When Is it Best to Evaluate the Results of a CSF Tap Test?
Abstract & Commentary
By John J. Caronna, MD, Professor of Clinical Neurology, Weill Cornell Medical College. Dr. Caronna reports no financial relationships relevant to this field of study.
Synopsis: The tap test has variable results as a tool to assess patients with a presumed diagnosis of normal pressure hydrocephalus.
Source: Virhammar J, et al. The CSF tap test in normal pressure hydrocephalus: Evaluation, reliability and the influence of pain. Eur J Neurol 2011; doi:10.1111/ j.1468-1331.2011.03486x.
Some adults with the clinical triad of gait disturbance, dementia, and urinary incontinence associated with normal pressure communicating hydrocephalus will improve after a cerebrospinal fluid (CSF) shunting procedure.1,2 The diagnosis of normal pressure hydrocephalus (NPH) is based on the clinical examination, brain imaging, and supplementary tests of CSF circulation. One commonly used prognostic test to select patients for shunt surgery is the CSF tap test (TT): Up to 50 mL of CSF are removed by lumbar puncture, and assessments of gait, cognitive function, and urinary symptoms are performed before and after CSF withdrawal. The TT procedure and the assessments of function after the procedure vary among centers, as does the reported sensitivity of the test, which ranges from poor to only moderate.3
These authors aimed to identify the optimal time for gait evaluation after TT, to assess the variability between two repeated measurements and the inter-rater agreement of the gait tests chosen, and to clarify whether post-lumbar puncture pain (backache and/or headache) affects gait performance.
Forty patients under evaluation for NPH at Uppsala University Hospital in Sweden underwent TT. Standardized gait analyses were performed before and at 2, 4, 6, 8, and 24 hours after TT. Each evaluation was reported twice by two independent investigators. Twenty-seven patients (15 men and 12 women; 69%), showed improvements in gait, speed, and number of steps after TT that were significant. Only nine patients were constantly improved at every time of assessment after TT. At 2 hours, only 15 patients were improved; at 4 hours the number was 18; at 6 hours 23; at 8 hours 26; and at 24 hours 27. Therefore, evaluations at more than one assessment time for the individual patient increased the chance of noticing improvement. The variability between two measurements was low and the inter-rater agreement was good. Pain was notable in 50% of the patients and correlated negatively with improvement in gait speed at 2 and 8 hours after TT.
Commentary
The authors did not determine the sensitivity of their assessments after TT in predicting which NPH patients would benefit from a CSF shunting procedure, and did not report the results of surgery in this series. The authors, however, have provided useful information to clinicians who use the TT to select patients for surgical shunting: The results of TT can be evaluated within the first 24 hours, but positive results may not be evident until the 8–24 hours period. In the first 8 hours after LP, back pain and orthostatic headache may limit the accuracy of the assessments of gait.
At present, most clinicians prefer to identify patients as either probable or improbable shunt responders on the basis of clinical history and neuroimaging techniques. A short history, a known cause of hydrocephalus, and the predominance of gait disorder rather than dementia, as well as the absence of both substantial cortical atrophy with extensive white matter involvement in a patient, predict a greater than 50% chance of responding to shunt surgery regardless of the TT results.
References
1. Hakim S, Adams RD. The special clinical problem of symptomatic hydrocephalus with normal cerebrospinal fluid pressure. Observations on cerebrospinal fluid hydrodynamics. J Neurol Sci 1965;2:307-327.
2. Adams RD, et al. Symptomatic occult hydrocephalus with "normal" cerebrospinal-fluid pressure. A treatable syndrome. N Engl J Med 1965;273:117-126.
3. Relkin N, et al. Diagnosing idiopathic normal-pressure hydrocephalus. Neurosurgery 2005;57:S4-16.
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