is increased.28'0 Prolonged periventricular ischaemia would eventually result in myelin disintegration and irreversible axonal loss, possibly explaining why some patients with NPH do not improve after a shunt. Precise epidemiological data on NPH are lacking. Katzman suggested that the total number of cases in the United States is 10 000 ± 5000, about one per neurologist.3' In published series, NPH accounts for 0 to 6% of cases of dementia,3' the variability of these percentages being partly explained by the use of different diagnostic criteria and referral biases. In a recent study, the incidence of clinical improvement after a shunt in 166 patients with presumed NPH was only 2-2 per million persons per year. 18 Assuming that the incidence of dementia in the same area might be estimated at 600 per million persons per year, shunt responsive NPH would represent only about 0 4% of the causes of dementia. Initially, NPH was thought to be a disease occuring mainly in the presenium7 but later many series contained many patients in the sixth decade or older.3438 A review of the literature including 21 articles and more than 1000 patients shunted for presumed NPH showed that substantial improvement occurred in about 30% of idiopathic NPH, in 50-70% where the aetiology was known. The total number with postsurgical complications was 30-40%, resulting in death or severe residual morbidity in 5-15%. 8