Hoover's sign: Clinical relevance in NeurologyMM Mehndiratta, M Kumar, R Nayak, H Garg, S Pandey
Department of Neurology, G.B. Pant Hospital, J.L.N. Marg, New Delhi, India
Hoover's sign was described by Dr. Charles Franklin Hoover more than 100 years back to differentiate between the organic and functional weakness of pyramidal origin. This test is usually performed in the lower limbs and is valuable when on bedside one is not sure about the nature of hemiparesis. A subject with hemiparesis of organic cause while asked to flex the hip of normal leg against resistance will not exert pressure on the hand of examiner placed under the heel on the affected side while in hysterical weakness heightened pressure will be felt on the examiner's hand. The presumed genesis of this sign could be the crossed extensor reflex or the principle of synergistic contraction. It is a useful clinical test in differentiating functional and organic paresis with moderate sensitivity (63%) and high specificity (100%), but there are some limitations which should be kept in mind while evaluating a patient.
Keywords: Hysterical, paresis, sign
The Hoover's sign is a simple bedside clinical test helpful in distinguishing organic from functional paresis, first described by Dr. Charles Franklin Hoover in 1908. Grasset and Gaussel described this phenomenon earlier in the setting of unilateral pyramidal lesions (1905) and Bychowski in subjects with epilepsy and multiple sclerosis (1907). Hoover for the first time described this phenomenon in great detail. Subsequently, Jean Lhermitte confirmed his findings and named the sign as "le phenomene de Hoover" or Hoover Phenomenon. 
The method of elicitation of this sign as originally described by Hoover was subsequently modified by Adams and Victor and is mentioned below in two steps. 
In normal subjects, in supine position, hip extension occurs on attempting flexion of contralateral hip against resistance. In organic hemiplegia, the test is elicited while doing hip extension and flexion:
The same procedure is used to elicit functional paresis as: 
Hoover's sign in the arms
Hoover described a similar phenomenon of "complementary opposition" in the arms. In a patient with functional weakness, flexion of an outstretched arm against resistance produces involuntary extension of the other arm.  In another method to detect functional weakness of upper limb, on testing shoulder adduction in one limb, the contralateral side will also adduct in patients with functional paresis of upper limb, while it will remain stable in organic weakness. 
Possible mechanisms of Hoover's sign
The Hoover's sign relies on the principle of synergistic contraction, and originally referred by Hoover as "complementary opposition." Another possible explanation for the genesis of Hoover's sign can be crossed extensor reflex as described by Sherrington in 1910.  The pathway for this reflex involves excitatory spinal interneurons, which traverse multiple levels of the spinal cord before producing an antagonistic contraction in the opposite limb. This spinal reflex mechanism probably helps in stabilizing the body or trunk and can be simply exemplified as; a pin prick to one leg will result in its flexion, while the hamstring(s) of the other leg contract to make it stable on the floor.
Usefulness and validity of the Hoover's sign
Hoover's sign has been evaluated in two small controlled studies. Ziv et al. carried out a controlled study of Hoover's sign using computerized myometry.  Patients were grouped into three diagnostic categories including patients with functional limb weakness, organic weakness, and healthy controls. Authors demonstrated that, maximal involuntary/voluntary force ratio (IVVR) of the lower limbs was similar in healthy subjects, patients with organic weakness and in the non-affected limbs of patients with functional weakness. In contrast, IVVR of the affected limbs in the functional paresis group was significantly higher (mean value 2.48 Vs 0.614, P < 0.001). Hence, quantitative measurement of the IVVR can be helpful in diagnosing non-organic weakness in either lower or upper limbs. Diukova et al., by using a simple weighing scale studied nine subjects with functional weakness and compared them with control groups having organic weakness, back pain, or no weakness. The results were similar as reported by Ziv et al. 
In another study, McWhirter et al evaluated Hoover's sign as a part of the diagnostic work-up in patients presenting with suspected stroke. Authors found moderate sensitivity (63%) and high specificity (100%) of Hoover's sign for a diagnosis of functional weakness. 
Other important clinical tests to discriminate between organic and functional paresis
Abductor sign (also known as Sonoo abductor sign)
It is another useful clinical test, which can distinguish between organic and non-organic leg paresis using synergic movements of the bilateral hip abductors.  To elicit the test, the examiner asks the patient to abduct each leg and oppose this movement with his hands placed on the lateral aspects of the patient's legs. The leg contralateral to the abducted one shows opposite actions for organic and functional paresis. When the weak leg is abducted, the normal leg remains stable in patients with organic paresis, but moves in the hyperadducting direction in functional paresis. In contrast to Hoover's sign, which depends on the subjective assessment of the examiner, the advantage of the abductor sign is that it gives a simple, visible result in terms of movement or steadiness of the unabducted leg. Similar to Hoover's sign, abductor sign has excellent specificity and sensitivity. 
In functional weakness, the contraction of an antagonist muscle can be appreciated on testing the agonist muscle. For example, contraction of triceps can be felt when the biceps is being tested. 
It has been demonstrated that because of the bilateral innervation to sternocleidomastoid muscle, patients with organic hemiparesis due to upper motor neuron lesions do not have sternocleidomastoid weakness, which is usually present in functional hemiparesis. 
Limitations of the Hoover's sign
False-positive result can occur in the following conditions
False-negative result can occur in the following conditions 
1. If applied to patients with bilateral symptoms.
2. The patient may not be concentrating on flexing their sound hip when examiner is testing involuntary extension of the weak hip.
Hoover's sign is a useful clinical test in differentiating functional and organic paresis. However, clinician should always keep in mind the limitations of the test as mentioned above. The patient may have a combination of organic and functional weakness, and the result of the test should be interpreted cautiously.  It is also important to emphasize that Hoover's sign does not differentiate conversion disorder from malingering or simulated weakness. Further case-control studies are needed to evaluate the validity of this important clinical sign, as previously performed studies included small number of patients and were not blinded.