Blan v. Ali
In Blan v. Ali, 7 S.W.3d 741 (Tex. App.-Houston 14th Dist. 1999, no pet.), the appeals court held that a neurologist was qualified to testify regarding the standard of care applicable to any physician who undertakes to treat and care for a patient suffering from a stroke along with the neurological complications of lupus cerebrids. 7 S.W.3d at 744-47.
"A medical witness who is not of the same school of practice may be qualified to testify if he or she has practical knowledge of what is usually and customarily done by other practitioners under circumstances similar to those that confronted the defendant charged with malpractice." Id. at 745.
Moreover, "if a subject of inquiry is substantially developed in more than one field, a qualified expert in any of those fields may testify." Id.