TROOPS in Afghanistan are suffering invisible wounds from bomb blasts that can leave them with brain damage, depression and post-traumatic stress.

The injury - caused by the pressure wave from a bomb explosion that leaves no visible external wound - accounts for about 12 per cent of all wounds inflicted on Australian troops in Afghanistan.

Mild traumatic brain injury - MTBI - is expected to rise as Australia suffers its share of what has been described in the US as a brain damage epidemic. While numbers are down this year, the Australian Defence Force expects they will rise as new tests, including one used on concussed AFL players, make it easier to detect.

Advertisement: Story continues below

Two soldiers were diagnosed with the injury this month after being hit by roadside bomb blasts four times in five weeks. They sought help only after the delayed onset of symptoms, which can include memory loss, headaches, a loss of balance, an inability to think clearly, irritability, mood swings and depression.

Concerned at the long-term impact on serving soldiers and discharged veterans, Australian and US military medical experts will meet at a conference on the injury in Canberra next month. The ADF has introduced compulsory testing of all soldiers involved in bomb blasts and is considering adopting the US practice of removing soldiers from combat if they have been in three bombings.

More than 200,000 US soldiers have been diagnosed with traumatic brain injury, ranging from concussion to serious brain damage. While most recover quickly, studies show up to 15 per cent could suffer long-term effects, including symptoms similar to post-traumatic stress.

Of the 192 Australians wounded in Afghanistan, 24 have been diagnosed with MTBI. Another three suffered the injury in Iraq.

Brain injury is described as the ''signature wound'' of the Iraq and Afghanistan wars. It is caused by the insurgents' weapon of choice - homemade bombs called improvised explosive devices.

Bombs have killed 14 of the 29 Australians lost in Afghanistan, and were behind about half of the injuries.

Despite its name, experts say the effects of MTBI are not always mild.

'''Mild' means the guy is still walking around talking, but the brain might have been significantly affected by the injury,'' Melbourne neurosurgeon Jeffrey Rosenfeld, a brigadier in the Army Reserve, said.

He and the psychiatrist Nick Ford, an Army Reserve major, sounded the alarm in an army journal in 2008, when soldiers suffering MTBI were often diagnosed as having a psychiatric condition.