The incidence of shoulder pain following soft-tissue injuries to the neck is variable. In a prospective study of 93 car-accident victims, 16 (18%) were found to have shoulder symptoms at follow-up . Others have quoted higher figures but it is not clear what proportion, if any, had impingement syndrome as a specific diagnosis. Chauhan and colleagues examined 102 patients for evidence of impingement syndrome . The incidence of shoulder pain was found to be 22% but only 9% had subacromial impingement. Following soft tissue injuries to the neck up to a third of the patients can be expected to develop shoulder pain. The incidence of subacromial impingement however is less well established. In our series 26% of patients had developed shoulder symptoms, which is comparable to figures quoted above, but only 5% were found to have clinical signs of impingement syndrome on an average of 13 months after injury.
All our patients were involved in litigation and may therefore have different characteristics. It has been shown that long-term disability following neck injury is unrelated to the physical insult and those pursuing compensation have the highest physical disability in terms of neck pain . Although this has not been specifically validated for impingement syndrome following neck injuries, a similar outcome can be expected.
In our review of the literature we identified two other studies that reported shoulder pain and subacromial impingement following whiplash injuries to the neck [6, 12]. Gorski  described asymptomatic impingement syndrome: where patients with neck pain alone responded to subacromial injections with a complete or a substantial relief of their neck pain. They postulated that chronic neck pain can be caused by subacromial impingement which should be considered in the differential diagnoses even if the shoulder is asymptomatic.
In our study clinical examination was the main tool for diagnosing subacromial impingement although some of our patients (table 1) did have radiological confirmation. Clinical tests in combination have been shown to have high post test probabilities for rotator cuff pathology . Muddu et al  have suggested that the primary pathology is due to a whiplash injury to the shoulder, as a separate entity, rather than impingement syndrome. In their series 15 out of 18 patients who were found to have 'shoulder symptoms' by a consultant orthopaedic surgeon had no significant shoulder pathology on MRI. In fact only 2 from 18 patients (11%) demonstrated rotator cuff tears and evidence of subacromial impingement. It is not clear however if their patients had positive clinical signs for subacromial impingement (despite their negative MRI) or they were merely complaining of generalised shoulder pain following their neck injury.
Pain radiating from the neck to the shoulder after whiplash injuries is common and difficult to treat. In contrast impingement syndrome can be helped with physiotherapy, injection of corticosteroids and even surgery. It is therefore important for clinicians to suspect and correctly diagnose subacromial impingement in patients complaining of shoulder pain following neck injuries instead of merely blaming radicular neck pain as the cause. In fact careful assessment can even identify and successfully treat a group of patients who may present with 'asymptomatic impingement' with pain outside the neck and at the medial aspect of the scapula but not in the shoulder itself .
In our series all the patients with subacromial impingement had consulted their family doctor but only 9% had been referred to a specialist and less than a third had had their diagnosis made prior to our medicolegal report. None were diagnosed by their general practitioners. This study highlights the fact that a potentially treatable condition in a small group of patients is diagnosed late or not at all due to lack of awareness of the association between neck injury and subacromial impingement.
The exact cause of impingement syndrome associated with whiplash injuries is subject to debate. In our study the seatbelt shoulder was involved in 83% of cases (X2, P = 0.021) suggesting direct trauma from the seatbelt as a possible cause. Moreover all 11 patients had developed their symptoms early and between 1 and 7 days after the injury further supporting direct trauma as an underlying cause. Only two (17%) patients had symptoms in the non-seatbelt shoulder. But even these patients were found to have evidence of direct trauma to the non-seatbelt side of their body. 'Patient 4' who was a driver with left subacromial impingement was noted to have 'bruising' around the left elbow and forearm on the day of the accident. 'Patient 9' who was a driver with bilateral impingement (left worse than right) also had severe bruising and tenderness on the left chest wall and axilla after the accident and was admitted to hospital for analgesia and observation. In our study therefore, all of the shoulders that had developed subacromial impingement had been subject to direct trauma, by the seatbelt or otherwise.
The average age in the group of patients who developed subacromial impingement was higher than those without subacromial impingement: 57.5 years versus 36.9 years. This difference is statistically significant (T-test, p = 0.002). This suggests that age or pre-existing degenerative change leading to a decrease in the subacromial space may be a risk factor for developing subacromial impingement following direct trauma to the shoulder.
This study has several limitations. It is based on patients in legal proceedings and may not truly reflect the general population. The diagnosis of subacromial impingement was made on clinical grounds only and although imaging was available in a number of cases (table 1) it was not used universally. Injection of local anaesthetic into the subacromial space would have been a useful adjunct to the assessment of the cohort.
Although a significant number of seat-belted shoulders were identified, the numbers involved were small and a larger study needs to be conducted to confidently link seatbelt trauma to the development of impingement syndrome.