S leep problems
A ppetite or weight change
D ysphoria or low mood
A nhedonia : lack of interest in pleasure
F atigue
A gitation/psychomotor retardation (movements slowed)
C oncentration problems
E steem problems
S uicidal ideas

Major depression would involve the presence of 5 or more of these 9 symptoms, including low mood or anhedonia over a period of more than 2 weeks. 

Another way of looking at depressive symptoms:

• Persistent sad, anxious or "empty" mood
• Sleeping too much or too little; middle-of-night or early-morning waking
• Reduced appetite and weight loss, or increased appetite and weight gain
• Loss of pleasure and interest in activities once enjoyed, including sex
• Irritability, restlessness or being slowed up
• Difficulty concentrating, remembering or making decisions
• Fatigue or loss of energy
• Feeling guilty, hopeless or worthless
• Thoughts of death or suicide
• In severe depression, psychotic symptoms such as hallucinations or delusions
• Persistent physical symptoms that do not respond to treatment (such as chronic pain or digestive disorders)*

* Obviously this is difficult to distinguish from the usual ongoing symptoms of arachnoiditis, but it is worth bearing in mind that depression may exacerbate the perceived pain level.