The terms "myotubular myopathy" and "centronuclear myopathy" are sometimes used interchangably. 

"Myotubular myopathy" specifically refers to the infantile, X-linked form of the syndrome.  "Centronuclear myopathy" is used to describe forms inherited autosomally and with later onset.

To preserve your sanity and mine, the term "CNM" will be used in this section to refer to any form of this myopathy.

Similar to the other "classical" congenital myopathies, the infantile form of CNM is often severe and rapidly progessive, leading to respiratory failure and death.  The childhood form of the disease are usually less severe, and the adult form may be mild.  

Distinguishing Clinical Features

CNM is associated with extraocular muscle weakness and ptosis, which are not characteristic of the other "classic" congenital myopathies.

Needle EMG studies of patients with CNM may show fibrillations or myotonic discharges.

Pathologic Findings

The defining pathologic feature is the presence of a large proportion of muscle fibers with centrally located nuclei.  Examples are shown below (click to enlarge):

There is often a preponderance of type 1 muscle fibers.

The gene associated with the X-linked, infantile form is MTM1.  Other, as-yet-unknown genes are responsible for the autosomally-inherited forms.

References:

Katirji, B., Kaminsky, H.J., Preston, D.C., Ruff, R.L., and Shapiro, B.E. (Eds.) (2002). Neuromuscular Disorders in Clinical Practice. Butterworth Heinemann, Boston. (ISBN:0750671696)  Pp. 1114-1127.