2.  Post-exertional symptom exacerbation: e.g. acute flu-like symptoms, pain and worsening of other symptoms.
 
3.  Post-exertional exhaustion may occur immediately after activity or be delayed by hours or days.

4.  Recovery period is prolonged, usually taking 24 hours or longer. A relapse can last days, weeks or longer. 

5.  Low threshold of physical and mental fatigability (lack of stamina) results in a substantial reduction in pre-illness activity level.

B. Neurological Impairments 
 
At least one symptom from three of the following four symptom categories: 
 
1.  Neurocognitive Impairments 

a.  Difficulty processing information: slowed thought, impaired concentration e.g. confusion, disorientation, cognitive overload, difficulty with making decisions, slowed speech, acquired or exertional dyslexia.

b.  Short-term memory loss: e.g. difficulty remembering what one wanted to say, what one was saying, retrieving words, recalling information, poor working memory.
 
2.  Pain

a. Headaches: e.g. chronic, generalized headaches often involve aching of the eyes, behind the eyes or back of the head that may be associated with cervical muscle tension; migraine; tension headaches .

b.  Significant pain can be experienced in muscles, muscle-tendon junctions, joints, abdomen or chest. It is non-inflammatory in nature and often migrates. e.g. generalized hyperalgesia, widespread pain
(may meet fibromyalgia criteria), myofascial or radiating pain.
 
3.  Sleep Disturbance 

a.  Disturbed sleep patterns: e.g. insomnia, prolonged sleep including naps, sleeping most of the day and being awake most of the night, frequent awakenings, awaking much earlier than before illness onset, vivid dreams/nightmares.
 
b.  Unrefreshed sleep: e.g. awaken feeling exhausted regardless of duration of sleep, day-time sleepiness
 
4.  Neurosensory, Perceptual and Motor Disturbances 

a.  Neurosensory and perceptual: e.g. inability to focus vision, sensitivity to light, noise, vibration, odour, taste and touch; impaired depth perception.
 
b. Motor: e.g. muscle weakness, twitching, poor coordination, feeling unsteady on feet, ataxia.

C. Immune, Gastro-intestinal & Genitourinary Impairments 
 
At least one symptom from three of the following five symptom categories:
 
1.  Flu-like symptoms may be recurrent or chronic and typically activate or worsen with exertion. e.g. sore throat, sinusitis, cervical and/or axillary lymph nodes may enlarge or be tender on palpitation.
 
2.  Susceptibility to viral infections with prolonged recovery periods.
 
3.  Gastro-intestinal tract: e.g. nausea, abdominal pain, bloating, irritable bowel syndrome.
 
4.  Genitourinary: e.g. urinary urgency or frequency, nocturia.
 
5. Sensitivities to food, medications, odours or chemicals

D. Energy Production/Transportation Impairments