Itemize inside longtable

The following is a working example. Because lcr are for one-line entries, itemize shouldn't work with them. Compare the first part, improved, with the second, unchanged.

\documentclass[a4paper,twoside,11pt,openright]{report}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{longtable}  
%\usepackage[inline]{enumitem}   
\usepackage{enumitem}   
\usepackage{booktabs}


\begin{document}

\renewcommand*{\arraystretch}{1.8}
%\begin{longtable}{lcl}
\begin{longtable}{p{4cm}p{4cm}p{6cm}}
    \caption{Summary of proven determinants for falling}\\ \toprule
    \label{tab:FallPredictionVariables}
    \textbf{Author} &\textbf{Subject count (M:F)} & \textbf{Determinants}\\
    \midrule

    \noindent Author 1 &
    1780 (?) & \vspace{-\baselineskip}
    \begin{itemize}[label={--},noitemsep,leftmargin=*,topsep=0pt,partopsep=0pt]
        \item Recent falls (last 2 months)
        \item Agitation
        \item Frequent toileting
        \item Visual impairment
    \end{itemize}\\

    \noindent Author 2 &
    311 (?) &
    \begin{itemize}%[label={--},noitemsep,leftmargin=*,topsep=0pt,partopsep=0pt]
        \item Posture sway
        \item Two or more falls in previous year
        \item Low hand grip strength
        \item Depressive state of mind
    \end{itemize}\\
        \bottomrule

\end{longtable}

\end{document}

Without a full MWE, I've taken some code from Przemysław Scherwentke's answer:

\documentclass{report}
\usepackage{longtable}% http://ctan.org/pkg/longtable
\usepackage{array,booktabs,enumitem}% http://ctan.org/pkg/{array,booktabs,enumitem}
\newcolumntype{P}[1]{>{\endgraf\vspace*{-\baselineskip}}p{#1}}

\begin{document}

\renewcommand*{\arraystretch}{1.8}
\begin{longtable}{p{4cm}p{4cm}P{6cm}}
  \caption{Summary of proven determinants for falling}\label{tab:FallPredictionVariables} \\
  \toprule
  \textbf{Author} &\textbf{Subject count (M:F)} & \multicolumn{1}{l}{\textbf{Determinants}} \\
  \midrule

  \noindent Author 1 &
  1780 (?) & 
  \begin{itemize}[label={--},noitemsep,leftmargin=*,topsep=0pt,partopsep=0pt]
    \item Recent falls (last 2 months)
    \item Agitation
    \item Frequent toileting
    \item Visual impairment
  \end{itemize}\\

  \noindent Author 2 &
  311 (?) & 
  \begin{itemize}[label={--},noitemsep,leftmargin=*,topsep=0pt,partopsep=0pt]
    \item Posture sway
    \item Two or more falls in previous year
    \item Low hand grip strength
    \item Depressive state of mind
  \end{itemize}\\
  \bottomrule
\end{longtable}

\end{document}

The P{<len>} column type (thanks to array) inserts a "vertical unskip" so that the first item aligns properly with the remainder of the table.

The assumption is that the last P-column only contains lists, of course. If not, a separate \multicolumn has to be issued like with the heading Determinants.

I thought you could achieve this effect without having to hack an itemize. Just use multiple rows of the table:

\documentclass{report}
\usepackage{longtable}% http://ctan.org/pkg/longtable
\usepackage{array,booktabs}

\begin{document}

\begin{longtable}{p{4cm}p{4cm}>{-- }p{6cm}}
  \caption{Summary of proven determinants for falling}\label{tab:FallPredictionVariables} \\
  \toprule
  \textbf{Author} &\textbf{Subject count (M:F)} & \multicolumn{1}{l}{\textbf{Determinants}} \\
  \midrule

  \noindent 
  Author 1 & 1780 (?) & Recent falls (last 2 months) \\
           &          & Agitation                    \\
           &          & Frequent toileting           \\ 
           &          & Visual impairment            \\[1.8\baselineskip]

  Author 2 & 311 (?)  & Posture sway                 \\
           &          & Two or more falls in previous year \\
           &          & Low hand grip strength \\
           &          & Depressive state of mind \\               
\end{longtable}
\end{document}