Acute Mountain Sickness (aka AMS, or altitude sickness) can bring you or a teammate to a halt with various mild symptoms such as loss of appetite, headache or nausea, Untreated, it can lead to more severe problems like cerebral or pulmonary edema.
In particular, ataxia, (doctor speak for "loss of coordination") can indicate more severe AMS, or may be a precursor to cerebral edema, which is extremely serious.
Here’s a quick field test can help determine the degree of ataxia, and can help you decide if someone needs to descend.
Ask the AMS affected person to stand in a flat, safe place. Have at least one other person standing alongside to catch them if they start to stumble. Have the AMS “patient” stand with their feet together (insides of the boots touching) have hands at their sides, and finally close their eyes.
If they can hold this position for 10 seconds without a stumble or fall, they are okay to continue. If they stumble, consider a prompt descent.
This tip was taken from from Rock and Ice magazine
A Gamow bag, used to treat severe altitude related illness, is NOT where you want to be on your climb. But it beats the alternative.