reading HTTP "REMOTE_USER from a shiny app

[ziffification]

I have a CAs authenticated apache server with redirects to shiny like this 

<VirtualHost *:80>

     <Proxy *>

         Allow from localhost

     </Proxy>

     RewriteEngine on

     RewriteCond %{HTTP:Upgrade} =websocket

     RewriteRule /shiny/(.*) ws://localhost:3838/$1 [P,L]

     RewriteCond %{HTTP:Upgrade} !=websocket

     RewriteRule /shiny/(.*) http://localhost:3838/$1 [P,L]

     ProxyPass /shiny/ http://localhost:3838/

     ProxyPassReverse /shiny/ http://localhost:3838/

     ProxyPass /shiny-admin/ http://localhost:4151/

     ProxyPassReverse /shiny/ http://localhost:4151/

</VirtualHost>

what i want to do however is read and use the REMOTE_USER http variable in the shiny scripts how can I get this from the apache httpd variables????