Have you heard the one about the rugby player who got concussed but couldn't remember how?

If only the injury saga of Hurricanes captain Dane Coles were a laughing matter. Instead the severity of his situation worsened on Wednesday, with confirmation that concussion symptoms are among the issues now keeping him from the field.

We were all told Coles injured knee ligaments in the Hurricanes' March 18 win over the Highlanders. Then one of his calf muscles - which he has a long history of tearing - went one day at training.

HAGEN HOPKINS/GETTY IMAGES Knee, calf and head injuries have kept Dane Coles out of the Hurricanes' line-up since this clash with the Highlanders on March 18.

Now Hurricanes head coach Chris Boyd has revealed Coles is suffering exercise-induced headaches and it's those, as much as the calf problem, that are keeping him from the paddock. The funny part - if you could call it that - is no-one knows how or when the head trauma occurred.

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NZN VIDEO The Hurricanes have looked at the video tapes from Dane Coles' last game and can't find where the hooker may have suffered a head knock.

"We genuinely don't know where he got those symptoms from, but those symptoms were not apparent to us until post his return-to-train from his knee injury. So he had no symptoms post that game, if in fact he incurred the injury in the game," Boyd said.

Players regularly receive head trauma without being concussed and headaches after or during exercise are often the first clue. In Coles' case, he was told not to exercise at all for two weeks because of his knee, meaning there was a delay before any head trauma symptoms became obvious.

Between the calf and the head, Coles is doing only minimal training, with Boyd unsure which of the ailments is primarily to blame.

HAGEN HOPKINS/GETTY IMAGES. Hurricanes captain Dane Coles hobbles off Westpac Stadium after injuring his knee against the Highlanders.

"It's a really interesting question and I asked that of the medical group this morning and I'm not a medical man but it's probably 50-50. We can't load his calf because he's getting headaches and in the end I'm not sure whether his concussion symptoms are going to go away first or whether his calf issue is going to go away first, but we can't test the calf until they do," said Boyd.

The longer this goes on, the more out of condition Coles becomes so even when he is unhindered by injury, he'll then need a couple of weeks to get fit.

The frustration from the outside is that on an almost-weekly basis since March 18, we've been told Coles wasn't far away from playing or could've been picked "if it had been a final."

Then there's the uncertainty around how the head injury happened.

"No [Coles has no idea]. We've had a look at the Highlanders game, which was the last game he played. There's no obvious point in time, he had no symptoms post that, so I'm not sure," Boyd said.

"I don't think there's any smoke and daggers there. I think at the end of the day he's started to return to train and developed some headaches."

Coles doesn't have a history of head trauma injuries but his one of three Hurricanes - along with prop Reggie Goodes and centre Matt Proctor - unable to play at the moment due to concussion symptoms. A fourth, lock James Broadhurst, retired last week as a result of continued head trauma symptoms stretching back to a concussion in 2015.

Players are certainly more aware and better educated about concussion than they were previously and there's a more cautious approach all round. Coles has been open with team-mates and associates about his situation and the team opted to make that public as a result.

Coles' injury would come as a further blow to the All Blacks who are beginning to feel the strain of Super Rugby on their selection prospects with many of their leadership group sustaining injuries in recent weeks.

Among the All Blacks to suffer an injury have been captain Kieran Read, flanker Jerome Kaino and fullback Ben Smith.