To understand what is meant by the terms SpPin and SnNout, we need to understand the notions of sensitivity and specificity.
When a sign, test or symptom has an extremely high specificity (say, over 95%), a positive result tends to rule in the diagnosis. For example, the specificity of 3 or more positive responses on a CAGE questionnaire in diagnosing alcoholism is >99% among internal medicine patients. Therefore, if a person does answer “yes” to 3 or 4 of the CAGE questions, it rules in the diagnosis of alcohol dependency.
When a sign, test or symptom has a high sensitivity, a negative result rules out the diagnosis. For example, the sensitivity of the loss of retinal vein pulsation in diagnosing high intracranial pressure is 100 per cent. Therefore, if a person displays retinal vein pulsation, it rules out important increases in intracranial pressure.
These terms are closely related to the measures of:
- Positive Predictive Value: The proportion of people with a positive test who have the target disorder; and
- Negative Predictive ValueThe proportion of people with a negative test who do not have the target disorder.
Often the best place to look for SpPins and SnNouts is at the highest (for SpPins) and lowest (for SnNouts) levels of multilevel likelihood ratios.
These can be calculated thus:
sensitivity = a/(a+c)
specificity = d/(b+d)
likelihood ratio (LR+) = sensitivity / (1-specificity) = (a/(a+c)) / (b/(b+d))
likelihood ratio (LR-) = (1-sensitivity) / specificity = (c/(a+c)) / (d/(b+d))
positive predictive value = a/(a+b)
negative predictive value = d/(c+d)
Table: Retinal veins, pulsation of, and increased intracranial pressure
retinal vein pulsation
retinal vein pulsation
HIGH by lumbar puncture (>190 mm H20), surgery, or evidence of herniation.
NORMAL by the absence of signs, symptoms, or suspicion of high pressure.
(Sensitivity and the loss of SRVP = 100% = SnNout!; presence of SRVP in normals = Specificity = 128/146=88%)
[Levin BE: The clinical significance of spontaneous pulsations of the retinal vein. Arch Neurol 1978;35:37-40]
Walsh et al checked this out by watching RVP during the Queckenstedt manoeuvre among a grab sample of 9 neurology patients who had normal pulsation prior to the LP:
- disappeared when spinal fluid pressure rose past 204 (+/- 17) mm H2O
- reappeared when spinal fluid pressure fell past 201 (+/- 17) mm H2O
[Walsh TJ, Garden JW, Gallacher B: Obliteration of retinal vein pulsations during elevation of cerebrospinal-fluid pressure. Amer J Opthalmology 1969;67:954-6.]