Captain Millers shaky hands in Saving Private Ryan PTSD or Parkinsons?

Captain Miller was suffering from PSTD only.

According to the wiki page of Saving Private Ryan :

Just before the Rangers embarked at Portsmouth, England, Miller's right hand began to shake uncontrollably for unknown reasons, though it hints he was suffering from post-traumatic stress disorder (PTSD). He kept this a secret from his men except for Horvath.

Definitely PTSD. I found this piece of dialogue in an early draft of the script (although I think it's in the final as well):

EXT. FRENCH COUNTRY SIDE - NIGHT

           The FINAL RUMBLES of the DISTANT ARTILLERY fade away.  The
           night is dark.  The band of six Americans makes their way
           warily along a French cart path.  Sarge eases up alongside
           Miller and speaks quietly to him.  The others don't overhear.

                                 SARGE
                     How long's your hand been shaking?

                                 MILLER
                     A couple of weeks.  It started in
                     Portsmouth when they brought us down
                     for loading.

                                 SARGE
                     Is it getting worse?

                                 MILLER
                     No.  It comes and goes.  It stops
                     when I look at it.

                                 SARGE
                     You may have to find yourself a new
                     line of work, this one doesn't seem
                     to agree with you anymore.

                                 MILLER
                     I'll be alright.

           Sarge looks at Miller, closely, evaluating him, they walk
           on.

Sarge's statement "You may have to find yourself a new line of work, this one doesn't seem to agree with you anymore" indicates that the shaking is a product of being in the war, which would definitely point to PTSD. Since it's the only time his shaking is ever addressed by anyone in the movie, there can be no other answer.

A very common reaction to the stresses of combat is adrenalin coursing through your veins. It will make you shake after a while. It is common when you experience the Fight or Flight response to life threatening situations. Though Miller gets is afterward, the adrenalin stays in your system for a while. Also, he could be getting very realistic flashbacks.

PTSD used to be called battle fatigue (which is when I studied its symptoms) and can set in pretty quickly, most likely in mild form. In the mild case, you have varying symptoms from developing a lisp, short term memory loss and stuff like that. I would not think it out of the question if shaking is involved.

Lets look at the facts as portrayed in the movie;

• Cpt Miller had an episode of facial masking when they landed on the beach, typical Parkinson's Disease in many people.
• Cpt Millers has a resting tremor that stops shaking when he looks at or holds his hand.
• Cpt Miller appeared to lose the ability to think strategically and questioned his own decisions at the bridge with Horvack.
• Cpt Miller was a School teacher, School teachers are over represented in many Parkinson's Disease studies
• Cpt Miller had a altered view of risk taking and pushed the limits, this can be a tell in Parkinson's Disease.
• Cpt Miller was somewhat a social individual and did not engage or push back when they took his platoon from him, this type of social behavior can Parkinson's Disease.
• Cpt Miller was happiest away from noise and was softly spoken which can be another sign of PD.
• General stiffness and soreness can be a symptom of Parkinson's Disease (PD)
• Cpt Miller didn't display signs of and swallowing issues or thermo regulation issues.