Evidence on diagnostic rule-in signs for functional neurological disorders (FND)

[cassava7]

Leading FND researchers (including Stone, Carson, Perez, Edwards) argue that the diagnosis of FND must now be a positive one, i.e. based on rule-in signs rather than being solely a diagnosis of exclusion. One implication, as David Tuller has shown, is that this reduces the overall prevalence of FND.

I have not seen so far a discussion thread on the evidence base pertaining to these rule-in signs, so I thought it might be worthwhile to create one. What I am specifically interested in is the validation studies of these rule-in signs. I have not been through the literature to find them but I suspect they may not have been replicated. If you find some, please link to the relevant thread in this one.

The only rule-in signs that I am aware of are Hoover’s sign and the hip abductor sign for functional limb weakness [1]:

Hoover's sign is positive if there is weakness of hip extension (left) which returns to normal with contralateral hip flexion against resistance (right). The hip abductor sign is similar test showing discrepancy between voluntary and automatic hip abduction strength.

This video shows testing Hoover’s sign on a stroke patient then a patient with functional limb weakness. The stroke patient’s right heel does not exert any downwards strength when pressure is applied on the other leg, contrary to that of the second patient:


For functional dystonia, a variety of signs are present [1]:

Functional dystonia usually presents with a fixed posture with ankle inversion (a), or flexion of fingers (b) or wrist. In the face, look for jaw deviation, contraction of platysma or orbicularis and sometimes tongue deviation (c).

For reference, other FNDs include [1]:

- Functional tremors and other abnormal movements
- Dissociative/functional seizures (PNES)
- Functional cognitive symptoms
- Urinary retention and ‘scan-negative’ cauda equina syndrome
- as well as a variety of symptoms:

speech problems (such as dysarthria, mutism or foreign accent syndrome); sensory symptoms (including numbness or positive sensory symptoms especially with motor symptoms); visual loss or diplopia; hearing loss or sensitivity; and globus.34,35 Persistent postural perceptual dizziness (PPPD) describes chronic dizziness as part of a functional disorder. It has specific diagnostic criteria and clinicians should be wary of diagnosing it too easily in anyone with persistent dizziness.36

[1] Bennett, K., Diamond, C., Hoeritzauer, I., Gardiner, P., McWhirter, L., Carson, A., & Stone, J. (2021). A practical review of functional neurological disorder (FND) for the general physician. Clinical medicine (London, England), 21(1), 28–36. https://doi.org/10.7861/clinmed.2020-0987

 

[ToneAl]

Most of these tests have very low specificity and can not be trusted as a main diagnostic tool. There is no gold standard to diagnose fnd.

Quote from why fnd is bulshit.

Poor, self-fulfilling, diagnostics: “Given the physiological evidence that functional movements are voluntary in nature” → This is based on things like the Hoovers test (see above). This is weak evidence at best.


Too many beliefs or assumptions regarding tests

 

[Mithriel]

The nervous system and the brain are so complicated - described as being "complicated beyond words" by one neurologist - that it is ludicrous to think that these neurological deviations from normal are caused by wrong thoughts or nerve dysfunction caused by psychological trauma.

Of course it is difficult to argue because they never explain exactly what they think is wrong, just airy fairy hand waving about brain plasticity or whatever. Everything is may or might or could then treated as proven.

These "rule in" signs may indicate something but there is no evidence it is FND as it is described. The research looked at FND patients and said they had them but it was the same group which had diagnosed them in the first place. These signs have been said to indicate hysteria for years so how many of the subject group were originally diagnosed because they had them?

They make a big thing of the neurological symptoms people have being "incongruent" but where is the congruence in these signs? They insist that the symptoms are real but the only validity for these tests is that the patient is faking, consciously or unconsciously. The unconscious was a big new idea a hundred years ago but is no longer spoken of publicly in medicine. None of it makes any sense.

I can see why the treatments help some people. Intensive physiotherapy works after strokes and other physical damage so it is plausible it would do the same for FND especially if there really is a physical problem. CBT could relieve anxiety and many people with MS say that stress and anxiety make their symptoms worse so it is possible that some people could benefit.

Dave Tuller has written useful articles about the way rule in is used to legitimise FND but then the prevalence rates include people who did not have or were never tested for these signs. However having the rule in signs is no proof that the patient has FND or will be helped by having such a diagnosis.

 

[alex3619]

The nervous system and the brain are so complicated

The brain is literally the most complicated thing studied in science. Astrophysics is simple by comparison. It aint rocket science, its much more difficult than that.

 

[cassava7]

Most of these tests have very low specificity and can not be trusted as a main diagnostic tool. There is no gold standard to diagnose fnd.

I do not know the sensitivities and specificities of FND diagnostic criteria (including tests like Hoover’s sign). My intent in creating this thread was to find the literature where they are reported to analyze it.

 

[dave30th]

i wrote this a couple years ago about the rule-in signs. it references some of the evidence for the claims.
https://www.virology.ws/2019/12/30/...r-signs-of-functional-neurological-disorders/

 

[ToneAl]

I remember Dave's post.
From the 2013 paper there is little evidence that the signs can identify people with fnd. Most of the signs have self fulfilling prophecies.

 

[cassava7]

Thank you @dave30th, I was not aware of your blog post. I will be glad to read it.

Importantly, I am concerned that FND rule-in signs may lump in people with side effects from psychotropic drugs such as tardive dyskinesia and akathisia. There has been some research, but at least one patient group has expressed their concerns.

“A functional neurological misdiagnosis, akathisia, and suicide”, response to the 2020 practice primer on FND in the BMJ: https://www.bmj.com/content/371/bmj.m3745/rr-0

“Tardive Dyskinesia—or Functional Stereotypy?” https://www.medpagetoday.com/resour...rdive-dyskinesiaor-functional-stereotypy/1788

 

[dave30th]

I was not aware of your blog post. I will be glad to read it.

it might be that there are much more updated studies of the accuracy of the signs since the 2013 look.