Q:

Is there a way to modify the theorem environment in order to have an horizontal rule immediately before and immediately after the statement? 

I've just seen this solution for the proof environment and I really liked it. 
How to draw a horizontal line at the end of proofs? 
I'm wondering if is there an elegant way to do it also with the theorem environment. 
Note
I don't want to use boxes or mdframed packages...

Edit: the following code for example doesn't work:
\documentclass[11pt,a4paper]{book}
    \usepackage[utf8]{inputenc}
    \usepackage[T1]{fontenc}
    \usepackage{fourier}
    \usepackage{lipsum}
    \usepackage{amsmath}

    \usepackage{thmtools} 
    \usepackage[thmmarks, thref, amsmath]{ntheorem}
    \theoremstyle{plain}
    \theoremprework{\bigskip\hrule\vspace{-1.5ex}\leavevmode\nobreak}%\leavevmode
    \theorempostwork{\vspace*{-1ex}\hrule\bigskip\leavevmode}
    \theoremheaderfont{\scshape}
    \theorembodyfont{\itshape}
    \theoremseparator{. } 
    \newtheorem{thm}{Theorem}
    \begin{document}
    \tableofcontents
    \lipsum[1]
    \begin{thm}[D’Alembert-Gauss]
    The field $\mathbf C$ is algebraically closed. 
    \end{thm}
    \lipsum[2]

 \addcontentsline{toc}{chapter}{List of Theorems}
 \listoftheorems[ignoreall,onlynamed={thm,lmm,prop}
\end{document} 

A:

Quite simple with the  \theoremprework and \theorempostwork commands from ntheorem:
\documentclass[11pt,a4paper]{book}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{fourier}
\usepackage{lipsum}
\usepackage{amsmath}

\usepackage[thmmarks, thref, amsmath]{ntheorem}%
\theoremstyle{plain}
\theoremprework{\bigskip\hrule\vspace{-1.5ex}\leavevmode\nobreak}%\leavevmode
\theorempostwork{\vspace*{-1ex}\hrule\bigskip\leavevmode}
\theoremheaderfont{\scshape}
\theorembodyfont{\itshape}
\theoremseparator{. } 
\newtheorem{thm}{Theorem}
\usepackage{thmtools}
\begin{document}
\tableofcontents
\bigskip

\lipsum[1]
\begin{thm}[D’Alembert-Gauss]
The field $\mathbf C$ is algebraically closed. 
\end{thm}
\lipsum[2]
\bigskip
\addcontentsline{toc}{chapter}{List of Theorems}
\theoremlisttype{allname}
\listtheorems{thm}%

\end{document} 

