SpPin and SnNout

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:

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

Intracranial Pressure
Intracranial Pressure
retinal vein pulsation
43 18 61
retinal vein pulsation
0 128 128
total 43 146 189

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:

[Walsh TJ, Garden JW, Gallacher B: Obliteration of retinal vein pulsations during elevation of cerebrospinal-fluid pressure. Amer J Opthalmology 1969;67:954-6.]